Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
2001-12-21
2003-11-04
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C075S744000, C423S027000, C423S029000
Reexamination Certificate
active
06641642
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable
STATEMENT REGARDING FEDERALLY FUNDED RESEARCH
Not applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the treatment of ores and ore concentrates to recover metal values, and in particular relates to the pressure oxidation treatment of sulfide ores and ore concentrates to enable the recovery of precious metal values including silver.
2. Description of Related Art
Silver is a valuable precious metal and can be found in precious metal ores such as acanthite (Ag
2
S). In addition, precious metals such as silver and gold are also found associated with other sulfide-containing ores.
There are many hydrometallurgical processes available for the treatment of silver-bearing sulfide ores to recover non-ferrous metal values (e.g., copper) as well as any gold that may be associated with the ore. However, the silver can be difficult to recover in an economically feasible manner using these processes.
Hydrometallurgical processes are generally preferred over methods such as smelting due to the environmental issues associated with smelting sulfide ores. Pressure oxidation is one known hydrometallurgical process for recovering metals from sulfide-containing ores and ore concentrates. During pressure oxidation, a slurry including the ore is subjected to elevated pressure and temperature while in contact with oxygen to decompose the minerals. The sulfide components of the ore are at least partially oxidized, liberating metals. The metals can then be recovered from the solids and/or the solution of the discharge slurry.
U.S. Pat. No. 5,698,170 by King discloses a method for the pressure oxidation of a copper-containing material followed by solvent extraction and electrowinning (SX/EW) to recover copper. The pressure oxidation step produces a high acid content solution, which is diluted after the pressure oxidation step and prior to recovery of the copper in a SX/EW circuit.
One of the problems associated with pressure oxidation of sulfide ores that also include iron is the formation of jarosite compounds. In particular, certain metals that can be found in the ore, including silver, preferentially form jarosite compounds during pressure oxidation. When the silver is associated with a jarosite compound, the silver is difficult to recover in an economical manner.
The article entitled “Pressure Oxidation of Silver-Bearing Sulfide Flotation Concentrates” by Thompson et al., (published in Mining Engineering, September 1993, pp. 1195-2000) discloses the pressure oxidation of sulfide flotation concentrates at a temperature of 160° C. to 225° C. It is disclosed that most of the silver in the autoclaved solids is associated with jarosites that are formed by hydrolysis of ferric sulfate. The silver associated with these jarosites is extremely refractory to cyanide leach treatment resulting in silver extractions of less than 5 percent. In order to recover higher levels of silver, the jarosites must be decomposed at an elevated temperature in the presence of lime (CaO), a process commonly referred to as a “lime boil.” However, a lime boil uses excessive quantities of lime, often in excess of 400 lbs. per ton of autoclaved solids, and adds significantly to the cost associated with recovering the silver.
U.S. Pat. No. 5,096,486 by Anderson et al. discloses a process for extracting silver from silver sulfide bearing solids by leaching a metal bearing mineral with an aqueous liquid including sulfuric acid and sodium nitrite. The silver is solubilized and is recovered from pressure oxidation discharge solution by precipitating silver chloride. However, sodium nitrite forms nitric acid and the associated off-gases are extremely harmful, if discharged, to the environment. It is also disclosed that maintaining 115 g/l or more of sulfuric acid in the aqueous mixture of sulfuric and sodium nitrite will prevent the formation of argentojarosite and plumbojarosite.
It would be useful to provide a method for treating silver-bearing sulfide ore and/or sulfide ore concentrates by pressure oxidation such that the silver is not combined in substantial quantities with refractory minerals such as jarosite and such that the silver is amenable to extraction from the solids portion using conventional cyanide leach methods without the need for a jarosite destruction step.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to the pressure oxidation of a mineral feed that includes at least iron, sulfide sulfur and silver wherein the pressure oxidation conditions are controlled to reduce the formation of jarosite mineral species in the solids portion of the discharge slurry.
During pressure oxidation of sulfide minerals according to the prior art, particularly those including iron, substantial quantities of jarosite compounds are typically formed and discharged from the pressure oxidation reactor in the solids portion of the discharge slurry. Equations 1 and 2 are representative of the reactions that are believed to normally occur in the formation of jarosite from pyrite during pressure oxidation.
4FeS
2
+15O
2
+5H
2
O→Fe
2
(SO
4
)
3
+Fe
2
O
3
+5H
2
SO
4
(1)
3Fe
2
(SO
4
)
3
+14H
2
O→2(H
3
O)Fe
3
(SO
4
)
2
(OH)
6
+5H
2
SO
4
(2)
Various metals and functional groups found in the mineral feed can substitute for the hydronium (H
3
O) group in the jarosite, including potassium (K), sodium (Na), rubidium (Rb), silver (Ag), thailium (TI), ammonium (NH
4
), lead (Pb) and mercury (Hg). When silver-containing jarosite species form, silver metal is very difficult to recover using conventional leaching methods without first subjecting the solids to a jarosite destruction step such as a lime boil.
In accordance with the present invention, the formation of jarosite species can be substantially inhibited by careful control over the pressure oxidation conditions. One way to control the pressure oxidation conditions is through the addition of a sulfate-binding material to the pressure oxidation step. The reactions that are believed to occur during the pressure oxidation step according to this embodiment of the present invention, when using calcium in the form of calcium carbonate as the sulfate-binding material, are illustrated by Equations 3, 4 and 5.
4FeS
2
+15O
2
+8H
2
O→2Fe
2
O
3
+8H
2
SO
4
(3)
CaCO
3
+H
2
SO
4
+H
2
O→CaSO
4
.2H
2
O+CO
2
(4)
CaSO
4
.2H
2
O→CaSO
4
+2H
2
O (5)
As is illustrated by Equation 4, the added calcium from the calcium carbonate preferentially binds sulfate by forming calcium sulfate and inhibits the formation of other sulfate species, such as jarosites and iron sulfate. The iron is converted to insoluble hematite (Fe
2
O
3
) and therefore the amount of iron solubilized in the discharge liquid is also reduced. The silver, which under typical pressure oxidation conditions would be associated with jarosite, is precipitated as elemental silver, silver sulfide and/or silver inclusions in hematite, all of which are now recoverable in a standard leaching step without the need for a lime boil or similar jarosite destruction step.
When calcium is used as the sulfate-binding material in the form of a calcium compound such as calcium carbonate, most of the calcium crystallizes to form crystalline anhydrite (CaSO
4
) in the discharge solids, which is more amenable to thickening and/or filtration than gypsum (CaSO
4
.2H
2
O). The conversion of most of the iron to hematite in the solids portion of the discharge slurry also simplifies filtration and other downstream processing steps that may be used.
Thus, according to one embodiment of the present invention, a method for processing a mineral feed comprising iron, sulfide sulfur and silver to facilitate recovery of silver is provided. The method includes the steps of: pressure oxidizing an aqueous feed slurry that includes the mineral feed wherein at least about 70 percent of sulfide sulfur in the mineral feed is converted
Gathje John C.
Simmons Gary L.
Andrews Melvyn
Marsh & Fischmann & Breyfogle LLP
Newmont USA Limited
LandOfFree
High temperature pressure oxidation of ores and ore... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High temperature pressure oxidation of ores and ore..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High temperature pressure oxidation of ores and ore... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3164322