Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
1998-03-04
2001-06-26
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C423S026000, C423S027000, C209S164000
Reexamination Certificate
active
06251163
ABSTRACT:
FIELD OF THE INVENTION
The process of the present invention is directed generally to the recovery of precious metals from refractory carbonaceous concentrates and specifically to the recovery of precious metals from refractory carbonaceous concentrates by pressure oxidation techniques.
BACKGROUND OF THE INVENTION
A significant quantity of gold is contained in refractory ores, which contain substantial amounts of sulfide minerals such as arsenopyrite, marcasite, and pyrite. Such sulfide minerals can encapsulate significant amounts of the gold. Pressure oxidation is a common technique to unlock the encapsulated gold. In pressure oxidation, the ore or concentrate is formed into an aqueous slurry and treated in an autoclave at elevated temperature and (oxygen) pressure to oxidize the sulfur to form sulfuric acid and render the gold soluble in a cyanide lixiviant.
During subsequent cyanide leaching of the oxidized ore, preg robbing can occur. Preg robbing occurs in carbonaceous ores when natural carbon in the ore captures the liberated gold once it has been dissolved into the aqueous phase of the slurry using cyanide. Blanking agents, such as kerosene and sodium lauryl sulfate, have been used with only limited success to prevent gold adsorption onto the fine natural carbon in the ore or concentrate.
In designing an effective process for recovering gold and other precious metals from refractory, carbonaceous ores, there are a number of considerations. First, gold and precious metal recovery should be as high as possible. As little gold and precious metals as possible should be lost through sulfide encapsulation or preg robbing. Second, pressure oxidation should be autogenous if possible. In this manner, expensive heat exchangers or heaters are not required to add external heat to the autoclave feed slurry during pressure oxidation. Third, the autoclave should have as small a size as possible for a selected autoclave feed slurry capacity to minimize capital and operating costs. Finally, the oxidized autoclave feed slurry, after pressure oxidation, should be cooled as economically as possible before leaching.
SUMMARY OF THE INVENTION
These and other design objectives are realized by the process of the present invention. The process includes the steps of:
(a) floating a first portion of a feed material including a precious metal to form a concentrate fraction and a tails fraction;
(b) combining at least a portion of the concentrate fraction with a second portion of the feed material to form a combined feed material;
(c) subjecting the combined feed material to pressure oxidation in an autoclave to produce an oxidized feed material comprising the precious metal constituents of the feed material;
(d) leaching the precious metal from the oxidized feed material to produce a pregnant leach solution containing dissolved precious metal and a waste material; and
(e) recovering the precious metal from the pregnant leach solution.
The feed material can be any refractory and/or carbonaceous precious metal-containing ore. The ore can contain a variety of precious metals, including gold and silver, and mixtures thereof with gold being most preferred.
The flotation of the first portion of the feed material in step (a) permits the autoclave in the pressure oxidation step to have a smaller capacity, particularly for relatively low grade ores. The use of a smaller autoclave results in a significant savings in capital costs.
Preferably, the pressure oxidation in step (c) is autogenous (i.e., self heating) so that there is no requirement that external heat be supplied to the combined feed material. The avoidance of heat transfer to the material provides further reductions in capital and operating costs by eliminating the need for heat exchangers or heaters, such as slurry preheat towers to preheat the feed material.
The autogenous operation of the autoclave is made possible by the combining step (b) in which the concentrate fraction (which typically has a sulfide sulfur content of greater than about 6 percent by weight) is combined with the second portion of the feed material (which typically has not been floated and therefore has a lower sulfide content than the concentrate fraction, i.e., the typical sulfide sulfur content ranges from about 0.5 to about 4% by weight). Blending of the materials is performed to provide a combined feed material preferably having a total sulfide sulfur content ranging from about 4 to about 10 percent by weight. Commonly, the volumetric ratio of the concentrate fraction to the second portion in the combining step ranges from about 1:2 to about 10:1.
In some applications, the second portion is a high grade ore and the first portion is a low grade ore. The division of the feed material into the two portions can be performed by known techniques, such as selective mining. In this manner, the recovery from the high grade ore is substantially maximized, i.e., there is no loss of gold in the flotation tailings fraction.
In pressure oxidation, the temperature preferably ranges from about 180 to about 240° C. while the pH of the slurried feed material in the autoclave preferably ranges from about pH 0.5 to about pH 2.0. Under these conditions, up to about 98% of the sulfide sulfur in the feed material may be oxidized if so required to liberate the gold from the sulfides and maximize subsequent gold recovery.
In the leaching step (d), the lixiviant is preferably a thiosulfate salt that is mixed with an aqueous slurry containing the oxidized feed material. Preferably, the concentration of the thiosulfate salt in the slurry ranges from about 5 to about 100 g/l. A catalyst, such as copper, can be included in the lixivant to promote solubilization of the precious metal as a thiosulfate complex.
To neutralize any carbonates in the feed material, the process can further include the step of contacting the feed material with an acid before pressure oxidation. The acid can wholly or in part be sulfuric acid generated during pressure oxidation.
To lower the temperature and raise the pH of the slurried oxidized feed material after pressure oxidation, the process can include the step of contacting the slurry with a diluent (e.g., water) to decrease the temperature of the slurry. The volumetric ratio of the slurry to the diluent preferably ranges from about 1:2 to about 1:15.
To recover the precious metal from the pregnant leach solution, resin-in-leach techniques are preferred. Carbon-in-leach and carbon-in-pulp techniques are not effective in adsorbing gold thiosulfate complexes from aqueous slurries. Recovery of gold as a precipitate using cementation with metals such as copper is also an alternative to using resin-in-pulp techniques for gold recovery.
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Hydrometallurgy;
King Roy
McGuthry-Banks Tima
Placer Dome Inc.
Sheridan & Ross P.C.
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