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CopperCopper4. 4. HydrotallurgyHydrotallurgy

Fathi HabashiFathi HabashiDepartment of Mining, Metallurgical, and Materials EngineeringDepartment of Mining, Metallurgical, and Materials Engineering

Laval University, Quebec City, CanadaLaval University, Quebec City, CanadaFathi.Habashi@arul.ulaval.caFathi.Habashi@arul.ulaval.ca

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• The roots of hydrometallurgy may be traced back to the period of alchemists when the transmutation of base metals into gold was their prime occupation.

• Some of these operations involved wet, i.e., hydrometallurgical methods. For example, when an alchemist dipped a piece of iron into a solution of blue vitriol, i.e., copper sulfate, the iron was immediately covered by a layer of metallic copper.

• This apparent transmutation of iron into copper is represented in modern terms by the equation: Cu2+ + Fe → Cu + Fe2+, but it was not known at that time that blue vitriol contained copper.

In the sixteenth century, the extraction of copper by wet methods received some attention. Heap leaching was practiced in the Harz Mountains area in Germany and in Río Tinto mines in Spain.

In these operations, pyrite containing some copper sulfide minerals was piled in the open air and left for months to the action of rain and air whereby oxidation and dissolution of copper took place.

A solution containing copper sulfate was drained from the heap and collected in a basin. Metallic copper was then precipitated from this solution by scrap iron, a process that became known as “cementation process”.

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HydroHydro-- versus versus pyrometallurgypyrometallurgy of copperof copper

In the past fifty years, In the past fifty years, hydrometallurgistshydrometallurgistshave been vigorously searching new have been vigorously searching new methods to replace the traditional smelting methods to replace the traditional smelting route for copper production. route for copper production.

Advantages of hydrometallurgyAdvantages of hydrometallurgy

Sulfur dioxide generation In the pyrometallurgical treatment of copper sulfide ores, if

SO2 formed is in high enough concentration, it must be used for making acid and nearby market for this acid must be found.

If the SO2 concentration is too low for making acid, disposal methods must be found. These are available but expensive. As a result, in many cases SO2 is simply emitted to the atmosphere.

On the other hand, copper sulfides can be treated by hydrometallurgical methods without generating SO2, thus the independence of sulfuric acid manufacture.

In addition elemental sulfur can be recovered; it can be easily stockpiled or transported at low cost.

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Material handling In a copper smelter, the metallurgist is forced to

transfer molten slags and matte from one furnace to the other in large, heavy, refractory-lined ladles.

Beside the inconvenience and the cost of handling these materials, there is also the inevitable gas emission from them because they are usually saturated with SO2 and during transfer they cool down a little resulting in decreased gas solubility hence the inconvenient working conditions.

In hydrometallurgical plants, solutions and slurries are transferred by pipelines without any problem.

Energy consumption Energy consumption Because of the high temperatures involved in a copper Because of the high temperatures involved in a copper

smelter, which is usually around 1200smelter, which is usually around 1200°°C, the reaction C, the reaction rates are high but much fuel will be needed. rates are high but much fuel will be needed.

To make a process economical, heat recovery systems To make a process economical, heat recovery systems are essential. Heat can be readily recovered from hot are essential. Heat can be readily recovered from hot gases, but rarely from molten material like slag or metal. gases, but rarely from molten material like slag or metal. Thus, a great deal of energy is lost. Thus, a great deal of energy is lost.

Further, the equipment needed for heat economy is Further, the equipment needed for heat economy is bulky and expensive. bulky and expensive.

In the hydrometallurgical process, on the other hand, In the hydrometallurgical process, on the other hand, less fuel is needed because of the low temperatures less fuel is needed because of the low temperatures involved (usually 100 involved (usually 100 -- 200200°°C). Heat economy is usually C). Heat economy is usually no problem. no problem.

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Dust formationDust formation Combustion of fossil fuels in furnaces results in the Combustion of fossil fuels in furnaces results in the

formation of large volume of gases that carry over large formation of large volume of gases that carry over large amounts of fine dust. amounts of fine dust.

This must be recovered to abate pollution and because This must be recovered to abate pollution and because the dust itself is also a valuable material. the dust itself is also a valuable material.

The technology of dust recovery is well established but The technology of dust recovery is well established but the equipment is bulky and expensive. the equipment is bulky and expensive.

In hydrometallurgical processes, this is no problem In hydrometallurgical processes, this is no problem because wet material is usually handled.because wet material is usually handled.

Treatment of complex oresTreatment of complex ores

Treatment of CuTreatment of Cu--PbPb--Zn complex ores by Zn complex ores by pyrometallurgicalpyrometallurgical method is unsuitable method is unsuitable because separation is difficult; this is, because separation is difficult; this is, however most suitable by hydrometallurgy.however most suitable by hydrometallurgy.

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Treatment of lowTreatment of low--grade oresgrade ores

Treatment of lowTreatment of low--grade copper ores in a grade copper ores in a smelter is unsuitable because of the large smelter is unsuitable because of the large amount of energy required to melt the amount of energy required to melt the gangue minerals. gangue minerals.

On the other hand it is especially suitable On the other hand it is especially suitable by hydrometallurgy if a selective leaching by hydrometallurgy if a selective leaching agent is used.agent is used.

Economics Economics The economics of a copper smelter is usually The economics of a copper smelter is usually

suitable for large scale operations and this suitable for large scale operations and this requires a large capital investment. requires a large capital investment.

On the other hand, hydrometallurgical processes On the other hand, hydrometallurgical processes are suitable for small scale operations that can are suitable for small scale operations that can be increased when needed and therefore be increased when needed and therefore requires low capital investment. requires low capital investment.

Hydrometallurgy may also fit a special need that Hydrometallurgy may also fit a special need that cannot otherwise be met.cannot otherwise be met.

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Disposal of residuesDisposal of residues It is true that residue of a copper smelter is coarse and It is true that residue of a copper smelter is coarse and

harmless. For example, harmless. For example, slagsslags, which are a silicate phase, can , which are a silicate phase, can be stored in piles exposed to air and rain without the danger ofbe stored in piles exposed to air and rain without the danger ofdissolution and contaminating the streams. They are just dissolution and contaminating the streams. They are just unacceptable from the esthetic point of view. unacceptable from the esthetic point of view.

On the other hand most residues of hydrometallurgical On the other hand most residues of hydrometallurgical processes are finely divided solids. If they are dry, they creatprocesses are finely divided solids. If they are dry, they create e dust problems when the wind blows and when wet they will dust problems when the wind blows and when wet they will gradually release metal ions in solution that will contaminate gradually release metal ions in solution that will contaminate the environment. However, disposal of waste solution in the environment. However, disposal of waste solution in hydrometallurgical operations is less troublesome than hydrometallurgical operations is less troublesome than disposal of gases in disposal of gases in pyrometallurgypyrometallurgy. Even in a . Even in a pyrometallurgicalpyrometallurgical process waterprocess water--scrubbing of a gas will scrubbing of a gas will eventually have a water disposal problem.eventually have a water disposal problem.

LEACHINGLEACHING

•• Methods and equipmentMethods and equipment

The method used for leaching an ore depends The method used for leaching an ore depends on the grade of the ore and the ease with which on the grade of the ore and the ease with which the mineral values are dissolved in a particular the mineral values are dissolved in a particular reagent. reagent.

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Leaching in placeLeaching in place

•• This method is also known as leaching This method is also known as leaching in situin situ or or solution miningsolution mining. The . The ore is simply shattered and leached in place over long periods oore is simply shattered and leached in place over long periods of time f time because it is usually too low in grade to justify mining and because it is usually too low in grade to justify mining and transportation expenses. transportation expenses.

•• The process involves spraying the acid on the exposed ore body oThe process involves spraying the acid on the exposed ore body or r injecting it when the ore body is underground, then collecting tinjecting it when the ore body is underground, then collecting the he pregnant solution from below. pregnant solution from below.

•• Because of its success, the method has been used for highBecause of its success, the method has been used for high--grade grade copper oxide ores as well, e.g., at Miami, Arizona. Dilute sulfucopper oxide ores as well, e.g., at Miami, Arizona. Dilute sulfuric acid ric acid is used for leaching copper oxide ores. When copper sulfide mineis used for leaching copper oxide ores. When copper sulfide minerals rals are also present, these are are also present, these are solubilizedsolubilized by the combined action of air and by the combined action of air and water. water.

•• The presence of pyrite enhances the leaching because of its oxidThe presence of pyrite enhances the leaching because of its oxidation ation and the formation of sulfuric acid and ferric sulfate. In this cand the formation of sulfuric acid and ferric sulfate. In this case the ase the oxidation reactions are exothermic and the heat generated facilioxidation reactions are exothermic and the heat generated facilitates tates continued oxidation.continued oxidation.

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Injection technique Injection technique •• In this method the leaching agent is injected in the ore In this method the leaching agent is injected in the ore

deposit by means of vertical perforated pipes arranged in a deposit by means of vertical perforated pipes arranged in a regular pattern. regular pattern.

•• For a group of injection pipes there is a recovery well with For a group of injection pipes there is a recovery well with an underground pump to collect and pump the pregnant an underground pump to collect and pump the pregnant solution to the surface. solution to the surface.

