Metal recovery process

The invention relates to a process for the recovery of a valuable metal(s) from its ore(s), that includes a method for removal of environmentally hazardous, volatile material prior to the removal of the metal(s) from its ore pulp or liquor carrier. The process comprises forming a pulp of particles of the ore(s) and leaching from that pulp the valuable metal component via the formation of soluble metal complexes. As elements of the leaching chemicals used are often volatile at least the liquor component of the metal containing pulp is contacted by flow of gas to remove any volatile content. In the case of the leaching chemicals volatile elements the recycling of the volatile element to reform the original leaching chemical is a substantial advantage. The metal is removed from the pulp, or its liquor component, by methods known in the art such as adsorption onto carbon or resin, electrowinning, zinc cementation, precipitation solvent extraction or the like. The metal is then recovered in its solid form.

The invention comprises a process for the recovery of valuable metals from 
ores thereof. 
Metals such as gold or silver may be recovered from ores by various 
chemical processes. Nickel, cobalt, copper, manganese and magnesium are 
examples of other valuable metals that may also be obtained by chemical 
recovery processes from their ores. 
In carbon-in-pulp type processes including the carbon-in-leach process as 
are used to recover gold or silver from their ores, the raw ores are 
finely ground and a slurry of the ground ore referred to as a pulp is 
formed. A chemical such as an alkali metal cyanide, for example NaCN, is 
added to the pulp to leach the gold from the ground ore by forming cyanide 
complexes of the gold. To prevent excessive volatilization of the cyanide 
from the pulp as hydrogen cyanide during leaching, the process is carried 
out under alkaline conditions at a pH of around 10.5. These alkaline 
conditions are typically achieved by the addition of lime to the pulp. The 
gold cyanide complexes are then recovered from what is referred to as the 
pregnant pulp by adding granular activated carbon, or other adsorption 
media such as a suitable adsorption resin, to the pulp or by passing the 
pregnant pulp through a column of activated carbon or resin, so that the 
gold complexes are adsorbed onto the carbon or resin adsorption media. In 
carbon-in-leach processes, which may be considered as a variation of 
carbon-in-pulp processes, some adsorption is carried out towards the end 
of leaching, by passing the carbon through the last one or more of a 
series of leach vessels for example. In either case the carbon granules 
are coarser than the particles of the pregnant pulp, and the carbon 
particles having adsorbed thereon the gold complexes may then be removed 
from the pulp by screening. The gold is then recovered from the carbon by 
elution and recovery of the gold from the resulting liquor. What is 
referred to as the barren pulp, comprising the balance of the pulp after 
removal of the gold complexes, is ejected to tailings ponds or the like. 
In carbon-in-pulp processes of this type, to ensure that all the gold or 
other valuable metal is recovered from the ground ore, excess leaching 
chemical, such as the NaCN, is employed. The barren pulp disposed of to 
tailings ponds thus contains the majority of the cyanide, which has not 
complexed with gold and other metals from the ground ore, and which is 
environmentally undesirable. Similar leaching processes to recover nickel 
and cobalt such as the Sherritt Gordon process, and processes to recover 
copper utilize excess ammonia to complex with the metal and similar 
environmental, economic and technical concerns can arise. These concerns 
also arise with other metal recovery processes using a volatile leaching 
chemical, such as sulphur dioxide used for the recovery of manganese and 
magnesium, and chlorine used for the recovery of base metals, copper, lead 
and zinc. 
In processes other than carbon-in-pulp type processes used to recover gold 
or silver or other valuable metals from their ores, leaching of a similar 
slurry of ground ore pulp is used to remove the metal(s) from the ore, and 
then the liquor component of the pulp containing the complexes is 
separated from the balance of the pulp comprising the solids. The barren 
solids are disposed of to tailings, while the valuable metal(s) is then 
recovered from the pregnant liquor by techniques such as precipitation, 
solvent extraction, zinc cementation or electrowinning, wherein the 
metal(s) is recovered from a solution of its salts by passing an electric 
current through the solution. Again, to ensure that all the gold or other 
valuable metals are recovered, excess leaching chemical is used, so that 
wastage of leaching chemical may occur. 
