GB2088413A - Process for Autogenous Oxygen Smelting of Sulphide Materials Containing Base Metals - Google Patents

Abstract

A base metal sulphide concentrate, especially of nickel or copper, is mixed with roasted concentrate and the mixture is autogenously smelted, e.g. by flash smelting, with oxygen-containing gas and a flux for iron oxides, e.g. silica, to give a matte, a siliceous slag and a strong sulphur dioxide off-gas. The addition of the roasted concentrate facilities control and improvement of the matte grade.

Description

SPECIFICATION Process for Autogenous Oxygen Smelting of Sulphide Materials Containing Base Metals Autogenous oxidation smelting of base metal sulphide concentrates has become a useful process which has been adapted in many countries for the treatment of a variety of sulphide concentrates.

Metal sulphide concentrates which may be treated by this technique commonly contain either chalcopyrite or pentlandite as the main copper and nickel sulphide minerals respectively, with minor amounts of other sulphides such as covellite, bornite, chalcocite in the case of copper, and violarite cobaltite in the case of nickel, and also large amounts of iron sulphides such as pyrite and pyrrhotite.

Sulphide concentrates are normally produced by grinding the ore and then carrying out froth flotation to concentrate the valuable metals so that the sulphide concentrates are normally obtained as finely divided material.

In autogenous oxidation smelting the finely divided metal sulphide concentrate in admixture with a flux material for iron oxide, e.g., silica, is first dried to eliminate water and then is injected along with an oxygen containing gas which can be oxygen enriched air or commercial oxygen by means of a device such as a burner. Part of the iron and sulphur contents of the concentrate burn with the combustion being supported by oxygen in the gas injected. The mixture of concentrate plus oxygen or oxygen enriched air is injected into a refractory furnace in a manner such that the oxidation of the sulphide occurs in the freeboard space of the furnace and the molten products of the combustion fall into the hearth of the furnace. The valuable metals are collected in the matte phase. The oxidized iron is fluxed by the silica to form a slag which collects on top of the molten matte.As desired, the matte and slag can be tapped at intervals. The process affords a means for smelting large quantities of sulphides on a continuous basis with generation of an off gas which can be 80% or more sulphur dioxide when the oxidizing gas consists of 100% commercial oxygen. The rich off-gas lends itself readily to treatment for recovery of liquid sulphur dioxide or for manufacture of sulphuric acid thereby making the operational highly advantageous from an environmental aspect. Another advantage of the process resides in the fact that the fuel for the process is iron sulphide which itself is not particularly valuable.

It is found that with any particular autogenous oxidation smelting furnace, it is necessary to arrive at a thermal equilibrium which is dependent upon the proportion of the sulphide concentrate burned.

The heat generated by the combustion of the furnace feed, essentially of labile S and FeS to SO2 and iron oxides, equals the heat content of the smelting products (matte, slag and off-gas) plus the furnace heat losses. This means that, for a given sulphide material and a given furnace, a sufficient amount of oxygen per unit weight of sulphides must be supplied to satisfy the heat balance of the operation.

When this is done, the matte grade is fixed, and the amount of oxygen cannot be altered without producing either an excess or deficiency of heat. In other words, the furnace balance, all other things being equal, determines the matte grade or the overall degree of conversion of the sulphide materials into a final product. This rigid interdependence of heat balance and degree of conversion is an important limitation of these processes. The present invention is directed to a means for controlling matte grade in flash smelting at will.

There is a well established prior art in regard to autogenous oxidation smelting and the technique is used throughout the world. As examples Canadian patents Nos. 503,446 and 934,968 may be mentioned together with the book "The Winning of Nickel" by J. R. Boldt and P. Queneau, Longman's Canada, at pages 244 to 247 and various articles including the paper, "Oxygen Flash Smelting in a Converter" by M. C. Bell, J. A. Blanco, H. Davies and R. Sridhar, J. of Metals, Vol. 30, No. 10, pages 9 14, 1978: "Smelting Nickel Concentrates in Inco's Oxygen Flash Furnace", by M. Solar et al, 107th AIME Annual Meeting, Denver, Colorado, Feb. 25-March 2, 1 978, "The Kivcet Cyclone Smelting Process for Impure Copper Concentrates" by Melcher, E. Muller and H.Weigel, J. of Metals, July 1976, pages 4-8, etc.

