Recovery of metal values from ores

Heap leaching of gold/silver ores with an aqueous alkali cyanide solution containing a surfactant hydrolyzable at the pH of the solution, the surfactant being present in an amount sufficient to increase the rate of metal value recovery without adversely affecting the carbon efficiency in the subsequent step of separating the metal cyanide value from the pregnant leach solution by absorption on carbon columns.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the recovery of metal values 
from ores. More particularly, the present invention relates to the 
recovery of metal values by the leaching of ores. Still more particularly, 
the present invention relates to an improvement in the treatment of 
gold/silver ores by the method of heap-leaching in which the metal values 
are recovered as a cyanide complex. 
2. Description of the Prior Art 
The recovery of gold and silver by heap-leaching low grade ores and 
tailings from other recovery processes is well known. In general, the 
procedure comprises spraying, trickling or pouring on, or otherwise 
applying to, a pile of low grade ore or tailings an aqueous alkali cyanide 
solution, e.g., aqueous calcium, sodium or potassium cyanide, so as to 
cause the solution to permeate and percolate through the pile thereby 
extracting the metal values as a cyanide complex. The resultant metal 
cyanide complex-bearing liquor is recovered from the bottom of the pile, 
of which there is usually a series in an extraction facility, and routed 
to a recovery system for separating the metal cyanide complex by 
adsorption on a column of activated carbon, usually coconut shell carbon. 
The metal cyanide complex-bearing carbon particles are then further 
treated, as by electrowinning, to separate and recover the elemental metal 
value. 
Various factors can affect the economics of heap leaching, a principal one 
being the physical nature of the crushed ore being treated. For instance, 
the presence of a high content of slimes or fines in a highly clayey ore 
can, in the presence of the aqueous leach solution, result in the swelling 
of the fines and the filling of the interstices of the ore particles, or 
even a coating of the particle surfaces. As a consequence, the porosity of 
the ore particles is reduced thereby preventing effective percolation of 
the leach solution with corresponding decreased leach solution percolation 
rate and decreased metal recovery rate. 
To minimize the effect of slimes or fines, various treatments, e.g. 
flotation, have been practice on the ore prior to being heap leached in 
order to reduce the fines content. Various means have also been use during 
the heap-leaching procedure itself in an effort to increase the rate and 
quantity of metal recovery. CA92(16):132551q, for example, reports an 
increase in the percolation rate in the heap leaching of gold/silver ores 
by using a cyanide leach solution containing a flocculant such as 
polyethylene oxide. On the other hand, the use of a nonionic 
ethylene/propylene oxide-based surfactant in a gold/silver cyanide heap 
leaching solution, as reported in "Gold and Silver Heap and Dump Leaching 
Practice", pp. 41-49, Soc. Mining Eng., Proc. Fall 1983 SME Meeting, 
showed no affect on percolation rate, metal recovery rate or reagent 
consumption. The use of the surfactant, moreover, resulted in a drastic 
drop in the carbon efficiency of the subsequent metal recovery system. 
SUMMARY OF THE INVENTION 
As the prices of gold and silver increase, interest in developing improved 
procedures for treating low grade gold/silver ores for the recovery of 
these metal values increases. It is a principal object of this invention 
to fulfill this interest. It is a particular object of this invention to 
improve the leach rate of recovery of metal values from low grade 
gold/silver ores by the process of heap-leaching with a metal cyanide 
leach solution. It is a further object of this invention to obtain an 
improved leach rate without adversely affecting the carbon columns used in 
the subsequent separation of the metal cyanide complex from the pregnant 
leach solution. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
These objects have been met in accordance with this invention by 
incorporating in the aqueous metal cyanide solution used for leaching a 
low grade gold/silver ore an amount of a surfactant effective to improve 
the metal value recovery rate. While the use of a surfactant in a metal 
cyanide leach solution has been reported in the prior art referred to 
above, various adverse effects were observed without, moreover, any 
improvement being realized in the metal value recovery rate. It is 
essential, therefore, in order for a surfactant to be effective in 
heap-leaching, that it not only improve the rate of metal value recovery, 
but accomplish this without adversely affecting the carbon efficiency of 
the carbon columns used in the subsequent metal cyanide absorption step. 