•• The basic criteria for leaching an underground deposit is The basic criteria for leaching an underground deposit is that the ore body must be enclosed between impermeable that the ore body must be enclosed between impermeable strata that will prevent the loss of solution or contaminating strata that will prevent the loss of solution or contaminating underground water and it must be permeable to the underground water and it must be permeable to the leaching solutionleaching solution

Production wellsProduction line

Monitor well

Inject ion line

Ce mentSubsoil

Up per aquifer

Ur aniumbearingaquifer

Impermeable clay

Impermeable clay

Impermeable clay

Pump

Casing

Screen

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Heap or dump leachingHeap or dump leaching

•• In this method an area is first cleared from all vegetation thenIn this method an area is first cleared from all vegetation then leveled at leveled at a slight inclination, compacted, and covered with an asphalt laya slight inclination, compacted, and covered with an asphalt layer or a er or a flexible plastic sheet. The crushed ore is then transported fromflexible plastic sheet. The crushed ore is then transported from the the mine to the prepared site by dump trucks or mobile conveyors to mine to the prepared site by dump trucks or mobile conveyors to a level a level of 10of 10––100 m high. 100 m high.

•• The leaching agent is then sprayed at the top of the dump througThe leaching agent is then sprayed at the top of the dump through h which it percolates and the leach solution is collected in streawhich it percolates and the leach solution is collected in streams at the ms at the bottom. bottom.

•• When the material is fully leached, the dump is either abandonedWhen the material is fully leached, the dump is either abandoned or or loaded on trucks for disposal and the site reloaded on trucks for disposal and the site re--used for leaching another used for leaching another batch. Main problems in operating a heap are the plugging with fbatch. Main problems in operating a heap are the plugging with fine ine clays, ferric hydroxide or basic sulfate, evaporation losses, leclays, ferric hydroxide or basic sulfate, evaporation losses, leakage of akage of solution at the bottom of the heap, and solution at the bottom of the heap, and channellingchannelling. .

•• The process was applied for leaching stock piled waste rock contThe process was applied for leaching stock piled waste rock containing aining small amounts of copper too small to be recovered by any other msmall amounts of copper too small to be recovered by any other means. eans. It was so economical that many heaps of lowIt was so economical that many heaps of low--grade copper ores were grade copper ores were constructed and leaching applied.constructed and leaching applied.

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Percolation or vat leachingPercolation or vat leaching

•• This process is suited to cases where the material This process is suited to cases where the material is porous and sandy, and is inapplicable to is porous and sandy, and is inapplicable to material that tends to pack into impervious material that tends to pack into impervious masses. masses.

•• Regularity in the size of the particles rather than Regularity in the size of the particles rather than their actual size is the chief factor governing good their actual size is the chief factor governing good percolation. The idea is that where the particles are percolation. The idea is that where the particles are of unequal size, the small ones pack in the of unequal size, the small ones pack in the openings between the larger ones, thereby openings between the larger ones, thereby clogging the channels. clogging the channels.

•• Extraction becomes slow, and Extraction becomes slow, and channellingchannelling of of solutions through the bed takes place. The method solutions through the bed takes place. The method is therefore unsatisfactory if much slime is present.is therefore unsatisfactory if much slime is present.

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•• The material to be leached is placed in a tank equipped The material to be leached is placed in a tank equipped with a false bottom covered with a filtering medium. with a false bottom covered with a filtering medium.

•• The leach solution is added at the top of the tank and is The leach solution is added at the top of the tank and is allowed to percolate through the material. These tanks are allowed to percolate through the material. These tanks are usually arranged so that a countercurrent system is usually arranged so that a countercurrent system is employed and thus a continuous operation is achieved; the employed and thus a continuous operation is achieved; the new solids being added to the last tank and the weak liquid new solids being added to the last tank and the weak liquid to the first and pumped successively from one tank to to the first and pumped successively from one tank to another till it reaches the last tank. another till it reaches the last tank.

•• Tanks having a capacity of 12Tanks having a capacity of 12 000 tons of ore are in 000 tons of ore are in common use. When leaching is finished, the tanks are common use. When leaching is finished, the tanks are emptied by mechanical shovels, and a new batch is emptied by mechanical shovels, and a new batch is introduced. Leaching time varies from 2 to 4 days. Its introduced. Leaching time varies from 2 to 4 days. Its advantages are minimum reagent consumption, the advantages are minimum reagent consumption, the production of highproduction of high--grade pregnant solution, and grade pregnant solution, and elimination of the use of expensive thickeners or filters.elimination of the use of expensive thickeners or filters.

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– Vat-leaching of copper oxide ore was improved by adopting the so-called “Thin Layer Leaching”. In this method, the finely crushed moistened ore is sprayed with a limited amount of concentrated H2SO4 in a rotating drum to permit agglomeration.

– The agglomerated material is then charged in the vat and left to cure for some time. This is then followed by normal heap-leaching by spraying dilute acid on the top.

Agitation or pulp leachingAgitation or pulp leaching

•• In this method, the leaching agent is added to the finely In this method, the leaching agent is added to the finely ground raw material such that the mixture forms a pulp ground raw material such that the mixture forms a pulp that has to be agitated continuously to prevent the solids that has to be agitated continuously to prevent the solids from settling, and to terminate the leaching process in the from settling, and to terminate the leaching process in the shortest possible time. It is generally used under the shortest possible time. It is generally used under the following conditions:following conditions:-- Raw material is of moderate or high grade.Raw material is of moderate or high grade.-- The metal values are of fine grain size and disseminated The metal values are of fine grain size and disseminated in the host rock therefore extensive crushing and grinding in the host rock therefore extensive crushing and grinding are necessary before leaching to expose the maximum are necessary before leaching to expose the maximum surface of the solids to the solution.surface of the solids to the solution.-- The metal values are difficult to dissolve and that is why The metal values are difficult to dissolve and that is why intensive agitation is needed to increase the rate.intensive agitation is needed to increase the rate.

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•• The equipment used are numerous and expensive, The equipment used are numerous and expensive, and high recoveries are mandatory. and high recoveries are mandatory.

•• Leaching conditions may be mild using dilute Leaching conditions may be mild using dilute solutions at ambient conditions or it may be severe solutions at ambient conditions or it may be severe using concentrated solutions and at high using concentrated solutions and at high temperature and pressure. temperature and pressure.

•• Agitation is usually accomplished mechanically by Agitation is usually accomplished mechanically by a motora motor--driven impellersdriven impellers

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•• Pulp leaching at high pressure is used for leaching Pulp leaching at high pressure is used for leaching copper sulphide concentrates and the high copper sulphide concentrates and the high pressure necessitates the use of pressure reactors pressure necessitates the use of pressure reactors (or autoclaves). (or autoclaves).

•• Air or oxygen must be added as oxidizing agent. Air or oxygen must be added as oxidizing agent. The use of oxygen instead of air is more The use of oxygen instead of air is more advantageous because for the same oxygen advantageous because for the same oxygen requirement the total pressure in the autoclave is requirement the total pressure in the autoclave is low, and as a result the autoclave design will be low, and as a result the autoclave design will be less demanding, or decreased in size. less demanding, or decreased in size.

Horizontal autoclaves • They are divided in the inside by partitions. In each chamber is

an electrically driven turbine mixer from the top. The body of the autoclave can be lined with lead, with alloy steel, or with rubber.

• Feed slurry is pumped into one end of the autoclave and cascades from one compartment to the next. The average degree of filling is 65–70 % static to allow sufficient disengagement space for the exhaust gases and to reduce the possibility of plugging the relief valve nozzle.

• Horizontal autoclaves are usually designed with four compartments, since too much volume will be lost in providing the static head required for the flow of slurry from one compartment to the next.

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Baking processBaking process

This process is supplied when agitation leaching becomes This process is supplied when agitation leaching becomes impossible because the ore slurry solidifies to a thick mass impossible because the ore slurry solidifies to a thick mass in the early stages of leaching. in the early stages of leaching.

Consequently the solidified mixture is allowed to react Consequently the solidified mixture is allowed to react without agitation at a temperature around 200without agitation at a temperature around 200°°C hence the C hence the term term bakingbaking is used. The reaction may be conducted in is used. The reaction may be conducted in pugmillspugmills. These are horizontal shallow tanks equipped with . These are horizontal shallow tanks equipped with numerous rotating blades that are capable to move a thick numerous rotating blades that are capable to move a thick paste or cut their way through a solidified cake. paste or cut their way through a solidified cake.

This method was suggested for the treatment of copper This method was suggested for the treatment of copper sulfide concentrates with concentrated Hsulfide concentrates with concentrated H22SOSO44 but was but was abandoned. The abandoned. The pugmillspugmills may be heated by a steam jacket.may be heated by a steam jacket.

A pugmill

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Leaching processes

Metallic copper

Copper oxide ores

Copper sulfides

Bioleaching of copper sulfides

Leaching of metallic copperLeaching of metallic copper

Metallic copper may be obtained from the following Metallic copper may be obtained from the following sources:sources:

•• Native copperNative copper. In Lake Superior district, native copper was . In Lake Superior district, native copper was found in abundance. At one time, the ore was smelted and found in abundance. At one time, the ore was smelted and the tailings from this process contained 0.4% metallic the tailings from this process contained 0.4% metallic copper. They were leached by ammonia. copper. They were leached by ammonia.