It is to be appreciated that in the smelting of copper, as an example, the matte generated in the smelting furnace must be subjected to further treatment to provide blister copper which can in turn be transformed into high purity copper products. The smelting furnace matte grade controls the supplementary operations which must be performed downstream so as to arrive at blister copper.

Thus, the higher the grade of the smelting furnace copper matte, the less needs to be done in converters or other equipment so as to provide blister copper and the less difficult are the problems in meeting environmental standards in regard to the evolution of sulphur dioxide in such downstream operations. In some cases, it may be desirable for example to provide a matte from the smelting furnace having the composition of white metal, almost pure Cu2S.

The above mentioned interdependence of heat balance and degree of conversion of the concentrate in autogenous oxidation smelting, in particular oxygen flash smelting, makes it difficult to obtain the sometimes desired matte grades, specially when the concentrate has a low copper content and a high iron content.

A number of methods have been proposed for controlling the matte grade in oxygen flash smelting. Among these are: adding to the concentrate revert materials, such as dust, ground matte and slag skulls, etc.; water injection into the smelting unit; air dilution of the oxygen. All these alternatives consist of introducing a coolant into the smelting unit to use up the excess heat generated when a matte grade higher than that normally obtained in autogenous flash smelting is desired. They provide a way of achieving the same end result as the process of the present invention but they are not as attractive because higher oxygen additions are required and the processes become wastefull in energy utilization.

The invention is based on the discovery that in autogenous oxidation smelting the matte grade generated in the smelting furnace can be controlled by dividing the concentrate stream to be smelted such that a portion of the stream is subjected to at least partial or even dead roasting, is then cooled and mixed with additional fresh sulphide concentrate before being fed to the smelting furnace along the flux in the usual manner. This techique permits an upgrading in the matte grade produced, and is particularly applicable to oxygen flash smelting.

It will be appreciated by those skilled in the art that process metallurgists involved in the milling and smelting of metal sulphide deposits will control the mill and smelter to provide the most efficient process which can be devised for treating the product of a particular ore body or available combinations of ore bodies. Despite the ingenuity of metallurgists involved in the recovery of the valuable minerals from ores the concentrate which is produced in the mill will vary greatly depending upon the nature of the ore. Thus, valuable copper minerals such as chalcopyrite, chalcocite, etc. usually occur in ore bodies wherein large quantities of iron sulphides which can be pyrite, pyrrhotite, etc. also can occur. In addition, certain copper sulphide minerals also include iron, as an example, chalcopyrite.

A similar situation occurs with nickel sulphide and other base metal sulphide minerals.

For example, if the ratio of iron sulphide to copper sulphide in the concentrate is high, the material will normally yield a low matte grade on autogenous oxygen smelting. In this case, the objective of the present invention is to adjust the ratio of iron sulphide to copper sulphide in the smelting furnace feed so as to obtain the desired matte grade. This is achieved by partial or dead roast of a portion of the concentrate. Similar considerations apply to nickel sulphide or other base metal sulphide concentrates.

It will be appreciated that the roasting step which forms part of the invention may be accomplished in equipment such as a fluid bed roaster. When this is done, a gas containing at least 10% of sulphur dioxide is produced which may be employed as feed for a sulphuric acid plant. In this way sulphur removed from the portion of concentrate which is roasted can be recovered and is not discharged to the atmosphere. Roasting in the fluid bed can be accomplished using air as the oxidant.

The blend of roasted and dry unroasted concentrate, mixed with silicious flux, is injected into the smelting furnace in a stream of oxygen. The desired composition of matte to be obtained can be controlled by adjusting the ratio of calcine to green sulphide material in the feed. For a given concentrate, heat balance calculations will dictate the relative proportions of calcine and green sulphide material which have to be fed to yield the desired product on autogenous smelting.

The process of the present invention makes it possible to autogenously smelt copper concentrates of any composition directly to white metal (Cu2S) or blister copper. In a similar manner, a low iron (~1 %Fe) matte can be produced directly from nickel concentrates. Since a richer matte grade is achieved, in respect of the metal value being recovered, less converting is required downstream of the flash smelter again with benefits in terms of reduced fugitive emissions of sulphur dioxide.

This invention provides advantages with respect to alternative methods for controlling the matte grade by adding coolants (reverts, scrap, water, etc.) to the smelting furnace. Less oxygen is required in the flash furnace since the fuel value of the concentrate is lowered to the required level by oxidation of a portion of its iron and sulphur content prior to the flash smelting operations. As a consequence there is an increase in the specific capacity of the furnace and less dusting due to the lower volume of gases produced.