It has now been found, in accordance with this invention, that an improved 
metal value recovery rate can be obtained in heap-leaching of gold/silver 
ores without affecting carbon efficiency in the subsequent metal cyanide 
absorption step by incorporating in the cyanide leach solution an 
effective amount of a surfactant readily hydrolyzable under alkaline 
conditions. Without intent to limit this invention by any theory of 
operation, it may be that hydrolysis of the surfactant prior to or during 
the metal cyanide absorption step keeps the activated carbon surfaces more 
completely available for metal cyanide absorption. 
An important class of surfactants that hydrolyzes readily at a pH of 10-11, 
i.e., the alkalinity of the cyanide leach solution, is ethoxylated fatty 
acid esters. Preferably, the surfactant comprises a fatty acid residue of 
8-30 carbons and about 4-20 units of ethylene oxide. Condensates of 
ethylene oxide and oleic or stearic acids and mixtures of these, e.g., 
polyethylene glycol 200 distearate have been found to be particularly 
suitable. 
Another class of surfactants effective in the practice of this invention is 
alkyl sulfosuccinates in which the alkyl is a fatty acid residue of 5-30 
carbons. An example of this class of surfactants is sodium diethylhexyl 
sulfosuccinate. 
Still another class of surfactants that may be used according to the 
present invention is long chain fatty alcohol sulfates in which the fatty 
alcohol residue contains about 8-30 carbons. Representative of such 
surfactants is sodium heptadecylsulfate. 
In the practice of the process of this invention, the surfactant, or a 
mixture of surfactants, is simply incorporated in the aqueous alkali 
cyanide heap-leach solution and kept at a concentration effective to 
improve the rate of recovery of the metal value. It is difficult to define 
a concentration range of surfactant effective, in the sense of this 
invention, for use in the heap-leaching of ores of varying compositions 
since it will depend to a large extent on the nature of the ore, e.g., its 
clayeyness, as well as the particular surfactant employed. The optimum 
concentration effective in each circumstance, therefore, must be 
determined through use. Accordingly, it can be stated that, depending on 
the surfactant to be used and the ore to be treated, the surfactant 
concentration may be low as 1 ppm of leach solution with an upper 
limitation of about 50 ppm governed principally by economics. Usually, an 
optimum concentration of surfactant will be found to be within the range 
of 10-30 ppm of leach solution. A further benefit derived from the process 
of this invention resides in the concomittant defoaming action that is 
obtained under conditions of certain surfactant use. Thus, the hydrolysis 
of esters used in accordance with this invention will yield intermediate 
length-chain fatty alcohols which are well known defoamers. For example, 
the hydrolysis of diethylhexylsulfosuccinate releases ethyl hexanol, a 
particularly active defoamer. In this case, since the hydrolysis is 
relatively slow, there will be a constant presence of ethyl hexanol in the 
heap-leach solution circuit. This presence will have a lowering affect on 
foam formed in the waste discharge water and can even effectively 
influence both the ore extraction and the subsequent metal cyanide 
recovery by minimizing foam and air froth during these processes. 
The process of the invention is further illustrated by the following 
examples in which all parts are by weight unless otherwise indicated.

EXAMPLE I 
Ore Leaching Tests 
A Nevada gold and silver ore identified as McCoy Ore was reduced to 80% 
minus 3/4 inch and the head (oz. Au/ton ore) calculated for each of five 
samples. Each sample was then loaded into a 6 in. I.D..times.6 ft. high 
column for leaching. 
The surfactants and their concentrations in the aqueous sodium cyanide 
leach liquors applied to the five columns were as follows: 
P1--11.25 ppm of polyethylene glycol 600 dioleate available under the 
trademark Drewsperse 739 from Drew Chemical Corporation. 
P2--9.75 ppm of sodium diethylhexyl sulfosuccinate available under the 
trademark DrewFax 0007 from Drew Chemical Corporation. 
P4--10 ppm of polyoxyethylene (20) sorbitan trioleate available under the 
trademark Tween 85 from ICI Americas Inc. 
P5--30 ppm of polyethylene glycol 600 dioleate available under the 
trademark Drewsperse 739 from Drew Chemical Corporation. 
P6--blank 
Leaching was conducted by applying an aqueous sodium cyanide leaching 
solution (2 lbs. NaCN per ton of solution) over the ore charge in each 
column. The application rate, controlled by a calibrated chemical feed 
pump, was 0.005 gpm/ft.sup.2, of column cross-sectional area. The pH was 
maintained during leaching by the addition of lime to the dry ore during 
column loading. 