•• Reduction of oxide or silicate ores. Reduction of oxide or silicate ores. LowLow--grade copper oxide grade copper oxide ores that are not economically amenable to acid leaching ores that are not economically amenable to acid leaching are first reduced to metallic copper then leached with are first reduced to metallic copper then leached with ammonia. This was the case in South Africa where the ammonia. This was the case in South Africa where the Bwana Bwana MM’’KubwaKubwa Company reduced the oxide ore with CO Company reduced the oxide ore with CO and leached the resultant metallic copper with ammonia. and leached the resultant metallic copper with ammonia. An oxide ore deposit in Australia is treated in a similar wayAn oxide ore deposit in Australia is treated in a similar way..

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• Metallic copper can be leached by dilute sulfuric acid in presence of oxygen but this is not economically possible if gangue minerals such as limestone, dolomite, or iron oxide are present in the ore since they are acid consuming.

• Under such circumstances leaching by ammonia in presence of oxygen is used since soluble ammine complexes [Cu(NH3)n]2+ are formed where n is a variable number that depends mainly on the free ammonia concentration in the solution.

The dissociation of ammonium hydroxide is small:

NH3 + H2O → NH4+ + OH–

and NH3 can be considered as the active complexingagent. The dissolution reaction can be represented by the equations:

Oxidation: Cu → Cu2+ + 2e–

Complex formation: Cu2+ + nNH3 → [Cu(NH3)n]2+

Reduction: ½ O2 + H2O + 2e– → 2OH–

The overall reaction:

Cu + nNHCu + nNH33 + + ½½ OO22 + H+ H22O O →→ [Cu(NH[Cu(NH33))nn]]2+2+ + 2OH+ 2OH––

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•• The rate of dissolution of copper depends on the oxygen partial The rate of dissolution of copper depends on the oxygen partial pressure when the ammonia concentration is low, and is a functiopressure when the ammonia concentration is low, and is a function of n of the ammonia concentration when the oxygen partial pressure is hithe ammonia concentration when the oxygen partial pressure is high.gh.

•• Ammonium ion alone does not dissolve these metals, but when addeAmmonium ion alone does not dissolve these metals, but when added d to ammonium hydroxide solution, it increases the rate of dissoluto ammonium hydroxide solution, it increases the rate of dissolution as tion as a result of inhibiting the ionization of ammonia. a result of inhibiting the ionization of ammonia.

•• For this reason a mixture of ammonium hydroxide and ammonium For this reason a mixture of ammonium hydroxide and ammonium carbonate is usually used. Ammonium carbonate is selected becauscarbonate is usually used. Ammonium carbonate is selected because it e it decomposes readily when the solution is boiled to recover the decomposes readily when the solution is boiled to recover the ammonia, and also because the carbonate ion is present any way iammonia, and also because the carbonate ion is present any way in the n the solution as a result of contact with air. solution as a result of contact with air.

•• The process is also known as the Caron Process after its inventoThe process is also known as the Caron Process after its inventor r originally for leaching nickel from originally for leaching nickel from lateriteslaterites..

•• Leaching is carried out under atmospheric pressure, Leaching is carried out under atmospheric pressure, but the tanks are closed to prevent the loss of but the tanks are closed to prevent the loss of ammonia. After filtering the gangue minerals, the ammonia. After filtering the gangue minerals, the solution is boiled to distill off excess ammonia and to solution is boiled to distill off excess ammonia and to decompose the ammine complex to precipitate a decompose the ammine complex to precipitate a hydroxide or a basic carbonate:hydroxide or a basic carbonate:

[Cu(NH[Cu(NH33))nn]]2+2+ + 2 H+ 2 H22O O →→ Cu(OH)Cu(OH)22 + nNH+ nNH33

2[Cu(NH2[Cu(NH33))nn]]2+2+ + 2OH+ 2OH–– + CO+ CO3322–– →→

Cu(OH)Cu(OH)22··CuCOCuCO33 + 2nNH+ 2nNH33

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•• The precipitate is filtered, washed, and The precipitate is filtered, washed, and calcinedcalcined to to yield a highyield a high--grade oxide containing 75grade oxide containing 75––78 % metal. 78 % metal.

•• The gases evolved are absorbed in water and The gases evolved are absorbed in water and recycled. Also, ammonia retained in the gangue recycled. Also, ammonia retained in the gangue residue is recovered by steam stripping. residue is recovered by steam stripping.

•• Instead of Instead of calcinationcalcination, the precipitated basic , the precipitated basic carbonate is sometimes dissolved in ammonium carbonate is sometimes dissolved in ammonium sulfate solution and the pure metal is precipitated by sulfate solution and the pure metal is precipitated by hydrogen under pressure.hydrogen under pressure.

Leaching of copper oxide oresLeaching of copper oxide ores

The main copper ores are sulfides, oxides, or a mixture The main copper ores are sulfides, oxides, or a mixture of sulfides and oxides. of sulfides and oxides.

The last type is the worldThe last type is the world’’s most extensive copper ore s most extensive copper ore bodies; they are disseminated, i.e., they are composed bodies; they are disseminated, i.e., they are composed of minute grains of copper minerals in large bodies of of minute grains of copper minerals in large bodies of rock. They contain on the average 2 % Cu and known as rock. They contain on the average 2 % Cu and known as porphyry ores. porphyry ores.

The sulfideThe sulfide––oxide ore is usually separated by flotation oxide ore is usually separated by flotation into two fractions: a highinto two fractions: a high--grade sulfide concentrate and a grade sulfide concentrate and a lowlow--grade oxide fraction. grade oxide fraction.

While the sulfide concentrate is usually treated by While the sulfide concentrate is usually treated by pyrometallurgicalpyrometallurgical methods, the oxide fraction is treated methods, the oxide fraction is treated by hydrometallurgical methods; copper sulfides are by hydrometallurgical methods; copper sulfides are insoluble in dilute acid. insoluble in dilute acid.

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Sulfuric acid process

Sulfuric acid is the usual leaching agent for oxidized copper ores;

copper is recovered from the leach solution by solvent extraction followed by electrolysis, With the exception of cuprite, all copper oxide minerals dissolve to give CuSO4.

Copper in cuprite is in the cuprous state; it dissolves according to:

Cu2O + 2H+ Cu2+ + Cu + H2Oi.e., half the copper in the mineral dissolves and the other half precipitates as metallic copper, a typical disproportionationreaction of Cu+ ion. In presence of oxidizing agents (air or ferric ion), cuprite dissolves completely:Cu2O + 4H+ + 1/2O2 2Cu2+ + 2H2O

Ammonium hydroxide process

Ammonia is used when the gangue is acid consuming. Dissolution takes place according to:

CuO + 2NH4+ + 3NH3 [Cu(NH3)4]2+ + H2O

2CuCO3 · Cu(OH)2 + 12NH3 3[Cu(NH3)4]2+ + 2CO32– + 2OH–

• After filtering off the gangue, the solution is boiled so that the above reactions are reversed. Ammonia is then absorbed from the gas phase and the basic carbonate recovered by filtration, which is then calcined to CuO.

• The process was in operation at Kennecott, Alaska, in the period 1916 to 1931; operation discontinued because the mine exhausted. Similar operation is presently in Australia.

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Copper from manganese nodules

► These nodules may be processes only for the recovery of copper, nickel, and cobalt, i.e., by selective leaching. The nodules may, therefore, be treated in practically the same way as copper oxide ores, i.e., leaching agents are selected to dissolve copper, nickel, and cobalt but not MnO2and Fe2O3.

► Any leaching agent that solubilizes MnO2 will also solubilize copper, nickel, and cobalt. Manganese dioxide is insoluble in dilute H2SO4, but when reduced to MnO, it readily dissolves to give a solution of MnSO4:

MnO + 2H+ Mn2+ + H2O► Since manganese is recovered from this solution by electrolysis, the spent

electrolyte can be recycled for leaching. ► Manganese dioxide can also be solubilized in dilute H2SO4 but only in

presence of a reducing agent such as sulfur dioxide, coal, or oxalic acid.

► The oxidation–reduction steps are as follows:Fe2+ Fe3+ + e–

SO2 + 2H2O H2SO3 + H2O SO42– + 4H+ + 2e–

C + 2H2O CO2 + 4H+ + 4e–

(COO)22– 2CO2 + 2e–

► MnO2 reacts according to:MnO2 + 4H+ + 2e– Mn2+ + 2H2O

► Because of the insolubility of MnO2 in dilute H2SO4, this reagent has been suggested for treating manganese nodules to selectively extract copper, nickel, and cobalt. However, the process is slow and few days are needed with intensive agitation.

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► Manganese dioxide dissolves in hydrochloric acid liberating chlorine:

2Cl– Cl2 + 2e–

MnO2 + 4H+ + 2e– Mn2+ + 2H2O► The large proportion of acid used per mole MnO2

necessitates its regeneration from the chlorine as well as from the aqueous solutions.

► Although this technology may be established for the treatment of manganese nodules yet it becomes costly because of the large number of circuits involved.

► Copper, nickel, cobalt, and iron oxides dissolve readily in hydrochloric acid, forming metal chlorides.

Leaching of copper sulfides

►Sulfide minerals are insoluble in water even at high temperature. They can be solubilized, however, by leaching under oxidizing conditions.