Direct production of very high-grade copper mattes, i.e. mattes over 60% copper, in the smelting unit will result in furnace slags which will require treatment for base metal recovery before being discarded. In the case of oxygen flash smelting of copper concentrates, the slag cleaning can be accomplished by a number of known processes such as treating the slag in a separate electric furnace as described by Brick et al in the article "Flash Smelting of Copper Concentrate", J. of Metals, vol.

10(6), 1958, pp. 39500; in a separate flash furnace with lower matte grade as described in US 503,446; or by slow-cooling as described by Subramanian and Themelis in J. of Metals, vol. 24(4), 1972, pp. 33-38. The low grade matte or concentrate obtained from the slag cleaning operation may be recycled to the primary smelting unit. In the case of nickel, the slags from the primary smelting furnace can be cleaned in an electric furnace as described in "The latest development in nickel flash smelting at the Harjavalta Smelter" by T. Niemela and S. Harkki, Joint Meeting MMIJ-AIME, 1972, Tokyo. Because nickel concentrates usually contain a significant amount of cobalt, which will report mainly in the slag of the primary smelting unit, the electric furnace slag cleaning operation will yield a secondary matte enriched in cobalt which can be processed separately by conventional methods to recover this metal as well as the nickel and other metal values.

Some examples will now be given: Example I A chalcopyrite type of copper concentrate analyzing (wt.%): 29.7 Cu, 1.0 Ni, 30.7 Fe, 35.2 S was roasted with air at 8000C to yield a calcine with the following composition (wt.%): 35.0 Cu, 1.2 Ni, 37.8 Fe, 0.8 S. The Cu and the Fe in the calcine were mainly as CuFe204. Minor amounts of CuO and Fe203 were also present. Blends of this calcine and green concentrate were oxygen flash smelted in a miniplant flash furnace with sufficient oxygen to simulate a commercial autogenous operation. The amount of oxygen required for this purpose was calculated from heat and mass balances which predicted the matte grades which would be obtained in the commercial furnace at the various experimental calcine/green concentrate ratios.The blends of calcine and green concentrate were fed to the miniplant furnace at a rate of 8-9 kg/h. The flashing space temperature was about 14000 C. The following table summarizes the results: Table % Calcine A dded by weight of Matte Grade, % (Cu+Ní)* Slag Composition {%) Green ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Concentrate Expected** Obtained SiO2 Fe Cu 0 40.0 42.4 30.7 37.0 0.7 5.5 48.5 50.8 30.6 40.8 0.67 11.1 58.5 58.6 35.0 34.7 1.19 22.5 77.0 75.0 33.5 34.9 4.77 * %Ni in mattes: 1.5 **Predicted from heat and mass balance calculations for an autogenous operation.

The slags were fluid in all the above tests. Excellent separation of mattes from slags was observed.

Example II One part of a nickel concentrate calcine analyzing (wt.%): 10.0 Ni, 2.9 Cu, 41.7 Fe, 6.8 S was mixed with four parts of a green concentrate analyzing (wit.%): 1 5.1 Ni, 1.9 Cu, 38.5 Fe, 32.0 S. The blend was oxygen flash smelted in a miniplant flash furnace at a rate of 8 kg/h and at a temperature of 1 4000C with an oxygen addition of 36.6% by weight of the calcine plus green concentrate blend. The matte obtained analyzed 55% Ni. The iron-silica slag was fluid and separated well from the matte. The nickel content of the slag was 2.8%. The results of this test demonstrated that oxygen flash smelting of nickel calcine-green nickel concentrate blends is technically feasible.

Claims (1)

1. Claims

   1. A process in which a metal sulphide concentrate is combusted autogenously with an oxygencontaining gas, characterised by roasting a portion of the metal sulphide concentrate to be smelted, cooling the roasted material, blending the portion of roasted material with further green metal sulphide concentrate, and autogenously smelting the resulting blend of roasted and green material with an oxygen-containing gas in a bounded space and in the presence of a flux for iron oxides to produce a molten matte, a molten silicious slag and a strong sulphur dioxide off-gas.

   2'. A process according to claim 1 in which the autogenous combustion is effected by flash smelting. ~~~~~~~~~~~~~~~~~~~~~