Process for improving the extraction yield of silver and gold in refractory ores

A process for the extraction of silver and gold from refractory ore is disclosed. The process includes first grinding the refractory ore. This ground refractory ore is subsequently treated with an alkali solution of a stoichiometric excess amount of sulphide in relation to the amount of gold and silver contained in the refractory ore, to thus form a pulp containing a gold and silver sulfide. The pulp and alkaline solution ore are aerated to oxidize the excess sulfide to sulfate and then a mixture of the pulp and cyanide liquor are formed. Finally this mixture is aerated to obtain a final gold and silver product.

The present invention relates to the extraction of silver and gold, with 
high yields, from oxidized pyrite ores, stripping ores or Gossan ores, 
argentojarosite ores and particularly those from which silver and gold are 
obtained with low extraction yields by cyaniding or ores containing 
slightly soluble silver or refractory ores. 
The current process for obtaining silver and gold from these ores consists 
of mining the mineral and preparing for crushing and grinding stages until 
a stage of 75% by weight with a particle size below 40 microns is reached. 
The ground ore is previously conditioned with lime, followed by leaching 
with a dilute sodium cyanide solution. Thereafter this pulp is subjected 
to a solid-liquid separation and the leaching liquors containing the 
silver and gold are cemented with a more electronegative metal such as 
zinc. The gold and silver powder is melted, and a gilded metal ingot or 
bullion is cast. 
The present extraction yield with this process is only about 40% for silver 
and 85% for gold, that is the already leached or treated ore or gossan 
residue or gossan tailings as it is known, still holds 60% of the silver 
contained in the fresh or gossan headings. 
The process according to the present invention allowing high percentages of 
silver and even to increase the percentage of gold to be extracted, is 
conducted as follows: the ore or the already treated ore tailings from 
which it is wanted to extract the residual silver still contained therein 
are contacted, as a stage prior to cyaniding, with an alkaline solution 
(NaOH, KOH or lime), of sodium sulphide at room temperature. There are 
several ways of contacting this alkaline sulphide solution with the ore in 
any degree of division, either in a reactor or in a thickener or by 
leaching in piles, etc. 
Further to sodium sulphide, other soluble or slightly soluble sulphides 
such as potassium sulphide, calcium sulphide, ammonium sulphide or any 
other organic or inorganic soluble sulphide compatible with an alkaline 
solution or any chemical providing sulphide (S.sup.2-) ions or 
hydrosulphide (SH.sup.-) ions to the solution may be used as sulphurizing 
agents. 
The silver, a large proportion of which appears to be as argentojarosite in 
these Gossan ores, is sensitive to the action of the sulphide so as to 
allow larger amounts to be extracted in the subsequent cyaniding process 
than those obtained without this prior sulphide treatment. 
It is believed that the reaction occurring is as follows, the gold and 
silver bearing ore being designated by (M). 
EQU 2M--Ag+S.sup.2- alkaline medium 2M.sup.- +Ag.sub.2 S 
This silver sulphide would be formed by displacement, since it is much 
more insoluble than the silver compound in the mineral. After the prior 
sulphide treatment, the Gossan pulp is transferred to a conditioning tank 
in which the excess sodium sulphide is oxidized to sulphate in an alkaline 
medium by blowing currents of air through it. This sulphate does not 
interfere with the subsequent cyanide operation and therefore avoids 
additional cyanide consumption. This excess sulphide in the pulp may also 
be burned or oxidized with any cheap oxidant which subsequently does not 
interfere with the cyanides. Once the excess sulphide has been removed, 
the ore pulp is fed to the known cyaniding stage in which the silver 
sulphide formed is dissolved with the aid of air and the cyanide, etc. 
Since the present invention represents an extra leaching step between the 
grinding and the cyaniding, stages in view of the layout of the existing 
plants, it may be easily incorporated into the established processes. 
Since the cyaniding iquors may be recycled, at least partly, after the 
cementation, the exhausted alkali in the preparation of the sulphide 
solutions may also be recycled, at least to a certain extent with the 
consequent alkali saving. 
In a similar way, the cyaniding may be replaced by a treatment with sodium 
chloride or calcium chloride lyes having a small amount of dissolved 
chlorine. The extraction yields are slightly higher than those attained by 
cyaniding. 
To facilitate the complete description of the invention, the following 
examples are given which are in no way limitative but only explicative of 
the process described.

EXAMPLE I 
One kilogram of gossan ore ground to particle sizes at least below 90 
microns was taken as a base. A typical composition of this gossan was: 2.7 
g gold per tonne and 50 g silver per tonne. 0.33 liters of a twice (2) 
molar solution in sodium hydroxide or 80 g of NaOH per liter also having 
0.25 moles of sodium sulphide, as Na.sub.2 S per liter or 20 g of sodium 
sulphide, as Na.sub.2 S per liter were added to this kilogram of gossan. 
In this way the 0.33 liters added to the gossan contained 26.7 g of NaOH 
and 6.67 g of Na.sub.2 S in solution at room temperature. 
The gossan and the solution were contacted for about six (6) days without 
stirring. At the end of these six days, the pulp was diluted to 50% solids 
or a similar level since this aspect is not critical and was aerated until 
the excess sulphide was oxidized with the air current. Thereafter the 
process followed the normal steps of cyaniding in the current plants, that 
is, a few grams of sodium cyanide were added to this sulphurized pulp and 
the air was blown through for 24 hours as usual, but now the extraction 
yields obtained by using the prior leaching with sulphurized alkaline 
solutions were: Gold extraction yield, 96%. Silver extraction yield, 76%. 
The gold and silver contents remaining in the gossan residues after 
treatment by the process described herein were: 0.1 g of gold per tonne of 
gossan tailings and 12 g of silver per tonne of gossan tailings, there 
having been recovered therefore 2.6 g gold and 38 g of silver per tonne of 
gossan treated. 
EXAMPLE II 
One kilogram of ground gossan ore having a richness of 2.1 gr of gold per 
tonne and 63 gr of silver per tonne was taken as a base. 0.66 liters of 
lime saturated water containing also 10 gr of sodium sulphide, as Na.sub.2 
S per liter were added to this kilogram. In this way the 0.66 liters added 
to the gossan contained 6.67 gr Na.sub.2 S in solution at room 
temperature. 
The solution and gossan were contacted for one day with moderate stirring 
(from 100 to 200 rpm). At the end of this day, the pulp was diluted to 50% 
solids and air was bubbled through it until the excess sulphide was 
oxidized. Thereafter the process followed the normal cyaniding channels. 
The gold extraction yield was 90% and the silver extraction yield was 64%. 
Having sufficiently described the nature of the invention, as well as the 
way of reducing it to practice, it should be stated that the above 
arrangements are liable to modifications of detail so as not to depart 
from the spirit and scope of the present invention.