InTech-Thermochemistry and Kinetics of the Reactions of Apatite Phosphates With Acid Solutions

8/2/2019 InTech-Thermochemistry and Kinetics of the Reactions of Apatite Phosphates With Acid Solutions

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Thermochemistry and Kinetics of the Reactionsof Apatite Phosphates with Acid Solutions

Mohamed JemalFaculty of Science, Chemistry Department, Applied Thermodynamics Laboratory,

Tunis El Manar University, 2092 Tunis El Manar,Tunisia

1. IntroductionThe main component of phosphate ore derive from fluorpapatite, Ca10(PO4)6F2, by a faintsubstitution of calcium by magnesium and sodium, phosphate ions by carbonate andfluorides by the latter and hydroxyl ions. Mineral component of calcified tissues (bone,dentine and enamel) derives from hydroxyapatie, Ca10(PO4)6(OH)2.Thermochemistry of these synthetic compounds and similar products began around themiddle of last century (Gottshall, 1958). During the next 25 year period, few works havebeen performed in order to determine thermochemical quantities of these compounds, but alot of work has been undertaken in the beginning of 70s on the synthesis andcharacterization of such products. Reactions of acid solutions on natural phosphates havealso been widely studied, this is because these reactions lead to phosphoric acid which is thebasic intermediate for phosphate fertilizer production. Fertilizers such as Mono-AmmoniumPhosphate, (MAP), NH4(H2PO4), Di-Ammonium Phosphate (DAP), (NH4)2HPO4, areproduced by the reaction of ammonia on phosphoric acid solution. The slurry is thengranulated at high temperature leading to the fertilizer product. Triple Super Phosphate(TSP) results from the reaction of phosphoric acid on the phosphate ore.High purity (food grade) phosphoric acid is usually produced in two steps. The first oneconsists in reducing apatite by carbon in presence of silica at a temperature up to 1500C, toproduce native phosphorus which is then oxidized into P4O10. The latter reacts with water toproduce H3PO4. This process is called dry process in opposition to the wet one in whichthe phosphoric acid results from the reaction of phosphate with sulfuric acid or with a

mixture of the latter with diluted phosphoric acid solution. Wet process produces an acidsolution containing large amounts of impurities, some of them coming from the phosphateore and the other from reacting acid. Depending on the reaction temperature and theconcentration of the acid, the wet process produces also a by-product which can be calciumsulphate dihydrate (gypsum), CaSO4,2H2O, or hemihydrate, CaSO4,0.5H2O, or anhydrousCa-sulfate, CaSO4, (Becker, 1989). Because of the exothermicity of the reactions occurring inthis process, a large amount of thermal energy is generated allowing to heat the reactionvessel at temperature up to 120C. Due to low cost and simplicity of acid production by wetprocess, fertilizers are manufactured using phosphoric acid resulting from this process.This paper deals with the determination of the formation enthalpies of a series of apatites,the general formula of which is M10(PO4)6Y2 in which M could be Ca, Sr, Ba, Cd or Pb or a


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Application of Thermodynamics to Biological and Materials Science548

combination of Ca with one of the other metallic elements. Y could be F, OH or Cl. A varietyof products have been synthesized, characterized then dissolved in acid solution using anisoperibol calorimeter in order to determine the enthalpies of solution. Combining thesequantities with others determined in the same device or picked from literature allowed to

determine the enthalpies of formation. Dissolution of the two metal compounds enables todetermine the enthalpies of mixing of the limit products in the solid state. Some otherexperiments have been recently performed on B-type Ca-carbonate apatites and allowed tocharacterize the substitution of PO4 by CO3 ions.Dissolution of samples of fluorapatite at different temperatures has also been followedmicrocalorimetrically using a C 80 SETARAM microcalorimeter and kinetic models havebeen proposed in accordance with the thermogenesis curves. In addition attack of aphosphate ore sample by phosphoric and phosphoric/sulphuric acid solutions has beenperformed and revealed various phenomena which have not been previously reported.