The invention provides an improved or at least alternative process for 
recovery of such valuable metals from their ores wherein either the level 
of environmentally undesirable elements of the leaching chemicals used in 
the leaching process present in the tailings, in for example a 
carbon-in-pulp type process, or the use of excess leaching chemicals, in 
for example solids-liquids separation type processes, and preferably both 
is minimize or substantially reduced. 
The process of the invention is applicable to the recovery of valuable 
metals such as gold, silver, cobalt, nickel, copper, lead, zinc, manganese 
or magnesium by leaching from their ores. The term `ore(s)` as used in 
this specification is to be understood as including all sources from which 
metals having commercial value may be extracted. 
In broad terms the invention may be said to comprise a process for 
recovering a valuable metal or metals from an ore or ores thereof 
comprising: 
forming a pulp of particles of the ore(s) including at least one leaching 
chemical which will react with the valuable metal(s) to remove same from 
the ore(s), 
contacting at least the liquor component of the pulp containing the 
valuable metal(s) with a flow of gas for removal into the gas of any of 
the leaching chemical(s) volatile residual elements and/or other volatile 
products of the leaching process, prior to recovery of any major portion 
of the valuable metal(s) from the pulp, and 
recovering the metal(s). 
In many leaching processes which use volatile residual elements of the 
leaching chemical(s) which have not reacted in the leaching process to 
form complexes with the valuable metals tend to be volatile under typical 
process conditions. In the process of the invention these volatiles are 
removed prior to removal of the valuable metal complexes, or at least any 
major portion thereof. Any volatile products of the leaching process may 
also similarly be removed. 
In carbon-in-pulp type processes, including carbon-in-leach processes, 
residual volatile elements of the leach chemical(s), such as the cyanide, 
may be removed or stripped from the pregnant pulp by passing a flow of air 
through the pulp prior to recovery of the valuable metals from the pulp by 
adsorption onto carbon or resin adsorption media. The cyanide content in 
the barren pulp disposed of as tailings after recovery of the valuable 
metal(s) will be substantially reduced. 
In processes where the liquor component of the pulp containing the valuable 
metal(s) is separated from the solids in a solid-liquid separation stage 
before recovery of the metal(s) from the liquor, the residual volatile 
elements of the leaching chemical(s) may be removed or stripped from the 
pregnant liquor by passing a flow of air through the pregnant liquor prior 
to recovery of the metal(s) by precipitation, solvent extraction, zinc 
cementation or the like. 
In the recovery of gold from ores with NaCN as the leaching chemical, for 
example, in either case the residual cyanide in the pulp or liquor after 
leaching may be removed or stripped as HCN, with the process of the 
invention. 
The cyanide or other volatiles such as HCN thus removed in either case may 
be recycled for reuse in the leaching process, to reduce consumption of 
the leaching chemical(s). For example, the HCN recovered from the pregnant 
pulp or liquor from gold or silver recovery with NaCN may be reconverted 
to NaCN and added back into the leaching pulp. 
It is most preferred that both leaching and stripping stages of the process 
of the invention are carried out in closed reaction vessels, and that any 
of the leaching chemical's volatile elements that volatilize during 
leaching are recovered for recycling. This enables the process to be 
carried out at reduced pH's, of for example 8 to 10, since it is not 
necessary to maintain the pulp pH at a higher level during leaching to 
prevent loss of the volatile component(s) of the leaching chemical(s). 
