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Gold Processing by CIL/CIP With Stacked Tailings Using INNOVAT Paste Thickeners

Prepared by INNOVAT Limited 760 Brant Street, Suite 405c Burlington ON L7R 4B8 Website: www.innovat.ca Email: [email protected]

1.0 GOLD PROCESSING BY CIL/CIP 1.1 Common Flow Sheet Agitated tank leaching requires that the ore be finely ground and of relatively higher grade. Leaching of gold and silver is accomplished with dilute cyanide solutions mixed with the ground ore in agitated tanks. Activated carbon particles are added at some point in the train of agitated tanks, where the gold in solution is absorbed on the carbon. At a later stage the carbon, which is now loaded with gold, is removed from the agitated tanks and the gold is then stripped from the carbon. Efficient separation of the solutions from the finely ground ore can be difficult and is expensive. As a result, most gold leach plants discharge the tailings to a holding dam, where the solids particles are allowed to settle out, while the solution is decanted and returned to the process plant.

F ro m G rinding P la nt T hic k e ne r Leach Tanks

S o lu tio n re u trn to p ro c e s s

C a rbo n-inP ulp T ank s

S a fe ty S c re e n

S o lu tio n R e turn to P ro c e s s T a iling s Po nd

A G ITA TE D TA N K GO LD L E A C H PLA N T

2.0 TAILINGS DISPOSAL 2.1 Current Practice Placement of tailings is normally as shown the following sketch Figure 1, where the residue is pumped to a pond, either thickened or not, and the solids are allowed to settle.

The solution remaining is collected in a decant tower, well or a raft and returned to the process.

S o lutio n R eturn T a iling s P ip eline

E arth D am

D ec ant T o w er or R aft

E arth D am S eep a g e Pond

F IG U R E 1 T Y P IC A L T AIL IN G S B AS IN

A small tailings disposal site As long as the dams remain competent, the cyanide solutions remain in the tailings basins and eventually the cyanide will be destructed by ozone, ultra violet rays from the sun, and by biodegradation. Problems do occur, however, if there is leakage from the dams, flooding conditions or liquefaction from earthquake events. 2.2 Cyanide Destruction It is good practice to destroy the cyanide in solution before placing it in a tailings pond or basin, even though the cyanide solution can be re-used in the process. There are about twenty methods of cyanide destruction. The best method currently in use is the INCO sulfur dioxide/air system, which involves mixing liquid or gaseous sulfur dioxide with air in an agitated tank in the presence of a small amount of copper sulfate that acts as a catalyst. Sulfur dioxide is normally produced as a by-product in smelting processes.

Where there is no immediate source of sulfur dioxide, a substitute chemical, sodium metabisulfite can be used. 2.3 Stacked Tailings In recent cases where the tailings are prepared for stacking instead of placing the tailings in a tailings pond, the liquid-solid separation is done with thickeners and vacuum filters. Vacuum filters are in the form of table filters, disc filters, drum filters, or belt filters. Tailings are dewatered to 10 ­ 20% liquid, depending on particle size. Filter cake is transported to the tailings disposal area in trucks or on belt conveyors with stackers. As in deposition in tailings ponds, the cyanide in the solutions can be destroyed using the INCO sulfur dioxide/air system.

T a iling s fro m C ya nid e De s truc tio n P la nt S o lutio n R e turn to P ro c e s s H ig h R ate T hic k e ne r P um p B e lt F ilte r

V a c uum R e c e ive r

V a c uum P um p T o T a iling s D is p o s a l F iltra te P um p

F IG U R E 2 T Y PIC AL ST AC K E D T A IL IN G S

2.4 Paste Tailings Newer technology involves placement of the tailings as a paste, which is created by thickening in special, modern thickeners, which do not require the use of filters, thus saving considerable energy. If the tailings contain sufficient fine material the tailings can be pumped to the deposition area instead of being conveyed by truck or conveyor.

Deposition of Tailings as Paste A paste is defined as slurry that has been dewatered to the point that it will slump (as in a concrete slump test) but that no water or solution will flow from the slumped material. A paste can be pumped, if it contains more than 20% minus 20-micron solids. By discharging the tailings into vertical columns as shown in the photograph, the paste will form a cone from which precipitation will run off and from which a slurry will not be recreated. The high cost of a tailings dam disappears. During high precipitation, rain merely runs off the tailings cones to ditches, where the runoff is directed to the environment. 2.5 Cyanide Destruction with Paste Thickeners It is best to dewater the tailings before cyanide destruction, because much of the cyanide is removed in the dewatering process. This can be done with a thickener and filter, but

the use of a paste thickener reduces the capital cost of dewatering by about 50% and the operating cost by as much as 75%. It is best to bookend the cyanide destruction process with a pair of paste thickeners, such as shown in the following diagram.

F ro m Grind in g P la nt T h icke ne r L e a ch Tan ks

C arb o n C o lum n s

T o G rin d ing C ircu it

C a rb o n-in P u lp Tan ks

S a fe ty S creen Air C o m p res so r T o Grin d ing C irc u it IN N O V A T P as te T h ick en er D eto x Tan ks

IN N O V A T P as te T h ick en er T o T ailing s S ta ck

N a2 S 2O 5 C u 2S O 4

IN N O V A T L im ite d B urlin g to n , Can ad a

C IL /C IP w/ CC D Pas te T h ic ke n er

2.6 Return of Tailings to the Open Pit Mine In some cases designing the mining sequence carefully and choosing the correct tailings deposition method can return the tailings to the mine, even while mining continues. Preparing this sequence along with co-deposition with waste can also provide a means of preventing acid mine drainage where sulfides are present in the rock.

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TAILINGS DISPOSAL

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