2. Synthesis of the apatitesTwo main procedures have been used in order to synthesize the products. The wet oneconsists in adding drop by drop a solution of metal nitrate solution into a boilingdiammonium phosphate solution at a pH between 9 and 11. Precipitation is then performedby maintaining ebullition for at least one hour, then the solid is separated and ignited atvarious temperature, depending on the product in concern. This procedure which is calledas direct method allows to synthesize hydroxyapatites with cadmium (Ben Cherifa, 2002)or lead (Ntahomvukiy, 1997)- It also permitted to obtain (Ca,Cd) and (Ca,Sr)hydroxyapatite solid solutions. The reverse method consisting in droping the metal solutioninto phosphate solution allows to synthesize hydroxyapatites containg only Ca or Pb, or Ca-Sr (Ben Cherifa and Jemal, 2004) or Ca-Pb (Ntahomvukiy, 1997) couples..Fluorapatites containing one or two metals have also been prepared according to one or theother method but the phosphate solution has been previously added with ammoniumfluoride. This procedure allowed to synthesize Ca, Pb and Sr fluorapatites or Ca-Sr(Khattech and Jemal, 1997), Ca-Cd (Ben Cherifa et al, 2001a) or Ca-Pb (Ntahomvukiye et al,1997a and 1997b) fluorapatites solid solutions.Chlorapatites have been usually obtained by the solid state procedure which consists inigniting during several hours at a temperature up to 1100C a mixture of trimetallicphosphate M3(PO4)2 with MCl2. This procedure allowed to synthesize Ca10(PO4)6Cl2 (BenCherifa 2002), Ba10(PO4)6Cl2 (Khattech, 1996), Cd10(PO4)6Cl2 (Ben Cherifa and Jemal, 2001b)together with Ca-Cd and Ca-Sr chlorapatite solid solutions.

3. Isoperibol calorimeter

The solids have been dissolved at 25C in a 9% weight nitric acid solution using anisoperibol calorimeter which is composed of a 500mL Dewar flask immersed in athermostated bath. In this device, the heat effect results in a variation of the temperature ofthe reactional medium, which is detected by a thermistance probe acting as one of the fourarms of a Wheatstone bridge connected to a recorder through a 177 Keitley currentamplifier. The device is provided with an electrical resistance connected to a stabilized DCcurrent supplier. The magnitude of the current was measured by a A.C power-meter GPM-8212 and the heat duration was accurately measured using a digital electronic timer with


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Thermochemistry and Kinetics of the Reactions of Apatite Phosphates with Acid Solutions 549

0.01 s resolution. 350 mL solution are introduced in the flask, and the solid is preliminarilyisolated in a Pyrex ball manufactured at the extremity of a 5 mm diameter tube which issupplied with a thin bar aimed at breaking the ball to start the reaction.Experiments started by searching a quasi-steady state in which the electrical current is

practically nil and the baseline deviates slightly from the horizontal line. Injection ofelectrical or chemical energy in the medium results in a deviation of the signal, which thenbecomes parallel to the previous baseline. Energy resulting from the reaction is calculatedconsidering the corresponding shift between the baselines and the mean calibrationcoefficient. Calibration coefficient E is calculated by dividing the calibration energy (J) overthe resulting shift d (mm) on the recorded paper. Three calibration experiments are usuallyperformed and the error on enthalpy is determined considering the scatter between theextreme values of E.Various amounts mi (mg) of each product were dissolved in the same volume of solution

and the molar dissolution enthalpy Hi was determined. Enthalpy per mass unit wascalculated according to a mathematical treatment in which the statistical weight of an

experimental result depends on the corresponding variance. This procedure leads to expressthe dissolution enthalpy as: solH = (iwimiHi)/(iwimi2), in which wi is the variance, itwas here assimilated to the reverse of the square of the error. The theoretical basis of thiscalculation was developed by Sands, 1974 and Pattengill and Sands, 1979.

4. Thermochemistry of the apatites

4.1 Formation enthalpy of non substituted apatites

In order to determine the enthalpy of formation of any compound from dissolution results,one procedure consists in considering a particular reaction involving the compound to bestudied and other solid or liquid reactants and products for which the formation enthalpies are

reported in literature. This reaction can be possible, impossible or even hypothetical.Measurements of the heat evolved on dissolution of the whole constituents in the same solventenable to derive the enthalpy of this reaction, and so, to deduce the formation enthalpy of theproduct in concern. This way of doing is useful particularly when dissolution enthalpy valuedepends on the concentration of the resulting solution that is when interaction of ions in thefinal solution is dependent on their concentrations even when the latter are very low. In thatcase the formation enthalpy of the compound have to be calculated by considering valuescorresponding to infinite dilution. This is the case when dissolving whitlockite,Ca18Mg2H2(PO4)14 . This compound can be involved in the following reaction:

16/3 Ca3(PO4)2 + 2/3 Mg3(PO4)2+ 2CaHPO4 Ca18Mg2H2(PO4)14 (R1)