►Elemental sulfur usually forms but because of its instability in neutral or basic media it oxidizes to sulfates.

►In acid medium, however, there is a narrow region where elemental sulfur can form.

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Valid in the temperature region 25 to Valid in the temperature region 25 to 150150°°C. Above 150C. Above 150°°C, the narrow stability C, the narrow stability region of elemental sulfur disappears and region of elemental sulfur disappears and no sulfur can form.no sulfur can form.

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When solid elemental sulfur is liberated during leaching it is uWhen solid elemental sulfur is liberated during leaching it is usually sually porous and does not form an obstacle. However, when it melts it porous and does not form an obstacle. However, when it melts it usually forms a viscous layer around the mineral particles and rusually forms a viscous layer around the mineral particles and retards etards the rate of leaching. the rate of leaching.

This can be solved by adding a surface active agent such as This can be solved by adding a surface active agent such as LignosolLignosolduring leaching which helps the sulfur globules to coalesce thusduring leaching which helps the sulfur globules to coalesce thus they do they do not cover the mineral particles. not cover the mineral particles.

Finely crushed coal, when added in small amounts (about 4 kg/tonFinely crushed coal, when added in small amounts (about 4 kg/tonconcentrate), was found to prevent the agglomeration phenomenon.concentrate), was found to prevent the agglomeration phenomenon.

In the elemental form, sulfur can be readily stockIn the elemental form, sulfur can be readily stock--piled, shipped long piled, shipped long distances, or transformed into Hdistances, or transformed into H22SOSO44 or SOor SO22 when needed. It is thus when needed. It is thus evident that leaching methods resulting in elemental sulfur formevident that leaching methods resulting in elemental sulfur formation ation are more attractive than the others. are more attractive than the others.

Since elemental sulfur may be a product of leaching, its physicaSince elemental sulfur may be a product of leaching, its physical and l and chemical properties will be briefly reviewed.chemical properties will be briefly reviewed.

Elemental sulfur is stable in acid medium but oxidizes slowly in water at high temperature (above 150°C) and oxygen pressure to form sulfuric acid:

S + 3/2O2 + H2O H2SO4

In basic medium, oxidation takes place in stages whereby, thiosulfate and other sulfur compounds are formed:

2S + O2 + 2OH– S2O32– + H2O

These finally oxidize to sulfate as follows:S2O3

2– + 2O2 + 2OH– 2SO42– + H2O

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• Other oxidizing agents such as HNO3 and aqueous chlorine solutions react readily with elemental sulfur even at room temperature transforming it into sulfate:

S + 2HNO3 H2SO4 + 2NOS + 3Cl2 + 4H2O H2SO4 + 6HCl

Concentrated H2SO4 above 200°C also attacks elemental sulfur:

S + 2H2SO4 3SO2 + 2H2O

Oxidation products of Oxidation products of sulfidessulfides in in neutral or alkaline mediumneutral or alkaline medium

• S2O32– Thiosulfate

• SnO62– Polythionate

• S2O62– Dithionate

• S2O42– Dithionite (or hyposulfite)

• SO22– Sulfoxylate

28

Leaching in Presence of Oxidizing Agents

• Most commonly used oxidizing agents are: Oxygen, ferric ion, nitric acid, concentrated H2SO4, chlorine water, and sodium hypochlorite.

• Leaching of sulfides in presence of oxidizing agents may be chemical or electrochemical. Chemical processes may lead to the formation of sulfates or elemental sulfur.

• Sulfates. These processes usually take place in neutral medium. They are slow at ambient conditions, but rapid at high temperature:

MS M2+ + S2–

S2– + 2O2 SO42–

• Elemental sulfur. This is usually the case for acid-soluble sulfides, e.g., ferrous sulfide:

FeS Fe2+ + S2–

S2– + 2H+ H2SH2S + 1/2O2 S + H2O

29

• Electrochemical processes usually take place with acid-insoluble sulfides and lead to the initial formation of elemental sulfur which may be oxidized further depending on the leaching conditions (temperature, pH, and electrode potential of the oxidizing agent).

• The electrochemical process may be represented by:Oxidation: MS M2+ + S + 2e–

Reduction: Oxidizing agent + ne– Reduced species

In neutral medium: In neutral medium: 11//22OO22 + H+ H22O + 2eO + 2e–– 2OH2OH––

In acid medium: In acid medium: 11//22OO22 + 2H+ 2H++ + 2e+ 2e–– HH22OOFeFe3+3+ + e+ e–– FeFe2+2+

•• Oxygen as an oxidizing agent has the advantage that it does not Oxygen as an oxidizing agent has the advantage that it does not need to need to be regenerated, and does not result in a foreign ion that contambe regenerated, and does not result in a foreign ion that contaminates inates the product. the product.

•• For example, when FeClFor example, when FeCl33 is used as a leaching agent, it is reduced to is used as a leaching agent, it is reduced to FeClFeCl22 during the course of reaction and has to be separated and during the course of reaction and has to be separated and oxidized back for recycle. oxidized back for recycle.

•• The aqueous oxidation of sulfides by oxygen at ambient conditionThe aqueous oxidation of sulfides by oxygen at ambient conditions is a s is a slow process; it is greatly accelerated if conducted at high temslow process; it is greatly accelerated if conducted at high temperature perature and high oxygen pressure. and high oxygen pressure.

•• Oxygen can be used in combination with water, dilute acid (usualOxygen can be used in combination with water, dilute acid (usually ly HH22SOSO44), and ammonium hydroxide. The latter case is used only for ), and ammonium hydroxide. The latter case is used only for leaching copper, nickel, and cobalt since these metals are capableaching copper, nickel, and cobalt since these metals are capable of le of forming ammine complexes in solution.forming ammine complexes in solution.

30

Oxygen and water

MS + 2O2(aq) MSO4

►The reaction takes place at a reasonable rate only at 200°C and an oxygen pressure of 4000 kPa; the reaction is complete in 2–3 hours.

Oxygen and ammonia

The process has been used successfully since 1953 for the The process has been used successfully since 1953 for the treatment of Nitreatment of Ni––CuCu––Co sulfide concentrate on large scale Co sulfide concentrate on large scale at the at the SherrittSherritt--Gordon Plant in Fort Saskatchewan, Gordon Plant in Fort Saskatchewan, Canada. It has minimum corrosion problems and any Canada. It has minimum corrosion problems and any pyrite present will not be attacked. The overall reaction is:pyrite present will not be attacked. The overall reaction is:

MS + MS + nnNHNH33 + 2O+ 2O22 [M(NH[M(NH33))nn]]2+2+ + SO+ SO4422––

► Oxidation of sulfides in a basic medium takes place in several stages and any of the intermediate compounds: Polysulfide, thiosulfate, etc., may be present in the leach solution.

► The leach solution is also free from iron, since hydrated ferricoxide is precipitated. In this process all the sulfur is oxidized and recovered as ammonium sulfate and marketed as fertilizer.

31

Oxygen and acid

• This method is usually conducted at 115–150°C and about 2000 kPa oxygen pressure so that elemental sulfur can be recovered in good yield. For chalcopyrite the reaction is:

2CuFeS2 + 4H+ + 5/2O2 2Cu2+ + 2FeOOH + 4S + H2O

– However, industrial operations are also conducted at high temperature where no elemental sulfur is formed. The method is most suitable when the metal values in the leach solution can berecovered by electrolysis or precipitation by hydrogen so that the acid generated during the recovery step can be recycled to the leaching step.

– Because of the high temperature and the acid medium involved, acid-resisting materials of construction must be used.

32

Ferric salts

Leaching of sulfides in acid medium in presence of ferric ion takes place readily with liberation of elemental sulfur. The disadvantage of this route is the formation of ferrous ion, which requires separation and re-oxidation to ferric for recycle. This method has found the following applications:

Leaching of copper sulfides in heaps or in situ. The dissolution of cuprous sulfide takes place in steps as follows:

5Cu2S + 2Fe3+ Cu9S5 + Cu2+ + 2Fe2+

Cu9S5 + 8Fe3+ 5CuS + 4Cu2+ + 8Fe2+

CuS + 2Fe3+ Cu2+ + 2Fe2+ + S However, due to the presence of microorganisms in the

heaps elemental sulfur is oxidized further to sulfuric acid.

33

Purification of molybdenite concentratesCopper sulfide impurities are removed from MoS2 by boiling with FeCl3 solutions.

Leaching of chalcopyrite concentrates.Leaching of chalcopyrite with ferric ion takes place readily at the boiling point:

CuFeS2 + 3Fe3+ Cu+ + 4Fe2+ + 2SCopper is solubilized in the cuprous form, but in presence of excess Fe3+ the cuprous is oxidized to cupric:

CuFeS2 + 4Fe3+ Cu2+ + 5Fe2+ + 2S Although CuCl has a low solubility in water, yet, due to the presence of Fe2+

ion in the leach solution it is completely soluble. The fact that CuCl can be obtained instead of CuCl2 under carefully controlled conditions is of importance in the subsequent recovery step, since Cu+ ion is easier to reduce than Cu2+ ion, e.g., by precipitation with iron:

2Cu+ + Fe 2Cu + Fe2+

FeCl2 + 1/2Cl2(aq) FeCl Ferrous chloride can be partly crystallized and treated further for HCl

regeneration while the remainder in solution is oxidized to FeCl3 for recycle. Oxidation of FeCl2 is usually conducted by chlorine.

