IE44838B1 - Waelz process for volatilizing zinc and lead from materials which contain iron oxide - Google Patents

Abstract

Cyclic process for volatilization of zinc and lead from materials containing oxide, in an inclined rotary tubular furnace, in which the material, together with solid carbon-containing reducing agent, is fed to the rotary tubular furnace at its upper end, the reduction and volatilization take place below the softening point of the load, the material containing metallic iron is discharged through the lower end of the rotary tubular furnace.

Description

This'invention relates to a Waelz process for volatilizing zinc and lead from iron-containing and oxide-containing materials in an inclined rotary kiln.

The Waelz process is used in the processing of 5. raw materials which contain volatilizable substances or metal compounds. It is particularly used for the · reduction and volatilization of zinc and lead from precursors having a relatively low metal content, such as ores,· intermediate products, lead furnace slag, and tetort residue. The process is conducted in a reactor constituted by an inclined rotary kiln, which is continuously operated and using a reducing agent consisting normally of coke breeze, which is fed together with the charge. Gaseous oxygen required for combustion in the form of air is sucked into the rotary kiln through the end from which the solids are discharged (discharge end) and the exhaust gases leave the rotary kiln through the end at which the solids are charged (charging end).

Two reactions ^re carried out in the charge in depend20 ence upon each other, namely, the reduction of e.g. zinc oxide by means of carbon monoxide according to the reaction:, . ZnO+CO*Zn+CO2 . and th^ re^cti^n of the resulting carbon dioxide in 4S -3accordance with the Boudouard reaction CO2+C-+2C0 to form new carbon monoxide for the reduction. Some of the resulting carbon monoxide and some of the zinc which becomes available above its boiling point, enter the free kiln space above the charge and are burnt there.

The resulting zinc oxide dust is collected from the exhaust gas.

Iron oxides contained in the raw materials are reduced and the resulting iron is contained mainly in metallic form in the material·discharged from the kiln.· Contrary to the process in which the charge has a pasty consistency in the final zone of the rotary kiln and the metallic iron forms lumps, the metallic iron produced by the present Waelz process is finely dispersed in the material which is discharged and if this material has a sufficiently high iron content, it can be used in the production of crude iron and of steel.

Coke breeze and anthracites can be used as reducing agents in the Waelz process whereas low-temperature coke or charcoal is not suitable, and all Waelz process plants Use coke or anthracite as the reducing agent unless sufficient carbon is contained in the retort residue which is charged. In Metall und Erz, 41st Year, 1944, No. 3/4 page 29, it is considered that only coke is suitable as an added reducing agent and that ordinary fine coal and coke from brown coal result in a high temperature rise and in a poorer yield of zinc. Die Metallverfluchtigungsverfahren mit besonderer Berucksichtigung der Herstellung von Zinkoxid, published by Wilhelm Knapp, Halle-on Saale, 1935 page 125, describes the use of coke or anthracite as a reducing agent and indicates that plants which are required only to volatilize metal compounds but need not reduce them, such as the plants used to volatilize -4lead sulphide7 (PbS) from high-silica ores, may use the dust and broken pieces from brown coal briquettes.

It is thus apparent that a carbonaceous material having a relatively low reactivity is used when it is desired to reduce and volatilise zinc and lead in the Waelz process. For this reason a sufficiently high reaction rate will not be achieved unless the minimum temperature in the reducing zone is about 1100°C.. On the other hand, the maximum temperature must generally not exceed 1130° to 1200°C., depending upon the melting point of the gangue and of the ash derived from the reducing agent. As a result,the favourable temperature range is very small and there is a danger of accretion.

It is an object of the invention to increase the reaction rate in the Waelz process carried out in a rotary kiln and to reduce the danger of accretion.

According to the present invention there is provided a Waelz process for volatilizing zinc and lead from an iron-containing and/or oxide-containing mat20 erial in an inclined rotary kiln, wherein the rotary kiln is fed at its upper end with the material together with a carbonaceous reducing agent, reduction and volatilization being effected below the softening point . of the charge, and a material which contains metallic iron being discharged from the lower end of the rotary kiln, and gaseous oxygen required for the combustion is sucked into the lower end of the rotary kiln, while exhaust gases are sucked from the upper end, and wherein the solid carbonaceous reducing agent charged into the upper end of the rotary kiln consists of a mixture of a carbonaceous material having a high reactivity index of above 3 and a carbonaceous material having a low reactivity index of below 1, the carbonaceous material of low reactivity index consisting of anthracite and/or 4483s -5coke breeze and amounting to 20 to 80% of the mixed reducing agent, based on fixed carbon.

The tendency of carbon contained in the various carbonaceous reducing agents to be gasified by reaction with carbon dioxide is defined by its reactivity index.

A coal is described as having a high reactivity index if its carbon reacts with carbon dioxide in accordance with the Boudouard reaction CO2+C+2 CO at 1000°C. in almost theoretical quantities. In practice, the reactivity index is determined by passing carbon dioxide gas at 1000°C over coal which has previously been devolatilized at 1000°C. The rate at which carbon is removed by its reaction with the carbon dioxide to form carbon monoxide is measured and the-reactivity index is then defined as CO (in cm. ) C (in grams) x~°C. x sec.