Dissolution of various amounts of withlockite and of stoichiometric mixture of Ca and Mgphosphates have been performed in a 46%weight HNO3 solution using a conductioncalorimeter (Calsol) and the results are reported in figure 1. Least square processing methodled to express the standard molar enthalpies y versus the final concentration x as y = ax2+ bx + c, from which the enthalpy of (R1) reaction was deduced at infinite dilution as

H(298)= -203.3 1.6 kJ mol-1, leading to -27.93.103 kJ mol-1 as the standard enthalpy offormation of whittlockite (Ben Abdelkader et al., 2004).Variation of dissolution enthalpy on final concentration can be neglected when the drawingof the enthalpy resulting from dissolving various amounts m of the same product is linearversus m. This was the case for more simple apatite compounds.


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Fig. 1. Variation of molar enthalpy of dissolution of whithlockite () and of thestoichiometric mixture () as a function of the solid concentration in the final solution.Solvent : 46%weight HNO3 aqueous solution.

Determination of the enthalpy of formation using dissolution results can also be performedby involving the entities or compounds appearing in the dissolution reaction in otherprocesses (dissolution, dilution or mixing) in order to get a succession of reactions the

summation of which leads to the formation reaction of the product in concern. This wayof doing allowed us to determine the enthalpies of formation of a number of hydroxy,chloro, carbonate and fluor-apatites.For example, in order to determine the formation enthalpy of Ca-fluorapatite, Ca10(PO4)6F2,one can imagine the following succession of processes.

{10Ca(NO3)2 + 6 H3PO4 + 2 HF}sln Ca10(PO4)6F2 (sd) + 20 {HNO3}sln (1)

20 [HNO3,35.35 H2O] 10 H2 + 10 N2 + 30 O2 + 707 H2O (2)

10[Ca(NO3)2,4H2O] + sln 10 {Ca(NO3)2 + 4 H2O} sln (3)

10 Ca + 10 N2+ 50 O2+ 40 H2 10 Ca(NO3)2,4H2O (sd) (4)

6 [H3PO4, 0.756 H2O] + sln {6 H3PO4 + 4.536 H2O} sln (5)

9 H2 + 6 P + 12 O2 + 4.536 H2O 6 [H3PO4, 0.756 H2O] (6)

2 [HF, 1.708 H2O] + sln 2 {HF, 1.708 H2O} sln (7)

H2 + F2 + 3.416 H2O 2 [HF, 1.708 H2O] (8)

47.952{H2O} sln 47.952 H2O+ sln (9)


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40 H2O 40 H2 + 20 O2 (10)

10 Ca(sd)+ 6 P(sd) + 12 O2(g)+ F2(g) Ca10(PO4)6F2 (sd) (11)

The subscript sln means in solution which is here a 9%weight HNO3 aqueous solution, or[HNO3, 35.35 H2O]In this succession, step (1) is the reverse of the fluorapatite dissolution. That dissolution issupposed to lead to a solution containing dissolved calcium nitrate, phosphoric andfluorhydric acids. Steps (2), (4), (6), (8) and (10) correspond to the formation of particularproducts or entities, the corresponding enthalpies are picked from literature (Handbook ofChemistry and Physics, 2006/2007). Steps (3), (5) (7) and (9) correspond to dissolution of Ca-nitrate tetrahydrate in the same solvent and dilution of H3PO4 and HF solutions and waterin the same solvent, respectively. The corresponding enthalpies are measured in the samedevice using the same procedure as for the apatite. Step (11) corresponds to formationreaction of Ca10(PO4)6F2 according to the definition. Enthalpy of formation of chlorapatite

has been determined with a similar succession but HF solution in step (7) has been replacedby a HCl solution.This way of doing enabled to determine enthalpies of formation of various apatites and theresults are gathered in table 1. However, due to lack of uncertainties on literature values,errors on these quantities can not be estimated.

M10(PO4)6Y2 Y = F Y = OH Y = Cl

Ca10(PO4)6Y2 -13548 -13305 -13179

Sr10(PO4)6Y2 -13604 -13373 -13233

Ba10(PO4)6Y2 -13564 -13309 -13246

Cd10

(PO4)6Y

2-8795 -8648 -8463

Pb10(PO4)6Y2 -8529 -8261 -8204

Table 1. Standard enthalpies of formation (in kJ mol-1) of the apatites at 25C

More recent determination of the formation enthalpies for Ca-hydroxy and chlor-apatitesled to -13399 kJ mol-1 and -13231 kJ mol-1 (Cruz et al., 2005). These values differ from that oftable 1 by 0.7% and 0.3% respectively.Neglecting the entropy factor, one can notice the following remarks:i. whatever is the metal, fluor compound is more stable than hydroxy one which is more

stable than the chlor- compound. This gives an explanation of what was observedexperimentally, that is, as soon as fluor is introduced in the precipitation medium, fluor

compound precipitates. On the other hand, except for lead, chlorapatites were preparedonly by solid state reactions because in the wet procedure hydroxyapatites precipitateeven in presence of choride ions.

ii. for the same Y, alkali earth compounds are more stable than cadmium one which ismore stable than the lead one.