Concentrated H2SO4

Leaching of chalcopyrite by concentrated H2SO4 has been developed to a large scale pilot plant known as the Treadwell Process.

• Below 150°C, no reaction.• Between 150°C and 250°C, there is a formation of

elemental sulfur, CuSO4, and FeSO4. Since chalcopyrite (Cu2S·Fe2S3) contains Cu+ and Fe3+ ions in the crystal structure, the formation of CuSO4 and FeSO4 (and not Cu2SO4 and Fe2(SO4)3) must be due to an oxidation–reduction reaction that takes place in the solid state during heating: Cu+ + Fe3+ Cu2+ + Fe2+

34

This can be represented by:Cu2S·Fe2S3 2(CuS·FeS)CuS·FeS Cu2+ + Fe2+ + 2S + 4e–

4H+ + SO42– + 2e– SO2 + 2H2O

Overall reaction:CuFeS2 + 8H+ + 2SO4

2– Cu2+ + Fe2+ + 2S + 2SO2 + 4H2O

• Above 250°C, both elemental sulfur and ferrous ion are oxidized:- Oxidation of elemental sulfur by concentrated

H2SO4:H2SO4 SO2 + 1/2O2 + H2OS + O2 SO2

2H2SO4 + S 3SO2 + 2H2O- Oxidation of ferrous ion by concentrated H2SO4:

Fe2+ Fe3+ + e–

4H+ + SO42– + 2e– SO2 + 2H2O

Fe2+ + 4H+ + SO42– 2Fe3+ + SO2 + 2H2O

Overall reaction:2CuFeS2 + 13H2SO4 + 10H+ 2Cu2+ + 2Fe3+ + 17SO2 + 18H2O

or2CuFeS2 + 18H2SO4 2CuSO4 + Fe2(SO4)2 + 17SO2 + 18H2O

35

It is obvious that the optimal reaction conditions are It is obvious that the optimal reaction conditions are between 150 and 250between 150 and 250°°C. C.

The residue containing the anhydrous The residue containing the anhydrous sulfatessulfates, , elemental elemental sulfursulfur, and the gangue minerals, is leached , and the gangue minerals, is leached with water to with water to solubilizesolubilize the the sulfatessulfates, precipitate , precipitate CuCNCuCN by HCN, crystallize FeSOby HCN, crystallize FeSO44··nnHH22O which is O which is then decomposed to recover the SOthen decomposed to recover the SO33 for recycle as for recycle as HH22SOSO44. .

The process has the advantage of forming elemental The process has the advantage of forming elemental sulfursulfur, but the disadvantage of , but the disadvantage of solubilizingsolubilizing iron and iron and the necessity of regenerating Hthe necessity of regenerating H22SOSO44 by decomposing by decomposing the ferrous the ferrous sulfatesulfate..

Chlorine and hypochloriteAqueous chlorine solutions solubilize copper sulfides at ambient conditions

with formation of elemental sulfur:CuS Cu2+ + S + 2e–

Cl2 + 2e– 2Cl–

At high concentrations, long contact time, or high temperature, elemental sulfur oxidizes to sulfate. The process is applied industrially for the treatment of Ni3S2–Cu2S (white metal) at the Falconbridge refinery in Sudbury. Both nickel and copper are solubilized, then separation is effected by adding fresh white metal to solubilize more nickel and precipitate the copper:

Ni3S2 + 3Cu2+ 3Ni2+ + CuS + Cu2S When nickel chloride solutions are electrolyzed to recover metallic

nickel, chlorine is generated at the anode which is collected and recycled to the leaching step.

36

Bioleaching of copper Bioleaching of copper sulfidessulfides

• Certain microorganisms known as Thiobacillus thiooxidansand Thiobacillus ferrooxidans isolated from mine water and others isolated from sulfur-bearing hot springs are capable of accelerating the rate of oxidation of sulfide minerals.

• These bacteria, known as autotrophic bacteria can also accelerate the oxidation of elemental sulfur and ferrous ion.

• They are minute, unicellular microscopic organisms 0.5–2 mm in size that reproduce by binary fission. Since they utilize oxygen for growth, they are classified as aerobic bacteria.

• In industrial leaching operation where the continuous flow of leach solution provides nutrient and a balanced environment, the microorganisms multiply following the logarithmic law:

n = n0ekt

A leach solution in a heap leaching operation may contain

as many as 10 million microorganisms per mL

37

►► They must have oxygen to carry out their function, i.e., the They must have oxygen to carry out their function, i.e., the oxidation of mineral substances with release of energy which oxidation of mineral substances with release of energy which is necessary for their growth.is necessary for their growth.

•• They utilize COThey utilize CO22 as the principal source of carbon required as the principal source of carbon required for the synthesis of organic compounds composing the cell. for the synthesis of organic compounds composing the cell.

•• They absorb certain nutrient from solution, e.g., POThey absorb certain nutrient from solution, e.g., PO4433–– ions ions

that are necessary for their metabolic function.that are necessary for their metabolic function.►► They have resistance to high concentrations of metal ions in They have resistance to high concentrations of metal ions in

solution which are usually toxic to most other forms of life.solution which are usually toxic to most other forms of life.•• They adapt to live and grow in strongly acidic They adapt to live and grow in strongly acidic

environments (pH 1.5environments (pH 1.5––3).3).

• They are destroyed by light, particularly ultraviolet light. ► Their maximum activity for Thiobacillus ferrooxidans occurs

at 35°C while for those isolated from sulfur-bearing hot springs (the thermophilic bacteria), the temperature may be as high as 85°C. Below these temperatures they are dormant while above, they are destroyed.

• They manufacture enzymes and with their aid they catalyze the oxidation process:

CuS(s) → Cu2+(aq) + S2–

(aq)S2– + 2O2 → SO4

2–

FeS2 + 7/2O2 + H2O → Fe2+ + 2SO42– + 2H+

► The adherence of the microorganisms on the sulfide surface makes possible the rapid oxidation of S2– ions liberated by the mineral and catalyzed by the enzymes.

38

►►Leaching is usually conducted in agitated Leaching is usually conducted in agitated tanks. tanks.

►►The process is cheap but slow and may The process is cheap but slow and may generate excessive amounts of generate excessive amounts of sulfuricsulfuric acid acid and ferrous and ferrous sulfatesulfate that must be neutralized that must be neutralized before disposal. before disposal.

39

SOLVENT EXTRACTION OF COPPERSOLVENT EXTRACTION OF COPPER

Extraction data may be represented in two ways: Extraction data may be represented in two ways: ►► Percentage extraction as a function of a certain Percentage extraction as a function of a certain

variable like acid concentration, pH, etc.variable like acid concentration, pH, etc.►► In form of extraction isotherms. The extraction In form of extraction isotherms. The extraction

isotherms are the plot of the concentration of the isotherms are the plot of the concentration of the metal ion in aqueous phase versus its metal ion in aqueous phase versus its concentration in the organic phase when the two concentration in the organic phase when the two phases are in equilibrium. Such a plot is useful, phases are in equilibrium. Such a plot is useful, since at each point it directly displays the since at each point it directly displays the concentrations in both phases and consequently concentrations in both phases and consequently the distribution coefficients can be evaluated. the distribution coefficients can be evaluated.

Extraction isotherm of copper from a sulfate solution

40

Extraction isotherms may be constructed in two ways:Extraction isotherms may be constructed in two ways:

•• An aqueous phase of a certain volume containing An aqueous phase of a certain volume containing a known amount of metal ion is contacted with an a known amount of metal ion is contacted with an organic phase of a certain volume until equilibrium organic phase of a certain volume until equilibrium is reached; the concentration of metal ions in both is reached; the concentration of metal ions in both phases is then determined. The process is phases is then determined. The process is repeated at least five times with variable volumes repeated at least five times with variable volumes of the organic phase. of the organic phase.

►► The process is repeated as in the previous case The process is repeated as in the previous case but keeping the organicbut keeping the organic––aqueous ratio constant aqueous ratio constant and varying the initial metal ion concentration in and varying the initial metal ion concentration in the aqueous phase.the aqueous phase.

McCabeMcCabe––Thiele diagramThiele diagram

►►With the addition of an operating line based With the addition of an operating line based on the volume ratio of the two phases, the on the volume ratio of the two phases, the extraction isotherm provides a McCabeextraction isotherm provides a McCabe––Thiele diagram for a stageThiele diagram for a stage--wise of wise of extraction in a countercurrent system.extraction in a countercurrent system.

41

A McCabe–Thiele diagram is useful for estimating the number of theoretical stagesrequired to obtain specified results in a solvent extraction system.

Extractants

• Oximes, i.e., compounds containing the group =N–OH, have been used for many years in analytical chemistry as precipitants for metals.

• The a-hydroxyoximes are well known as specific for precipitating copper, e.g.:

HO

C—C

H

-Benzoinoxime

N

OH

OH

C=N–OH

H

Salicylaldoxime

42

• The hydroxyoxime, 5,8-diethyl-7-hydroxy-6-dodecanone oxime, is used in extracting copper from leach solutions, under the trade name LIX-63 introduced to the market by General Mills Corporation, now Henkel Corporation.