This then enables a reduction in the use of pH modifying chemical 
reagents. However, the process of the invention with air stripping of the 
leaching chemical(s) residual volatile elements may alternatively be 
carried out at pH's in excess of 10. Removal of residual leaching 
chemical's volatile element(s), or leaching process volatile products, by 
air stripping may also be carried out at such a higher pH, although the 
efficiency will be lowered, or alternatively the pH of the pulp after 
leaching and prior to stripping may be reduced to ensure optimum stripping 
of undesirable components. It has also been found that carrying out 
leaching and stripping at such lower pH levels leads to less fouling of 
the carbon or other adsorption media in carbon-in-pulp type processes 
where the metal(s) is recovered by adsorption, so that less frequent 
cleansing and reactivation of the carbon is required. It has also been 
found that carrying out leaching and stripping at such low pH levels leads 
to greater recovery of cyanide. 
In accordance with the invention removal or stripping of residual elements 
of the leaching chemical(s) from the pulp or clear liquor is carried out 
prior to recovery of at least the major portion of the valuable metal(s) 
from the pulp or liquor. It is intended to include within the scope of the 
invention the possibility of adsorption to recover a minor part of the 
valuable metal(s) before the air stripping stage. In the case of 
carbon-in-leach processes for example, stripping would not be carried out 
until leaching is complete or substantially complete, but carbon 
adsorption may begin towards the end of leaching and prior to stripping. 
For example, leaching may be carried out in a series of leach tanks 
followed by stripping followed by passage of the stripped, pregnant pulp 
to a series of adsorption tanks. Some adsorption before the completion of 
leach may be effected by addition of carbon to the last few of the leach 
tanks, to adsorb a minor portion of the valuable metal complexes, with 
removal of the carbon prior to passage of the pulp to stripping, and 
removal of the remaining major portion of the metal(s) in the subsequent 
adsorption tanks. 
The process of the invention will be further described with reference to 
its application in the recovery of gold from ores thereof employing an 
alkali metal cyanide particularly NaCN as the leaching chemical, by way of 
example, but it will be apparent that the process of the invention is 
equally applicable to leaching processes for the recovery of silver using 
NaCN or the like, cobalt, nickel, or copper using ammonia and so forth as 
referred to previously.

In both processes shown in FIGS. 1 and 2, raw ore from which the gold is to 
be recovered is first finely ground with water in a mill. The grinding 
media may be made of an alloy of iron or non-ferrous material or an 
artificial ceramic material. The ore is ground to an average particle size 
typically 80% finer than 50 microns in the presence of lime to raise the 
pH of the slurry to the level to be achieved, as is known in the art. 
The slurry of ground ore known as the pulp is then passed into leaching 
vessels such as a series of covered tanks having internal agitation, to 
which is added the NaCN leaching chemical. Leaching may be conducted at 
higher pH's such as 10 or above as in conventional processes but 
alternatively leaching may be carried out at a lower pH in the range 8 to 
10 so that less lime to raise the pulp to the desired pH need be added. 
Air is passed through the pulp in the leaching vessels. An arrangement of 
fans or blowers and ducts carries away any volatile HCN from the pulp 
during leaching. The cyanide thus recovered during leaching is reconverted 
to NaCN for re-use in the leaching process by the HCN adsorber as will be 
described. As the cyanide leaching chemical is recycled, excess leaching 
chemical may thus be used, even at the lower pH's that may be employed 
with the process of the invention, to maximize the leaching of the gold 
from the ore. 
The pregnant pulp containing the leached ore contains among other things 
the complex cyanides of gold and residual cyanide leaching chemical which 
has not reacted in the leaching process, as free cyanide, which is removed 
by the process of the invention before recovery of the gold. In the 
carbon-in-pulp type process of FIG. 1 the free cyanide is then removed 
from the pregnant pulp in its gaseous form as HCN by contacting the pulp 
with a flow of air to remove the residual cyanide into air i.e. air 
stripping. This may be achieved by passing air through the pulp as the 
continuous phase, or by passing the pulp through the air as the continuous 
phase, either cocurrently or countercurrently. Particular air stripping 
arrangements that may be mentioned include tanks and columns with bubble 
diffusers, and packed tower, grid tower, and spray tower arrangements. 