The reactivity index is, e.g. below 1 for anthracite, between 1 and 2 for long flaming gas coal, and below 0.5 for coke.

The carbonaceous reducing agents of high reactivity which are used in carrying out the present process have reactivity indices above 3 and preferably above 5. Such reducihg abends are e.g. brown coal or lignite. The reducing agents of low reactivity index which are used in carrying out the present process have reactivity indices below 1.0. Such reducing agents include coke breeze and anthracite. Gaseous oxygen, preferably in the form of air, is drawn into the open lower end of the rotarykiln by suction, the suction being conveniently produced by an exhaust gas blower, which follows a purifier ffer the exhaust gas. Air is supplied at the required rate under the control of the vacuum which is applied. -6The proportion of the carbonaceous material of low reactivity index in the mixed reducing agents is preferably 40 to 60%,based on fixed carbon. Particularly favourable conditions are obtained in this range with respect to the throughput rate, the volatilization, the reduction of iron, the reducing agent requirement, and the prevention of a reoxidation of metallic iron, in conjunction with a small carbon surplus in the material which is discharged.

At least 60% of the carbonaceous material of low reactivity index which is charged preferably has a particle size above 1 mm. and below 20 mm. In such a case, the operating conditions are maintained in favourable ranges and reoxidation of metallic iron is prevented.

The carbonaceous material of high reactivity index which is charged preferably has a particle size Sip to 50 mm., more preferably 5 to 15 mm., and its content of particles having a size below 1 mm. is desirably as low as possible. This will promote the throughput (rate and reduce the danger of secretion.

The maximum temperature in the rotary kiln is desirably from 900° to 1050°C., preferably 950° to 1OOO°C. This will greatly decrease accretion in spite of a high throughput rate. in carrying out the present process materials with a high-iron content, such as calcined pyrites and'waste materials resulting from processes-of ferrous metallurgy, may be charged to the kiln in such proportions that the iron content of the charge exceeds 30%. The material which is discharged in this case can well be used in blast furnaces and materials which are not normally used in the Waelz process can thus be processed in an ecologically satisfactory manner. *4838 -7An advantage of the present process is that the combined use of reducing agents of high and low reactivityindices enables a higher throughput rate to be achieved and reduces the danger of accretion, while sintering of the charge and reoxidation of the resulting metallic iron can be substantially prevented whereas the material discharged from· the rotary kiln contains a relatively low surplus of carbon. Even if more heat is supplied from the kiln gases, the temperature in the charge does not increase in the reducing zone until the removal of oxygen from the oxidic constituents has been substantially terminated. In this region virtually all the heat which is supplied by the kiln gases and the kiln wall is consumed for the hj.gh.ly endothermic Boudouard reaction within the charge so that no heat is available for the charge.

In this region the curve representing the temperature variation in the charge along the kiln is virtually horizontal and, toward the discharge end, does not begin to rise until .the removal of oxygen has been substantially terminated. The use of reducing agents which differ in reactivity indices results in a decrease in temperature because the reducing agents of high reactivity index tend to lower the temperature whereas the remaining reducing agents of low reactivity index ensure that there will be no depletion of carbon in the final zone.

Claims (10)

1. A Waelz process for volatilizing zinc and lead from an iron-containing and/or oxide-containing material In an inclined rotary kiln, wherein the rotary, kiln is fed at its upper end with the material together with a carbonaceous reducing agent, reduction and volatilization being effected below the softening point of the charge, and a material which contains metallic iron being discharged from the lower end of the rotary kiln, and gaseous oxygen required for the combustion is sucked into the lower end of the rotary kiln, while exhaust gases are sucked from the upper end, and wherein the solid carbonaceous reducing agent charged Into the upper end of the rotary kiln consists of a mixture of a carbonaceous material having a high reactivity index of above 3 and a carbonaceous material having a low reactivity index of below 1, the carbonaceous material of low reactivity index consisting of anthracite and/or coke breeze and amounting to 20 to 80% of the mixed reducing agent, based on fixed carhon.

2. A process 'as claimed in Claim 1, wherein the proportion of the carbonaceous material of low reactivity index in the mixed reducing agents amounts to 40 to 50% based on fixed carbon.

3. A process as claimed in claim 1 or 2, wherein at least 60% of the carbonaceous material of low reactivity index whioh is charged has a particle size above 1 mm. and below 20 mm.

4. A process as claimed in any one of claims 1 to 3, wherein the carbonaceous material of high reactivity index which is charged has a particle size up to 50 mm. and its content of particles having a size below 1 mm is as low as possible.

5. A process as claimed in claim 4, wherein the carbonaceous material Of high reactivity index has a particle size of 5 to 15 mm. 44S38 -9ί

6. A process as claimed in any one of claims 1 to 5, wherein the maximum temperature in the rotary kiln is from 900° to 1O5O°C.

7. A process as claimed in any one of claims 1 to 6, 5 wherein the maximum temperature in the rotary kiln is from 950° to 1000°C.

8. A process as claimed in any one of claims 1 to 7 wherein a material having a high iron content is charged in such a proportion that the iron content of the charge 10 exceeds 30%. /

9. A process as claimed in claim 8, wherein the material of high iron content is calcined pyrites or a waste material resulting from a process of ferrous metallurgy.

10. A Waelz process in accordance with claifn 1 sub15 stantially as hereinbefore described.