O

R–C–C–R

H

LIX 63

N

OHH

R is CH–C4H9

C2H5

43

►►HydroxybenzophenoneHydroxybenzophenone oximesoximes are more are more efficient than the efficient than the hydroxyoximeshydroxyoximes, also , also introduced by Henkel. introduced by Henkel.

►►Other groups of similar structure were Other groups of similar structure were introduced by Imperial Chemical Industries introduced by Imperial Chemical Industries ((AcorgaAcorga extractantsextractants), and by Shell (SME ), and by Shell (SME group). group).

►►The commercial products are usually The commercial products are usually mixtures and not pure compounds. mixtures and not pure compounds.

44

Trade name Chemical name Formula

LIX-64 (Henkel)

2-Hydroxy-5-dodecylbenzophenone oxime

C

N

OHOH

C12H25

+ LIX 63

LIX-64N LIX 65N + 1% by volume LIX 63

LIX-65N (Henkel)

2-Hydroxy-5-nonylbenzophenone oxime

C

N

OHOH

C9H19

LIX-70 (Henkel)

2-Hydroxy-3-chloro-5-nonylbenzophenone oxime

C

N

OHOH

C9H19

Cl + LIX 63

LIX-71 LIX 70 + LIX 65N

LIX-73 LIX 70 + LIX 65N + LIX 63

LIX-860 (Henkel)

5-Dodecylsalicyl aldoxime

H–C

N

OHOH

C12H25

Acorga P50 (ICI)

5-Nonyl salicylaldoxime

H–C

N

OHOH

C9H19

SME 529 (Shell)

2-Hydroxy-5-nonyl acetophenone oxime

H3C–C

N

OHOH

C9H19

45

Quinolines, e.g., 8-Hydroxyquinoline, also known as oxine, is a colorless solid, m.p. 75 °C, slightly soluble in water:

OHN

8-Hydroxyquinoli Keelex-100 is -alkenyl-8-hydroxyquinoline:

OHN

H3C–C C–C C–C

H

CH

H

H

CH3

CH3

H

H

CH3

CH3

CH2 It is marketed by Ashland Chemicals for the extraction of copper. It has a copper loading capacity of 0.7 g Cu per 1 volume percent. Keelex-120 is composed of 20 % volume of Keelex-100 in para-nonyl phenol.

Extraction of copper from H2SO4 by Keelex at organic/aqueous ratio 1/1

• 20%Keelex + 10% nonylphenol, o 20% Keelex + 20% nonylphenol

46

Extraction of copper by Keelex from queous phase containing 33.6 g/L Cu and 13.5 g/L H2SO4

• 20%Keelex + 10% nonylphenol, o 20% Keelex + 10% iso-decanol

Mechanism of Extraction

The chemical process in which copper ions interact with the extractant to form an organometallic complex soluble in the organic phase at the same time an ion from the extractant is transferred stoichiometrically to the aqueous phase:

ON

8-Hydroxyquinoline copper complex

ON

Cu

OHN

8-Hydroxyquinoline

47

Cu2+ + H2X CuX + 2H+

Where H2X represents the extractant. ► It can be seen that one copper ion is exchanged for two

hydrogen ions. ► In stripping, the reaction is reversed by adding concentrated

acid. ► Extraction is usually fast at room temperature, equilibrium

being achieved within a few seconds. ► Extraction of copper ions from aqueous solutions is a slightly

exothermic process. Consequently, the distribution ratio decreases with increasing temperature, but the rate of extraction increases with increasing temperature.

The Organic Phase

► A certain weight of an extractant has a limited capacity for extracting copper ion from solution.

► When this limit is reached, the extractant is then saturated with copper and is said to have reached its maximum loading.

► Maximum loading varies widely from one extractant to the other. ► The organic phase increases in viscosity with increasing metal ion loading

and sometimes it is necessary to operate at a solvent loading below the maximum loading of the extractant.

► Saturation concentration can be determined by contacting the organic phase several times with fresh aliquots of the aqueous phase until the amount of metal in the organic phase becomes constant.

► Sometimes the extracting power of a mixture of extractants exceeds the sum of the extracting powers of the components. This phenomenon is known as synergism.

48

Role of diluentsRole of diluents

►► The diluents themselves are unable to extract The diluents themselves are unable to extract copper ions from the aqueous phase, but they copper ions from the aqueous phase, but they greatly affect the extraction behavior of the greatly affect the extraction behavior of the solvent. solvent.

►► The reason is the interaction that may take place The reason is the interaction that may take place between the between the diluentdiluent and the and the extractantextractant. .

►► Further, some diluents favor the polymerization of Further, some diluents favor the polymerization of the the extractantsextractants while others do not. while others do not.

Third-phase formation

Third-phase formation depends on the diluent. The addition of a long-chain alcohol such as n-decanol overcomes the difficulty of third phase formation. The amount of decanol required can be determined by equilibrating the organic phase with the aqueous phase and allowing it to settle, the two organic phases are separated and titrated with the alcohol till a single phase is formed. Usually, about 3 % alcohol is sufficient to prevent third-phase formation.

49

The Aqueous Phase

► Extraction by cationic solvents is greatly affected by the pH of the aqueous phase since H+ ions are taking part in the process.

► Decreasing the hydrogen ion concentration shifts the equilibrium from left to right thus favoring the extraction.

► In many other cases, the variation of distribution coefficient with pH may be due to the effect of the latter on the stability of the complexes in the aqueous phase.

► During the extraction of metal ions by organic solvents, it is found that most inorganic acids are extracted as well. Water is also co-extracted.

EquipmentEquipment

Only clear filtered solutions can be extracted Only clear filtered solutions can be extracted by organic solvents. by organic solvents.

►►Mixer settlersMixer settlers►►Columns Columns

50

Mixer settlersMixer settlers►► These are composed of a mixing chamber where the These are composed of a mixing chamber where the

aqueous and organic phases are mixed together by a aqueous and organic phases are mixed together by a rotating impeller, and a settling chamber where the mixed rotating impeller, and a settling chamber where the mixed phases are given enough time to separate.phases are given enough time to separate.

►► Each such unit composes a stage. The apparatus is very Each such unit composes a stage. The apparatus is very efficient but its principal disadvantage is the large space efficient but its principal disadvantage is the large space required per stage. required per stage.

►► Usually many stages are used (3 to 5) and are operated in Usually many stages are used (3 to 5) and are operated in countercounter--current in the extraction as well as in the stripping current in the extraction as well as in the stripping steps. steps.

►► Sometimes, a washing step is inserted between extraction Sometimes, a washing step is inserted between extraction and stripping to remove loosely bound metal ions. and stripping to remove loosely bound metal ions.

51

ColumnsColumns

►► The packed columnThe packed column consists of a vertical tower packed with consists of a vertical tower packed with ceramic or plastic rings for increasing the surface area of ceramic or plastic rings for increasing the surface area of contact between the aqueous and organic phases.contact between the aqueous and organic phases.

►► Pulse column is that with screen partitions instead of packing.Pulse column is that with screen partitions instead of packing.Efficiency of contact may be improved by introducing a Efficiency of contact may be improved by introducing a pulsating action in the column.pulsating action in the column.

►► In this column, the two phases are dispersed in each other by In this column, the two phases are dispersed in each other by pulsating action through the screens. The pulsating action is pulsating action through the screens. The pulsating action is supplied by leading a pulse leg from the bottom of the column supplied by leading a pulse leg from the bottom of the column to a reciprocating pump. The pulsating action forces the two to a reciprocating pump. The pulsating action forces the two phases through the holes in the plates, forming bubbles. They phases through the holes in the plates, forming bubbles. They rise (or fall) to the next plate, where they coalesce to await rise (or fall) to the next plate, where they coalesce to await the next pulse. the next pulse.

►► In both cases the organic phase is introduced at the bottom In both cases the organic phase is introduced at the bottom and the aqueous phase near the top. Separation of the phases and the aqueous phase near the top. Separation of the phases takes place at the top.takes place at the top.

Interface

Interface

Solventinlet

Solventinlet

Aqueousinle t

Aqueousinlet

Solve ntoutlet

Solventout let

Aqueousoutlet

Aqueousout let

Piston

platesPerforated

52

Applications

• The recovery of copper from dump leaching solutions containing around 1 g/L Cu was usually achieved by displacing the copper ion by scrap iron — the cementation process.

• Solvent extraction with subsequent electrodeposition of a marketable product offered a convenient method of recovery.

• There is the added advantage that acid is liberated in the recovery step and can therefore be used as a stripping solution in the extraction process. Extractants commonly used are LIX-64 and Keelex-100.

53

RECOVERY FROM LEACH SOLUTION

54

Crystallization

• Copper sulfate, a by-product of electrolytic copper refining is used in agriculture as a herbicide.

• Solubility of copper sulfate increases with increased temperature and decreases with increased acid concentration

55

Hydrolytic precipitation

• Metal ions in aqueous solution are solvated, i.e., they are bound to a number of H2O molecules so oriented that the negative end of water dipole is directed toward the positively charged metal ion.

56

The loss of H+ from the bound water molecules takes place readily.

That is why copper sulfate solutions are acidic:

[Cu(H2O)6]2+ [Cu(H2O)5·OH]+ + H+

or simply:Cu2+ + H2O CuOH+ + H+

• Hydrolysis continues until an uncharged species is formed:CuOH+ + H2O Cu(OH)2 + H+

• In this case the ionic forces keeping the ions apart are thus removed, and therefore precipitation takes place.