Alternative to air another stripping gas may be employed. Air removal of 
volatiles in most processes will be quite adequate but it is possible that 
other gases may be more effective in some situations. 
Sufficient stripping of volatiles may be achieved at higher pH's of 10 or 
above but preferably the pH of the pregnant pulp is first reduced prior to 
stripping, if necessary, to around pH 8 or even to as low as pH 2 if there 
are many complex cyanides such as those of copper, zinc, and iron to be 
decomposed. Reduction of the pH may be achieved by the addition of a 
suitable acid, such as sulphuric acid. 
As is most preferred, the cyanide which has been air stripped from the 
pregnant pulp is recycled back into the leaching stage. For example the 
HCN gas with stripping air and that from the leach tanks may be 
reconverted to NaCN in the liquid phase by passing the air stripped gas 
into a highly alkali solution of NaOH at a pH of at least 12. 
After removal of the residual cyanide the pregnant pulp is then contacted 
with carbon or a resin adsorption media for removal of the gold cyanide 
complexes. In the process shown, granular activated carbon is then added 
to the adsorption tanks to adsorb the gold cyanide complexes. 
Alternatively carbon column techniques where the carbon is contained in 
columns through which the pulp flows may be employed, for example. After 
adsorption the pulp containing the carbon particles passes to screening 
wherein the coarser carbon adsorption media is separated from the balance 
of the pulp. For example the carbon may be of an average particle size 
2000 microns and be separated by a screening system of aperture size 600 
microns. The gold is then recovered from the carbon by elution as is known 
in the art and the carbon subjected to regeneration for reuse. 
The barren pulp resulting may be passed through a safety screen and then to 
tailings ponds. The tailings contain a reduced level of environmentally 
undesirable cyanide. 
In the process of FIG. 2 wherein the liquor component of the pulp 
containing the valuable metals is separated from the solids before 
stripping, the pregnant pulp passes after leaching to a solid-liquid 
separation stage. Separation of the pregnant liquor from the solid pulp 
may be achieved by thickening, filtration and washing. The balance of the 
pulp separated from the pregnant liquor, referred to as barren solids, may 
be disposed of as tailings whilst the pregnant liquor is then subjected to 
stripping by a flow of air to remove the residual cyanide into air. Any 
suitable air stripping technique as referred to previously may be 
employed. Again, whilst sufficient stripping of volatiles may be achieved 
at higher pH's, preferably the pH of the pregnant liquor is reduced to 
around pH 8 or even to as low as pH 2, which may be achieved by the 
addition of a suitable acid such as sulphuric acid to the liquor before 
stripping. 
As is again most preferred, the cyanide which has been air stripped from 
the pregnant liquor is recycled back into the leaching stage. The HCN gas 
in the air from the leach tanks and the stripping air may be reconverted 
to NaCN in the liquid phase by passing the air into a highly alkali 
solution of NaOH at a pH of greater than 12. 
After removal of the residual cyanide the metals are then recovered from 
the pregnant liquor. Zinc cementation wherein zinc, typically as a dust, 
may be employed to precipitate the gold out of the pregnant liquor, but 
precipitation and solvent extraction processes may be used too. 
In the FIG. 1 flow chart shown, in the carbon-in-pulp process the pulp is 
subjected to leaching and the pregnant pulp is then subjected to air 
stripping followed by adsorption. In accordance with the invention the 
major portion of adsorption of the gold from the pulp is carried out after 
stripping but it is possible for carbon to be added to the last few of the 
leaching tanks to adsorb a minor portion of the gold and the carbon then 
removed before the pregnant pulp is subjected to air stripping, followed 
by passage of the pregnant pulp to the adsorption tanks for adsorption of 
the remaining major portion of the gold. 
The following examples further illustrate the invention: 
EXAMPLES 
Ore samples for the test runs were supplied by Waihi Gold Mining Co Ltd 
from Coromandel Peninsula, New Zealand. The two bulk samples of ore were 
crushed separately in a hammer mill and roll crusher and reduced to a 
grain size of less than 1 mm. The crushed material was blended by coning 
and quartering and then divided into representative samples by riffling. 