The aqueous cyanide leach solution was percolated through the ore charge in 
each column and collected in a pregnant solution reservoir daily. The 
volume of effluent solution was measured and a sample taken for precious 
metal analysis by conventional atomic adsorption methods. The pH and 
cyanide concentration of each effluent solution were determined. Fresh 
leach solution was prepared and applied to the ore charge daily. An equal 
amount of pregnant solution was saved daily from each column for the 
purpose of running carbon activity kinetic tests. 
Results of the leaching tests appear in the following Table I. 
TABLE I 
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Results P1 P2 P4 P5 P6 
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Au Extraction (%) 
1st Effluent 
54.5 61.2 59.0 66.2 58.7 
5 days 66.6 73.5 69.6 76.4 69.4 
20 days 78.6 84.6 83.4 86.6 79.5 
30 days 80.7 86.9 86.2 89.5 82.1 
35 days 80.7 87.3 86.5 90.4 82.5 
Au Extracted 
.051 .052 .051 .043 .047 
(oz/ton ore) 
Tail Assay .013 .014 .018 .010 .010 
(oz Au/ton ore) 
Calc. Head .064 .066 .069 .053 .057 
(oz Au/ton ore) 
Cyanide consumed 
.50 .47 .52 .49 .46 
(lb NaCN/ton ore) 
Lime added 3.50 3.30 3.30 3.50 3.50 
(lb/ton ore) 
Final sol. pH 
11.0 11.0 10.8 10.8 10.8 
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Following 35 days of leaching, the above data show that gold extractions 
varied from 90.4% for P5 to 80.7% for P1 as compared to 82.5% for P6 
(Blank). The failure to show comparable improved extraction for P1 appears 
to be the result of the low concentration of surfactant employed as 
compared, for instance, to P5 in which a greater amount of the same 
surfactant was employed with significantly improved results. The cyanide 
requirements ranged from 0.49 to 0.83 pounds NaCN per ton of ore compared 
with 0.46 pounds/ton for the blank. Cyanide consumption, therefore, was 
not significantly affected by the use of a surfactant. The lime 
requirements were low for all of the columns at 3.3-3.5 pounds CaO per ton 
of ore. 
Statistical testing was conducted in order to compare the means of each 
column with the blank column. The testing allows for the elimination of 
standard errors introduced in each column and also takes into 
consideration the effects of rate of recovery of gold in each column. Both 
the F-Test and the T-Test treatments measure the 95% confidence as to 
whether the means of two populations are significantly different. 
Statistical results appear in the following Table II. 
TABLE II 
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Statistical Results 
P1 P2 P4 P5 
______________________________________ 
Variance 41.8 38.5 52.5 33.9 
Mean 75.4 81.7 80.1 84.3 
Mean Variance 37.5 35.9 42.8 33.6 
(Compared to Blank) 
Variance of Means 
1.4 10.9 4.5 26.2 
(Compared to Blank) 
F 1.289 10.310 3.604 26.574 
T 1.135 3.548 1.428 5.155 
95% Significantly 
No Yes No Yes 
Different 
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The above data show that at the 95% confidence level, a one way analysis of 
variance shows that P2 and P5 are significantly better than the blank for 
both the rate and absolute extraction. 
EXAMPLE II 
Carbon Kinetics Tests 
Tests were conducted using 1 gram (Calgon GRC-22, 6.times.16 mesh) of 
coconut shell carbon in 1 liter of pregnant solution saved from each 
column for this purpose from the extraction tests of Example I. The tests 
were conducted by agitating the carbon in the pregnant solution for 24 
hours while the solutions were withdrawn at different time intervals and 
tested for gold and silver. The results from the 24 hr. testing appear in 
Table III. 
TABLE III 
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Vol. Preg. Sol. Barren Sol. 
Carbon Loading 
Col. (1) (ppm Au) (ppm Au) 
(oz Au/ton C) 
______________________________________ 
P1 .93 0.84 0.02 25.770.sup.(1) 
P2 .93 0.84 0.04 25.142.sup.(2) 
P4 .93 0.84 0.01 26.084.sup.(4) 
P5 .93 0.84 0.04 25.142.sup.(5) 
P6 .93 0.84 0.01 26.084.sup.(6) 
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Carbon Assay Checks (oz Au/ton C): (1) 25.091, (2) 25.335, (4) 25.513, (5 
23.356, (6) 25.778 
The above data show that the presence of the surfactants does not have a 
diminishing effect on the adsorption capacity of the coconut shell carbon.