• It is evident that precipitation of a hydrolytic product can be favored by adding OH– ions that react with H+ ions formed thus shifting the equilibrium to the right.

• Hydrolytic reactions may lead to the precipitation of oxides, hydrated oxides, hydroxides, or hydroxy salts depending on temperature, and concentration of solution.

• High temperature increases greatly the rate of hydrolysis.

57

Cuprous chloride when reacted with calcium hydroxide precipitates cuprous oxide due to hydrolysis:

CuCl(s) Cu+ + Cl–

Cu+ + OH– CuOH2CuOH Cu2O + H2O

The process was used on a commercial scale in Germany in connection of the recovery of copper from pyrite cinder.

Under certain conditions of pH, temperature, and concentration of metal ion in solution, hydroxy or basic salts of copper are obtained instead of hydroxides.

For example, pale blue hydroxy copper sulfate is precipitated according to:2Cu2+ + SO4

2– + 2H2O Cu(OH)2·CuSO4 + 2H+

Similarly, the hydroxy chlorides and hydroxycarbonates:

2Cu2+ + 2Cl– + 2H2O Cu(OH)2·CuCl2 + 2H+

2Cu2+ + CO32– + 2H2O Cu(OH)2·CuCO3 + 2H+

58

Hydroxy carbonates are also precipitated when a copper ammine complex in an ammoniacal ammonium carbonate solution is heated to distill off NH3:

2[Cu(NH3)n]2+ + 4OH– + (NH4)2CO3 Cu(OH)2·CuCO3 + 2(n+1)NH3 + 2H2O

HydroxyHydroxy salts of copper salts of copper

Carbonate Cu(OH)2·CuCO3

Sulfate Cu(OH)2·CuSO4

Chloride 3Cu(OH)2·CuCl2

59

Ionic precipitationIonic precipitation

Ionic precipitation is based on the fact that Ionic precipitation is based on the fact that when a reagent is added to a solution when a reagent is added to a solution containing metal ions, a compound is containing metal ions, a compound is formed whose solubility is very low under formed whose solubility is very low under these conditions that precipitation takes these conditions that precipitation takes place immediately. place immediately.

Chlorides

The solubility of cuprous chloride in water is low, about 0.062 g/L, and this property is sometimes utilized to separate copper from leach solutions:

Cu+ + Cl– CuCl(ppt.)

When copper is present as Cu2+ ions in leach solutions, reduction to Cu+ is necessary before precipitation. This is achieved by heating with SO2 or metallic copper in absence of air:

2Cu2+ + SO2 + 2H2O 2Cu+ + 4H+ + SO42–

Cu2+ + Cu 2Cu+

Precipitation of cuprous chloride is practiced at Mantos Blancos in Chile since copper ore there contains appreciable amounts of chlorides and at Duisburger Kupferhütte in Germany for copper recovery from pyrite cinder leach solution.

60

Cyanides

In acidic or neutral medium, cuprous cyanide is practically the only cyanide insoluble in water. Consequently, processes have been suggested for copper recovery from a leach solution by precipitation as cyanide, as follows:

The low-grade ore is leached with sulfuric acid, SO2being added to the leach solution to reduce cupric to cuprous sulfate; then HCN is passed through the solution to precipitate cuprous cyanide:

HCN H+ + CN–

Cu+ + CN– CuCN

• The low-grade ore is leached by sodium cyanide solution:Cu2O + 6CN– + H2O 2[Cu(CN)3]

2– + 2OH–

On acidifying the leach solution with dilute sulfuric acid, CuCN is precipitated, with simultaneous generation of HCN for recycle:

[Cu(CN)3]2– + 2H+ CuCN + 2HCN

There is interference, however, in this case due to traces of iron which may co-precipitate as copper ferrocyanide, Cu2[Fe(CN)6], and contaminate the product.

This can be minimized by careful pH adjustment to isolate the complex cyanide prior to precipitating CuCN.

Cuprous cyanide is a white, easily filterable precipitate. Both methods are based on processing CuCN to metallic copper by reduction with hydrogen at about 400°C whereby HCN is regenerated for recycle:

2CuCN + H2 2Cu + 2HCN

61

Sulfides

• Copper is sometimes precipitated from leach solutions as sulfide when its concentration in the solution is low.

• Hydrogen sulfide, ammonium sulfide, and sodium sulfide are the most common precipitating agents used for this purpose.

Hydrogen sulfide is a poisonous and corrosive gas. In certain concentrations, it explodes in air. In precipitating metal sulfides by H2S, the following points should be taken into consideration.

• Acid is generated during precipitation:M2+ + H2S MS + 2H+

and can be used in the leaching circuit, or must be neutralized before disposal.

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• Sometimes, it is desirable to float the precipitated sulfide to economize the cost of filtration. This is especially the case when leaching low-grade copper ore containing fine particles (slimes). Precipitation can be conducted directly in the slurry and CuS is recovered by flotation thus by-passing a filtration step.

• Precipitation of sulfides can be selective. For example, at Outokumpu, in Finland, pyrite cinder is leached with water to get a solution at pH 1.5 analyzing 20 g/L Co, 6–8 Ni, 7–8 Cu, 10–12 Zn, and trace amounts of iron.

• The solution is purified by selective precipitation with H2S to remove first copper, then zinc, nickel, and finally cobalt sulfides.

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Sulfites

Copper sulfites are of interest in hydrometallurgy because they can be readily precipitated from leach solutions in a pure form and can be treated in a variety of ways to yield pure metallic copper

Cu2SO3·7(NH4)2SO3·10H2O

Cu2SO3·(NH4)2SO3

Cu2SO3·CuSO3·2H2O (Chevreul’s salt)

Ammonium copper sulfites

• Ammonium sulfite solution (natural pH about 8) extracts copper from oxidized copper ores.

• When SO2 is bubbled through the solution at room temperature until pH 7 is reached, the cupric species are reduced to the cuprous. The same behavior takes place when SO2 is bubbled into a copper ammine solution (pH 10.5): the dark blue solution becomes colorless because of the oxidation–reduction reactions:

SO2 + H2O H2SO3 SO32– + 2H+

SO32– + H2O SO4

2– + 2H+ + 2e–

• At pH 7in sulfite medium the following equilibrium is established:[Cu(NH3)4]+ + 4H+ Cu+ + 4NH4

+

[Cu(NH3)4]2+

blue

+ e–

[Cu(NH3)4]+

colorless

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Two products can be obtained from these solutions:• On cooling, transparent crystals of cuprous ammoniumsulfite,

Cu2SO3·7(NH4)2SO3·10H2O, are formed. These needle crystals are water-soluble and when acidified, they precipitate metallic copper by disproportionation:

2Cu+ Cu + Cu2+

while the sulfite ion forms SO2:SO3

2– + 2H+ SO2 + H2O• When SO2 is bubbled further until pH 3, then a double salt

Cu2SO3·(NH4)2SO3 is formed. These cream colored shiny hexagonal platelets are insoluble in water and when acidified, they also precipitate metallic copper by disproportionation:

Cu2SO3·(NH4)2SO3(s) + 4H+ Cu + Cu2+ + 2SO2 + 2NH4+ + 2H2O

When heated with water at 150°C in an autoclave in absence of acid, complete conversion to metallic copper takes place:

Cu+ + e– CuSO3

2– + H2O SO42– + 2H+ + 2e–

SO32– + 2H+ SO2 + H2O

Overall reaction:Cu2SO3·(NH4)2SO3 2Cu + SO2 + 2NH4

+ + SO42–

Chevreul’s salt

Named after its discoverer the French chemist Michel Eugène Chevreul, the salt is a red crystalline solid having the composition Cu2SO3·CuSO3·2H2O in which copper is present in two valency states. It can be obtained in a variety of ways. For example:

• When copper oxide ore is dissolved in sulfurous acid, then the solution is heated:

CuO + H2SO3 Cu2+ + SO32– + H2O

SO32– + H2O SO4

2– + 2H+ + 2e–

Cu2+ + e– Cu+

Overall reaction:3CuSO3 + 3H2O Cu2SO3·CuSO3·2H2O + H2SO4

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RECOVERY FROM LEACH SOLUTION

Precipitation by reductionPrecipitation by reduction

Precipitation by reduction is an oxidation–reduction process:Oxidation:

Reduced species → Oxidized species + ne–

Reduction: Mn+ + ne– MOverall reaction:

Mn+ + Reduced species M + Oxidized species The process may be homogeneous (the reduced species is

ionic or non-ionic) or heterogeneous (reduction takes place on a solid surface). Heterogeneous processes may be electrochemical or electrolytic.

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Ionic reduction

Sulfite ion may be used as an aqueous solution of sulfur dioxide:

SO2 + H2O H2SO3 2H+ + SO32–

SO32– + H2O SO4

2– + 2H+ + 2e–

On passing SO2 into a solution of copper sulfate at room temperature, copper sulfite will precipitate. However, if precipitation is carried out at 150°C and 350 kPa, metallic copper is precipitated according to:

Cu2+ + SO32– + H2O Cu + 2H+ + SO4

2–

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The drawback to this process is the low yield of copper, corrosion problems due to the acidic environment, and the presence of small amounts of sulfur in the copper produced.

