GB1568362A - Heavy metal recovery in ferrous metal production processes - Google Patents

Description

(54) HEAVY METAL RECOVERY IN FERROUS METAL PRODUCTION PROCESSES (71) We, UNIVERSITY COLLEGE CARDIFF, of C.U.I.C. University College, P.O. Box 78, Cardiff CFl IXL, a British University, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- production of ferrous metals or alloys thereof in which iron-bearing flue dust, containing Zn and optionally other heavy metals, is generated in a furnace. Ironcontaining dusts produced by blast furnaces, arc furnaces and steel-making plant contain zinc and lead and vary in composition depending on the nature of the charge used and the proportion of scrap and upon the degree to which the zinc and lead levels have been allowed to build up in the furnace.Such dusts generally have a particle size of 95--5 microns, or finer.

According to current blast furnace practice, blast furnace dusts are in the first instance recycled through the blast furnace; zinc levels tend however to build up in the dust, and these high levels of zinc cause corrosion of the furnace linings. Therefore, after a certain period of operation those dusts which contain excessively high zinc levels cannot be recycled further without damage and they are disposed of by dumping, which causes environmental pollution.

In oxygen steel making processes, dusts also arise and these are not generally recycled, but are merely dumped; the zinc level in the dumped dust also represents an environmental pollution hazard, and depends on the quality of the scrap used.

Similarly, arc furnace flue dusts vary in composition depending on the furnace charge. Zinc levels in arc furnace flue dusts are in general higher than those of blast furnace dusts, and similar problems arise in dumping of the dusts.

The dust which has been dumped may be treated to recover the heavy metal values, either for commercial utilisation or to remove them as an environmental hazard.

Known methods for extracting these heavy metals include pyrometallurgical techniques and acid extraction. The disadvantages of pyrometallurgical techniques are the high capital expenditure on plant, the high running costs due to energy consumption and the air pollution resulting from the process. Acid extraction techniques have the disadvantage that acids which cannot economically be regenerated are consumed, and that waste materials which involve disposal costs are produced.

In addition, both pyrometallurgical processes and acid extractions have the disadvantage that only limited ranges of materials can be mixed and processed at one time.

Previous attempts have been made to use an alkaline extracting process for the extraction of heavy metals but these attempts have not resulted in a sufficiently high extraction efficiency, and have not been pursued. Such previous attempts utilised a very concentrated alkaline extraction which was not sufficiently selective and did not provide an economically viable percentage extraction.

An object of the present invention is therefore to provide a process for the extraction of Zn and optionally other heavy metals from materials containing them which provides sufficiently high extraction efficiency, involves relatively low capital and operating expenditure and permits a range of materials to be mixed and processed at one time. It is also an object of the invention to provide a cyclic process for the extraction of heavy metals by alkaline extraction in which the components can be recovered and reused.

In the process according to the invention the ability of alkali metal hydroxides, in particular sodium hydroxide to dissolve heavy metals and their compounds, in particular where the metals are zinc and lead to form, for example, sodium zincate and sodium plumbate in solution, is utilised to give an extraction process of high efficiency and selectivity.

According to this invention a process for the production of ferrous metal or alloys thereof in which iron bearing flue dust containing zinc and optionally other heavy metals is generated in a furnace, includes a process for the production of ferrous metals or alloys thereof in which iron-bearing flue dust containing Zn and optionally other heavy metals is generated in a furnace, and including an extraction stage which comprises the steps of: separating the ironbearing dust from the furnace flue gases; leaching the dust in an alkali metal hydroxide solution having a concentration greater than 5M, and a pulp density of 1:1.5 to 1:8, preferably 1:3 to 1:8, at a temperature in the range 800C to the boiling point of the solution, to dissolve Zn and other heavy metals present; separating the pregnant alkali metal hydroxide solution from the residue; washing the residue to remove the alkali metal hydroxide and returning the iron dust residue to the furnace; purifying and clarifying the pregnant alkali metal hydroxide solution; receiving the Zn and other heavy metals present to regnerate the alkali metal hydroxide solution; and recycling the alkali metal hydroxide solution for re-use in the leaching step.The pulp density is a measure of the solid content of the pulp and expressed either as a ratio (by weight) of the solid to liquid content or as a percentage of the total weight of pulp.

