Continuous vat leaching

Gold/silver leaching

THE LATEST TECHNOLOGY

BY

INNOVAT Limited 760 Brant Street, Suite 405c

Burlington, Ontario Canada L7R 4B8

Phone: 905-333-7133

Email dan.mackie@danmackie.com

Website www.innovat.ca

Gold/Silver 1/8

LEACHING OF PRECIOUS METALS ORES

OVERVIEW

CONTINUOUS VAT LEACHING

The use of continuous vat leaching is a preferred method of leaching gold and silver ores over both agitated tank leaching and heap leaching for both economic and environmental reasons. An exception is when high economical recoveries can be earned in run-of-mine heap leaching. This is a rare situation, and the environmental risks involved can be serious.

Heap leaching occupies huge land areas that must be reasonably flat. In dry climates there are high evaporation losses. In cold climates the heaps must be protected against freezing. In high rainfall areas the heaps must be covered. Following leaching, the heaps must be rinsed and detoxified, a lengthy process of doubtful long-term effectiveness. The following photograph shows a small heap leach operation. Some are many times larger, occupying literally hectares of land.

Gold/Silver 2/8

A Small Heap Leach Operation Agitated tank leaching, using carbon-in-leach, carbon-in-pulp, or Merrill-Crowe methods is commonly employed for higher grade ores. Merrill-Crowe is the preferred flow sheet for high silver/gold ratios with CIL/CIP normally used for gold ores. Each of these methods normally employs tailings ponds. Detoxification of slurries discharged to tailings ponds is only done in some regions of the world. During the past thirty years, there has been an average of one major tailings dam failure each year, with about one third of them involving cyanide. This has translated into extreme environmental pressure being put on the industry by the public and by the press, which makes financing of gold mines difficult. Cyanide destruction prior to tailings disposal has become mandatory in many parts of the world, while in other areas, the use of cyanide has been banned altogether. INNOVAT technology discharges dewatered tailings that have been fully detoxified, using the INCO SO2/Air method, all as an integral part of the system.

Gold/Silver 3/8

Figure 1TRADITIONAL GOLD LEACH PLANT

Tailings Pond

Carbon-in-Pulp Tanks

Leach Tanks

Thickener

CarbonColumns

SafetyScreen

ReturnSolution

From GrindingPlant

There is a growing trend towards dewatering tailings and stacking them in the environment. This is an expensive proposition with conventional flow sheets, because it requires the use of thickeners and filters. The INNOVAT system has dewatering as an integral part of its discharge. Vat leaching has been around since Roman times, but had become viable in 1896, when cyanide leaching began in earnest with the Merrill-Crowe Process. In an article in the Engineering and Mining Journal in December 1999, Lou Cope described the first vat leaching at Homestake in 1901, which later improved the process in 1906 by adding a desliming circuit. Traditional vat leaching is a batch process, however, with materials handling being of high cost. Some vat leaching continues to this day, especially in the copper industry, where such plants as Mantos Blancos in Chile continue to operate. The method used is the same as was first introduced in 1927 on nitrates, with ore loaded into the vats for batch leaching, then discharged by gantry-mounted clamshells. It is a leach/drain/wash/drain cycle that conceivably could be used with a detoxification cycle in gold leaching and would allow for stackable tailings. The operating costs in most cases, however, would be prohibitive.

Gold/Silver 4/8

Traditional Vat Leaching of Copper Traditional Vat Leaching of Copper Mantos Blancos, Chile, 1995 Mantos Blancos, Chile, 1995

INNOVAT LimitedOakville, Canada

INNOVAT Limited

SIZE FSCM NO DW G NO REV

Figure 2 ASCALE SHEET 1 OF 1

Gioas ProjectContinuous Vat Leaching Flow Sheet

EcoVat ®

REV. DESCRIPTION DATE BY

PasteThickener

InductionFurnace

Circulating Pumps

Detox.Pumps

Make-upWater Na2S2O5

Cu2SO4

Air NaOCl

To G

ravi

tyP

lant

WashSection

DetoxSection

To TailingsDisposalFrom

GravityPlant

Leach Section

CarbonColumns

HoldingTank

ElectrowinningCell

CarbonStrippingColumns

CarbonReactivation

ToCarbon

Columns

NaOH

NaCN

HeadTank

CaO

Figure 2 Continuous Vat Leaching Flow Sheet with Carbon Columns

Gold/Silver 5/8

Continuous Vat Leaching, introduced by INNOVAT in 1991, dramatically reduces materials handling costs. Ore simply is introduced into the feed end of the vat, is transported through the vat by intermittent fluidization, and is discharged as a stackable product. Referring to Figure 2, which is a typical flow sheet using carbon columns for gold extraction, three distinct sections can be seen: leaching, washing, and detoxification, all operating as a continuous process in a single vat. Ore is intermittently fluidized by high energy up bursts of solution that provide intimate mixing and the fluidity to transport the ore through the vat. The ore behaves like quicksand and flows to level the bed as ore is put in and taken out. Ore in the vat is naturally classified and washed of slimes, which are collected separately in a paste thickener. Coarser ore left in the vat travels through in a near sanded out condition so that when it is lifted out and decanted by the discharge mechanism it is well drained to paste consistency. Other discharge methods patented by INNOVAT can filter the discharge for drier disposal. Ground ore tailings discharged from the vats are typically below 20% water by weight for normally grind sizes (200 mesh) with slimes discharged separately at 50% water by weight or less. The advantage of this scenario is flexibility in final disposal, particularly if pozzolanic mixes are planned.