The low yield may be due to the intermediate formation of cuprous ion which disproportionates precipitating half of the copper and regenerating the other half as cupric ion:

Cu2+ + e– Cu+

2Cu+ Cu + Cu2+

The decomposition of sulfurous acid at the reaction temperature to H2SO4 and elemental sulfur according to the equation: 3H2SO3 2H2SO4 + S + H2Oaccounts for the presence of small amounts of sulfur in the copper produced.

This process can be improved by adding an ammoniacal solution of ammonium sulfite instead of SO2, i.e., neutralizing the acid as soon as formed. The following reaction takes place:

Cu2+ + SO32– + 2NH3 + H2O Cu + 2NH4

+ + SO42–

This results in complete precipitation of copper and has the advantage of operating under basic conditions thus eliminating corrosion problems. However, it has the inconvenience of producing ammonium sulfate that has to be crystallized and marketed as fertilizer.

In a similar way, cuprous ion in a copper ammine sulfite solution can be reduced to metallic copper when the pH is adjusted to 3 by sulfurous acid to precipitate the double salt Cu2SO3·(NH4)2SO3 which, when slurried with water and heated at 150°C in an autoclave:

Cu+ + e– CuSO3

2– + H2O SO42– + 2H+ + 2e–

SO32– + 2H+ SO2 + H2O

Overall reaction:Cu2SO3·(NH4)2SO3 2Cu + SO2 + 2NH4

+ + SO42–

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Non-ionic reduction

NonNon--ionic reducing agents such as hydrogen, ionic reducing agents such as hydrogen, carbon monoxide, formaldehyde, and carbon monoxide, formaldehyde, and hydrazine may be used to precipitate copper hydrazine may be used to precipitate copper from aqueous solutions. from aqueous solutions.

Reducing

agent Reaction

Hydrogen H2 2H+ + 2e–

Carbon

monoxide CO + H2O CO2 + 2H+ + 2e–

Form-

aldehyde

HCHO + H2O HCOOH + 2H+ + 2e–

HCHO + 3OH– HCOO– + 2H2O + 2e–

Hydrazine H2N·NH2 N2 + 4H+ + 4e–

2H2N·NH2 N2 + 2NH4+ + 4e–

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Hydrogen

Hydrogen at high temperature and pressure leads to the precipitation of copper. Reduction is usually carried out in horizontal stainless steel autoclaves equipped with agitators, baffles, heating or cooling coils, and the necessary connections for feed and gas inlets and outlets. The product of this technique is a high-purity powder that can be used as such or hot pressed and rolled in form of strips.

For the reaction:Cu2+ + H2 Cu + 2H+

the precipitation of copper from CuSO4 solution takes place through the formation of cuprous ion which has been identified in the course of reaction:

2Cu2+ + H2 2Cu+ + 2H+

2Cu+ + H2 2Cu + 2H+

Copper can be precipitated from acidic solutions [140 g/L sulfuric acid ]

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When reduction is conducted in ammoniacal medium the following reactions will take place:

[Cu(NH3)n]2+ nNH3 + Cu2+

Cu2+ + H2 Cu+ 2H+

H+ + NH3 NH4+

It can be seen that increasing the ammonia concentration has twoopposing effects:

• Precipitation is favored due to the neutralization of the liberated acid.• Precipitation is hindered because of the decrease in the reducible metal

ions Cu2+ due to the complexing action.

Therefore, there must be an optimum [NH3]/[Cu2+] ratio at which these opposing effects are balanced. The amount of ammonia in solution also influences the rate of precipitation. The rate of precipitation achieves a maximum when the ratio = 2.

After each reduction, copper particles are allowed to settle to the bottom of the autoclave, while the spent solution is drawn off and replaced with fresh pregnant solution. In this way, copper particles grow to the desired size, at which point the suspension is discharged and the nickel powder then separated.

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The presence of certain organic or inorganic substances in the aqueous phase greatly affects the physical nature of metal precipitated.

It is possible to precipitate copper powder of certain physical property by simply adding a certain amount of additive.

However, when organic additives are used, the carbon content in the powder produced is increased and a special heat treatment is necessary to lower it to 0.01 %.

Application

Copper scrap is dissolved either in ammoniacalammonium carbonate at 60°C at atmospheric pressure with continuous aeration, or in dilute sulfuric acid.

When ammoniacal medium is used, the molar ratio [NH3]/[Cu2+] should be equal to 2.4. After filtration to remove insoluble material, a small amount of anti-agglomerating agent is added, then solution is heated to 200°C under 6000 kPa hydrogen.

The copper powder precipitated is filtered off, washed, and then dried in a reducing atmosphere at 600°C.

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Carbon monoxide

Carbon monoxide has been used for precipitating copper from [Cu(NH3)4]2+ solutions obtained by leaching brass scrap in ammoniacal ammonium carbonate:

[Cu(NH3)4]2+ Cu2+ + 4NH3

Cu2+ + CO + H2O Cu + CO2 + 2H+

Precipitation takes place at 150°C with CO partial pressure of 5,000 kPa. The solution is then boiled to precipitate zinc as basic carbonate.

RECOVERY FROM LEACH SOLUTION

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Electrochemical

Precipitation of copper from copper sulfate solution by iron is known as cementation, because copper is usually cemented on the surface of iron.

Iron, having more positive (oxidation) potential will pass into solution and displace copper that has a less positive potential. The process is electrochemical in nature.

Reactions take place through transfer of electrons at the surface of the solid.

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Iron used for cementation of copper is in form of scrap, cans, or sponge.

Cans are heated in a rotary kiln to remove the tin plating and paint from the surface, and the solder from the seams. They are then passed through a hammer mill, a toothed roll, or a shredding machine to make them more compact and less bulky.

Stockpiling of iron usually presents a problem: it occupies large areas and it oxidizes with time. This oxidation not only means an iron loss, but also a decrease in the quality of cement copper produced from such material.

Recovery of copper from sulfuric acid leach solution by this technology has been practically abandoned although it was an important process during the 20th century.

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Side reactions during cementationSide reactions during cementation

Excessive dissolution of iron A part of the iron usually dissolves by a secondary reaction without affecting precipitation, thus increasing metal consumption:

Fe + 2H+ Fe2+ + H2

The presence of ferric ion in solution may also contribute to the dissolution of iron by the reaction:

Fe + 2Fe3+ 3Fe2+

In practice, two to four times the theoretical amount of iron is consumed. In all cases, more metal dissolves when it is in powder form than when it is coarse.

• Re-dissolution of the cemented copper Unless precautions are taken, the precipitated copper may re-dissolve on standing:

Cu + Cu2+ 2Cu+

Cu + 2H+ + 1/2O2 Cu2+ + H2O

Therefore, the precipitate should be removed as soon as possible from the solution, and excessive agitation should be avoided since it enhances these reactions

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Precipitaion of hydrolytic products. The pH should be about 2 to prevent the hydrolysis of Fe2+ and Fe3+

ions and formation of hydroxides or basic salts that may take place and contaminate the copper precipitated thus decreasing its purity.

Formation of toxic gases. Due to the presence of trace amounts of arsenic and phosphorus as impurities in iron, the conditions are favorable for the formation of arsine, AsH3, and phosphine, PH3, by the following reactions:

Fe3As2 + 6H+ 3Fe2+ + 2AsH3Fe3P2 + 6H+ 3Fe2+ + 2PH3

Both gases are highly poisonous and were detected when using iron for precipitating copper.

Formation of alloysFormation of alloys. Under certain conditions, . Under certain conditions, an alloy is precipitated instead of the pure an alloy is precipitated instead of the pure metal. For example, during the precipitation of metal. For example, during the precipitation of copper by iron, the main impurity is iron. copper by iron, the main impurity is iron. There is an indication that this iron impurity There is an indication that this iron impurity occurs in alloy with copper. occurs in alloy with copper. Cement copper containing 2Cement copper containing 2  % Fe is magnetic, % Fe is magnetic, and it is not possible to separate iron and it is not possible to separate iron selectively by magnetic methods.selectively by magnetic methods.

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Factors affecting the physical properties of precipitated copper

For the cementation of copper by iron, a finer For the cementation of copper by iron, a finer powder is obtained at high Cupowder is obtained at high Cu2+2+ ion ion concentration.concentration.

At a constant CuAt a constant Cu2+2+ ion concentration, a fine ion concentration, a fine powder is obtained at high Hpowder is obtained at high H22SOSO44

concentrationconcentration

Morphology of copper precipitate from sulfate solution

on aluminum

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Morphology of copper precipitate from sulfate solution

on iron

RECOVERY FROM LEACH SOLUTION

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Precipitation by substitution

Copper is extracted by organic solvents by forming a coordination bond. When this loaded organic phase is treated by hydrogen at high temperature and pressure in an autoclave, copper precipitates in powder form and the organic phase is regenerated. The process may be described as precipitation by substitution since no ionic species are taking part in the reaction as compared to precipitation by hydrogen from an aqueous phase. The substitution reaction can be represented as follows:

H2(g) H2(org)R2Cu(org) + H2(org) 2RH(org) + Cu(s))

where RH is the organic solvent. where RH is the organic solvent.

The precipitation of copper powder from hydroxyl quinoline–kerosene phase containing copper

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