Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
2001-11-29
2004-03-02
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S010620
Reexamination Certificate
active
06699302
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the treatment of metal sulphide concentrates.
PRIOR ART
Roasting for Sulphur Removal
Oxidative roasting of pyrite (FeS
2
) is a standard way of producing sulphuric acid. Roasting is used on an industrial scale, e.g. for the production of zinc, copper, and nickel, even tin, molybdenum, and antimony and, in many cases, takes place in conjunction with one or more leaching or smelting operations. Sulphide roasting is used to oxidize some (or all) of the sulphur. The resulting SO
2
is treated further, most commonly producing sulphuric acid. Other options for recovery of sulphur include the production of elemental sulphur, or liquid SO
2
.
Modem roasting processes usually use fluidized-bed reactors, which are energy-efficient, and have a high productivity because of their favourable kinetic reaction conditions. The SO
2
content in the off-gas is typically 8 to 15% by volume.
For pyrometallurgical processing, the usual purpose of roasting is to decrease the sulphur content to an optimum level for smelting to a matte. Partial (oxidizing) roasting is accomplished by controlling the access of air to the concentrate; a predetermined amount of sulphur is removed and, for example in the recovery of copper, only part of the iron sulphide is oxidized, leaving the copper sulphide (for example) relatively unchanged. Total, or dead, roasting involves the complete oxidation of all sulphides, usually for a subsequent reduction process.
There are many modem pyrometallurgical processes in which roasting is not a separate step, but is combined with matte smelting. Flash furnaces employ sulphide concentrate burners that both oxidize and melt the feed, and are used extensively in the copper industry. Autogenous bath smelting is another alternative. Here a lance blows air or oxygen, together with concentrates and reductant, into a molten bath, and the energy released by the oxidation of the sulphur provides much of the required energy for the smelting process.
The roasting process has several effects:
a) Drying the concentrates
b) Oxidizing a part of the iron present
c) Decreasing the sulphur content by oxidation
d) Partially removing volatile impurities, for example arsenic
e) Preheating the calcined feed with added fluxes (for example, silica or limestone), in order to lower the energy requirement of the downstream process
Environmental concerns have highlighted the need to lower the emissions of sulphur from smelters treating sulphidic raw materials. These emissions emanate primarily from the furnaces and converters, either as fugitive emissions or as process gases vented up a stack. It should be noted that the typical 1 to 2% SO
2
in the off-gas from reverberatory furnaces (for example) is too low for effective acid production.
The general trend in recent years has been to eliminate as much as possible of the iron sulphides (usually pyrrhotite) during the milling and flotation stages, in order to minimize the sulphur input to smelters.
Dead roasting, i.e. close to 100% sulphur removal, has the benefit of removing essentially all the sulphur at the beginning of a smelting process. Furthermore, in comparison with the intermittent nature of SO
2
produced in a converting operation, a steady and almost optimum SO
2
content of off-gas from a roaster requires a smaller and less expensive acid plant.
Copper
Various roasting techniques in the recovery of copper are described in the literature.
1-16
Copper—Brixlegg
In the Brixlegg process, copper was produced by electric smelting of dead-roasted chalcopyrite concentrate in a circular AC (alternating current) submerged-arc furnace, using coal as a reductant.
Brixlegg reports a 95% recovery of copper to blister, and levels of copper in the slag of less than 1% have been claimed. The crude copper averaged only 95% copper, and the operation has been discontinued
1
. Disadvantages of this process are the relatively high copper losses in slag, and the high electrical energy consumption.
An undesirable aspect of the Brixlegg process is the fact that lead passes into the final copper anodes and makes them fragile if the concentration is too high. On the other hand, the exceptionally high recovery of other metals related to copper makes the process of particular interest for treating ores which contain nickel and noble metals. (The nickel can be separated from the anode mud.)
A submerged-arc furnace has been used for treating dead-roasted calcine in a process developed by the US Bureau of Mines
14
, as was also used in the Brixlegg process. It was found that in order to produce a high-purity blister (2.2% total impurities) and low-copper-content slag in a submerged-arc furnace, a two-cycle procedure was required. Using this rather inconvenient and non-continuous procedure, recoveries as high as 98% were attained.
Nickel
In the nickel industry, Falconbridge
17-24
and Inco
25-29
have worked on processes involving the smelting of roasted sulphide concentrates. These processes use six-in-line furnaces, commonly employed in that industry, which generally operate at temperatures around 1400° C. The reduction reactions needed to provide the appropriate conditions for recovering metals from the oxides tend to raise the operating temperature of these furnaces. Consequently, large volumes of air are drawn into the furnace to cool the freeboard space of the furnace. This tends to result in high losses of the feed materials as dust. Dust losses of up to 25% of the feed have been mentioned
20
.
Nickel production has however been accompanied by a level of SO
2
generation which is environmentally unacceptable. It has been recognised that a major means to reduce SO
2
emissions is to increase the degree of sulphur elimination in the fluidized-bed roasters. However, the existing furnace technology is limited in the degree to which highly roasted concentrates can be handled. The higher degree of roast demands more strongly reducing conditions in the furnace to smelt more oxidized calcine feed, and to counteract slag losses. Higher coke addition rates are needed. Extra energy is generated by the additional coke combustion products, resulting in a higher temperature in the furnace freeboard. This requires greater amounts of cooling air to control the temperature. The furnace off-gas handling system capacity would have to be expanded to handle the greater quantities of gas. Also, the more metallized matte melts at higher temperatures, demanding superheated slags to control matte temperatures and bottom build-up. Refractory erosion in the slag zone with higher temperature slags must be controlled by cooling the refractory with copper coolers.
About 25% of the calcine escapes the six-in-line furnace; as much as possible of this is recycled back to the furnace
20
.
Inco's roast-reduction smelting process
25-29
involves deep roasting of nickel concentrate in fluidized-bed roasters. The roaster off-gas is treated in a sulphuric acid plant. The low-sulphur calcine is reduction smelted with coke in an electric furnace to yield a sulphur-deficient matte. This sulphur-deficient matte is converted to Bessemer matte in Peirce-Smith converters, with minimal evolution of sulphur dioxide (because of its sulphur-deficient nature), and the converter slag is returned to the electric furnace. Excellent recoveries of nickel were obtained, and the process was developed up to commercial-scale testing at the Thompson smelter during 1981 to 1982. Flash smelting of bulk copper-nickel concentrates was considered superior at Inco's Copper Cliff smelter, but it was seen that in other circumstances the roast-reduction process could be an attractive option.
Sulphur is eliminated from the concentrate mainly in the roasters, running at 830 to 850° C. The high temperatures promoted high oxygen efficiency, of approximately 95%. Slurry feeding permitted excellent control of the air to concentrate ratio in the roaster, and good control of sulphur elimination (approximately 80%). The process resulted in higher furnace temperatures, as well as highe
Barcza Nicholas A.
Hannaford Anthony
Jones Rodney T
Kaiura Gary
O'Connell Gary
Andrews Melvyn
Brinks Hofer Gilson & Lione
Mintek
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