INNOVAT LimitedOakville, Canada

INNOVAT Limited

SIZE FSCM NO DW G NO REV

SCALE SHEET 1 OF 1

1000 tpd Leaching Plant Flow DiagramWith Merrille-Crowe

EcoVat ®

REV. DESCRIPTION DATE BY

Leaf Clarifier

PasteThickener

Plate and FrameFilters

InductionFurnace

Zn

VacuumPump

Crowe Tower

RecirculatingPumps

Detox.Pumps

Make-upWater

NaCN

Na2S2O5

Cu2SO4

Air

BarrenSolution

Tank

NaOCl

To G

ravi

tyP

lant

WashSection

DetoxSection

To TailingsDisposalFrom

GravityPlant

HeadTank

Leach Section

CaO

Figure 3

Typical Continuous Vat Leach with Merrill-Crowe

Gold/Silver 6/8

Economics of Continuous Vat Leaching™

Comparison with Flotation and Agitated Tank Leaching A recent capital cost estimate prepared for Greater Lenora Resources on their 4000 t/d Goldfields project, compared the two cases. The original flow sheet had been conceived as a gravity/flotation plant at an estimated cost of $30 million. Following laboratory work on leaching and based on autogenous crushing to a P80 of 10-mesh, the resulting comparison was made. Item Agitated Leaching

Case US $ Continuous Vat Leaching Case US $

Crushing Plant 5,000,000 5,000,000Gravity Plant 1,200,000 1,200,000Leach Plant 4,070,000 3,000,000Carbon Strip Plant 3,546,000 3,546,000Detoxification Plant 762,000 IncludedElectrical and Controls 400,000 400,000Tailings Disposal 2,780,000 200,000Subtotal 17,758,000 13,346,000Contingency 15% 2,664,000 2,002,000EPCM 15% 2,664,000 2,002,000Contractor Indirects 900,000 703,000Total Capital Cost 23,986,000 18,053,000 A great deal of the capital savings is in the detoxification plant and tailings disposal. A significant advantage in operating cost savings, in the range of $0.35 to $0.60 per ton, can be realized by using vat leaching. Power savings, ranging from $0.15 to $0.30 per ton, depending on power rates, result from the use of hydraulic fluidization versus the powerful agitators required in conventional leaching plants, most of which power is consumed simply to keep the agitated tanks from sanding out. The coarser the grind, the more dramatic this becomes. Savings in disposal transportation can also be earned by using the mine trucks to back-haul the detoxified waste to the dump. One has to realize that the same trucks that deliver the ore to the crushing plant can take the ore to the dump; the trucks never need run empty. Comparison with Heap Leaching It is not just the environmental needs that will drive the use of Continuous Vat Leaching, but the improved profitability. As shown in the following table, where a 2 gpt gold ore is processed at a rate of 1.65 million tons per year (5000 t/d), where heap leaching at a crush size of –3/8” is compared with vat leaching at –1/4”, the economic benefits are significant. This data, based on an existing heap leach, assumes a heap leach cycle of 60 days versus a vat leach cycle of 36 hours. Ore is treated in two vats.

Gold/Silver 7/8

Item Heap

Leach Continuous Vat Leach

Benefits Comment

Recovery 70% 90% +20% Continuous Vat Leaching has better process control.

Capital Cost

$7,300,000 $6,600,000 -$700,000 No agglomeration system, no ponds.

Operating Cost Differential

+$2.38/ton +$0.60/ton -$1.78/ton Minimum materials handling, labor and water losses

Inventory Cost

$1,800,000 $45,000 -$1,755,000 Work in Process inventory: Heap Leach requires 300,000 tons of ore; continuous vat leaching requires 7,500 tons of ore.

What is seldom considered in heap leaching are the tons of ore and tons of solution in inventory, a value estimated at over $1.7 million in the previous example. The important issue is the cash flow implications for the business. In addition, there is more cash flow at risk; ore sits there until it has been leached and recovered from solution, losing potential value for the metal as the market rises and falls during recovery. Some heap leaches take up to 350 days for full recovery, examples being Greenstone’s Minas Santa Rosa and Viceroy’s Brewery Creek. Consider the cost of money for the time invested; consider the business cash flow.

Summary of Continuous Vat Leaching™ Benefits

You win hands down with better economics and other benefits, which include but are not limited to:

• Up to 6 mm as well as ground ores (typically P80 75 microns) can be handled • Can be applied to economically benefit existing heap leach operations • No tailings ponds; tailings completely detoxified to international standards • Minimum water losses through evaporation as compared with heaps and tailings

ponds. • Operation year round in freezing conditions • Smaller footprint than heap leaching and agitated tank leaching • Ideal for underground operations

Go Modern Go INNOVAT!!!

Gold/Silver 8/8