Values in this table have been used by Flora et al. (2004) to determine the lattice enthalpiesof these compounds and to derive consistent values for the enthalpy of formation of (PO43-,g) using the cycles reported in figure 2.Using the correlation of Jenkins and Glasser (2003), Flora et al. calculated standard Gibbsenergy of dissolution of a series of apatites including those which are not yet synthesized. Anegative value of this quantity indicates a significant solubility for the apatite. This is the


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case for zinc and magnesium apatites, leading the authors to conclude on the uncertainexistence of these apatites. In fact a significant solubility of an apatite suggests impossibilityof synthesizing it by wet procedure, but this does not eliminate the possibility of synthesisby solid state reaction.

10M(s) + 6P (White, c) +12O2 (g) +X2

f H[M10(PO4)6X2(s)]

M10(PO4)6X2(sd)

10M2+(g) +6PO43-(g)+2X-(g) 10M3+(aq)+6PO43-(aq)+2X-(aq)

Cycle A : Hlattice= Hdiss-10 hydH(M2+,g)- 6hydH(PO43- ,g)- 2hydH(X-, g)Cycle B : Hdiss=10 fH(M2+,aq)+6fH(PO43- ,aq)+2fH(X-, aq)- fH(M10(PO4)X2,s)Cycle C : fH(PO43- ,g)= 1/6[H lattice+fH(M10(PO4)X2,s)-10fH(M2+,g)- 2 fH(X-, g)]

10 fH(M2+,g)+6 fH(PO43- ,g)+2 fH(X-, g)

Cycle C Cycle B

10 fH(M2+,aq)+6fH(PO43- ,aq)+2 fH(X-, aq)

Cycle AHlattic Hdiss

Fig. 2. Thermochemical cycles to derive the lattice enthalpies of the apatites and theformation enthalpy of gaseous PO43- at 25C and to calculate again the dissolution enthalpies(Flora et al., 2004).

4.2 Estimation of standard Gibbs energy of formation

Comparing the stability of similar compounds on the basis of the formation enthalpies

supposes the entropy factor be non significant compared to enthalpy. This way of doingremains valid when the enthalpies of formation are significantly different. It also supposes

the entropy of formation of the compounds be equal zero. Taking into account the

discrepancy between the formation enthalpies of alkali earth apatites and those of cadmium

or lead one can compare their stability on the basis of this approach. But when the formationenthalpies are close to each other, entropy factor becomes important and can modify the

stability order.Instead of affecting the zero value to the formation entropy of the compound, an other

approach can be developed for better estimating this quantity. This approach consists ininvolving the compound in a particular reaction for which the formation entropies of

reactants and products are tabulated. This leads to determine the upper value of the

formation entropy of the compound in concern. By assimilating fS be equal to this limit,one can derive a new value of the Gibbs energy.For example Ca-fluorapatite , CaFap, can be involved in the following reaction:

9 CaO + 3/2 P4O10 + CaF2 Ca10(PO4)6F2 (R2)

Taking into account the literature data ( Handbook, 2006-2007 and JANAF, 1971 for P4O10) ,

the entropy of this reaction is derived as: R2S(T0) = S(T0, CaFap) 904.6 J mol-1 K-1, where


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T0 is the standard temperature (298.15 K) and S(T0, CaFap) the standard entropy of CaFapat 298.15 K. Entropy of R2 reaction is negative, consequently S(T0,Fap) < 904.6 J mol-1 K-1.The formation entropy of Fap is the entropy of the following reaction:

10 Ca(sd)+ 6 P(sd) + 12 O2(g)+ F2(g) Ca10(PO4)6F2 (R3)It can be expressed as fS(T0, CaFap) = S(T0, CaFap) - 3327,6 J mol-1 K-1. Taking intoaccount the previous inequality one can derive the following inequality as fS(T0, CaFap)

InTech-Thermochemistry and Kinetics of the Reactions of Apatite Phosphates With Acid Solutions

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