The one sample was used for Examples 1, 2 and 3, and the other samples for 
Example 4. 
EXAMPLE 1 
A 2 kg charge of crushed ore was placed in a 15 l ceramic ball mill and 2 
kg of a water and sufficient lime added to raise the pH to 8.9. The sample 
was then ground to 85% passing 73 microns. A further 2 kg of water was 
then added to transfer the ground pulp to a 20 l open necked glass jar. 
The pulp density in the jar was 33% solids. 
2120 mg of cyanide as 4000 mg of sodium cyanide was added and the jar 
rolled slowly for 30 hours. 
The pregnant pulp was then stripped of its residual cyanide in a column 1 
meter high by 150 mm in diameter. Sulphuric acid was added to reduce the 
pH to approximately 8.0 and compressed air passed through the column using 
fans from the bottom via a diffuser at a constant flow rate of 100 
l/minute for 10 minutes per kilogram of pulp. To recover the cyanide, the 
cyanide laden air was then ducted to a second column containing 5 l of 
0.04 N caustic soda solution. 
The stripped pregnant pulp was then placed back in the open necked glass 
jar and 6 g/kg of conditioned activated carbon with an average extended 
length of 3500 microns, added. The jar was then rolled for 8 hours. The 
carbon was then screened from the pulp, via screens with an aperture size 
of 500 microns, washed with a minimum amount of water and air dried. 
The cyanide recovered in the air-stripping process was 1680 mg. Cyanide 
remaining in the barren pulp for disposal as tailings was therefore 440 
mg. 
The gold and silver recovered from the ore was 7.12 mg, from a 
theoretically available 7.76 mg, and 11.6 mg from a theoretically 
available 23.6 mg respectively. 
EXAMPLE 2 
The process of Example 1 was repeated with the lime added to the crushed 
ore being sufficient to raise the pH to 9.5. Sulphuric acid was again used 
to lower the pH to approximately 8.0 for air-stripping. 
In this case the cyanide recovered by air stripping was 1730 mg. Cyanide 
remaining in the barren pulp for disposal as tailings was therefore 390 
mg. 
The gold and silver recovered was 7.16 mg, from a theoretically available 
7.50 mg, and 9.14 mg, from a theoretically available 23.5 mg, 
respectively. 
EXAMPLE 3 
The process of Example 1 was repeated with the water and lime mixture added 
to the crushed ore being sufficient to raise the pH to 10.4. Sulphuric 
acid was again used to lower the pH to approximately 8.0 for 
air-stripping. 
The cyanide recovered was 1730 mg. The cyanide remaining in the barren pulp 
for disposal as tailings was therefore 390 mg. 
The gold and silver recovered was 6.76 mg, from a theoretically available 
7.08 mg, and 14.5 mg, from a theoretically available 25.2 mg, 
respectively. 
EXAMPLE 4 
The process of Example 1 was again used with a 0.6 kg charge of a different 
crushed sample and 0.6 kg of water and sufficient lime added to raise the 
pH to 9.5, ground in a 5 l ceramic ball mill. Following grinding, the pulp 
was transferred with 0.6 kg of water to a 2.51 open necked, glass jar. 
640 mg of cyanide as 1200 mg of sodium cyanide was then added. 
Sulphuric acid was again used to lower the pH to approximately 8.0 for 
air-stripping. 
The cyanide recovered was 530 mg. The cyanide remaining in the barren pulp 
for disposal as tailings was therefore 110 mg. 
The gold and silver recovered was 0.95 mg from a theoretically available 
1.02 mg, and 2.33 mg, from a theoretically available 4.0 mg, respectively. 
The foregoing describes the invention including preferred forms and 
specific examples thereof. The application of the process with various 
alterations and modifications will be obvious to those skilled in the art 
and is intended to be incorporated within the scope hereof as defined in 
the following claims.