For optimum operating conditions, the alkali metal hydroxide concentration is 8 to 16M and preferably 10 to 14M.

The boiling point of the alkali metal hydroxide solution depends on the concentration, and with hydroxide concentrations at the higher end of the preferred range, the boiling point may be as high as 140"C. The operating temperature may be increased by working under pressure. Operating at high temperature is preferred, because the extraction tends to be more selective at higher temperatures, the rate of reaction is increased and the viscosity of hydroxide solution is reduced, making the materials easier to handle. Filtration of the pregnant alkali metal hydroxide solution is preferably effected at temperature in excess of 60"C.

The precise conditions required to give optimum results depend on the material being treated. Where impurities other than zinc are present in the pregnant alkali metal hydroxide solution, purification may be initiated before the non-ferrous metal is recovered. A preferred purification technique is by cementation with metallic zinc preferably in the form of dust.

Suitable techniques for heavy metal recovery from the sodium hydroxide solution include electrowinning; precipitation in the form of the carbonate with carbon dioxide or carbonic acid or precipitation in the form of the sulphide with hydrogen sulphide or sulphide ions.

In a preferred embodiment of the process according to the invention, the flue dust is a basic oxygen steelmaking (BOS) dust, a blast furnace dust or an arc furnace dust.

Embodiments of the invention will now be further described with reference to the following Examples and with reference to the accompanying drawings, of which: Figures 1 to 3 are graphs showing the extraction obtained under various conditions, in Example 1; Figures 4 to 6 are graphs showing the extraction obtained under various conditions, in Example 2; Figure 7 is a flow diagram of an extraction stage; and Figure 8 is a flow diagram of a steel production process including an extraction stage such as illustrated in Figure 7.

Example 1 A BOS dust of the following weight composition was obtained from British Steel Corporation: FeO 85% Mn 1.65% Fe2O3 0.4% S 0.14% SiO2 3.5% P 0.25% CaO 5.7% Zn 2.5% MgO 0.9% Pb 0.25% Al203 0.5% Cu 0.01% The material was a fine dark magnetic powder of specific gravity of 4.4, and was pyrophoric, confirming X-ray powder photographs indicating the iron to be present as FeO. The material was extremely fine, 73% being less than 45 micron.

Figure 1 shows the % zinc and % iron extraction versus log molar sodium hydroxide concentration, at a temperature of 1200C and a pulp density of 40%.

Figure 2 shows the % zinc extraction versus log pulp density, expressed in gilitre, at a temperature of 90"C and a sodium hydroxide concentration of lOM.

Figure 3 shows the % zinc extraction versus temperature, at a pulp density of 40% and a sodium hydroxide concentration of 10M.

From this data, it was decided that a sodium hydroxide solution concentration of 10M (Log molar conch=1.0) at a temperature of about 120"C (Log Temp=2) would result in satisfactory zinc extraction, resulting in an extraction per step of about 70% at a pulp density of 40%; the efficiency of a three-step extraction would therefore be 75%.

The percentages of zinc and iron were determined by analysis, using atomic absorption spectrophotometry.

Example 2 An arc furnace flue dust having the following composition was investigated.

Because the furnace was charged with low grade scrap, the sample had a high zinc content.

Fe 20% Ca 4.3 /n Zn 28.0 /n Cd 0.24% Cu 0.26% Si 4% Pb 2.5% The dust was a fine, light brown material formed under oxidising conditions indicating the presence of well oxidised products, such as ZnO and Fe2O3.

Figure 4 shows the /" zinc extraction versus % pulp density, at a temperature of 90"C and a sodium hydroxide solution concentration of 10M.

Figure 5 shows the /n zinc and % iron extraction versus log molar sodium hydroxide concentration at boiling temperature of the caustic soda solution and 20% pulp density.

The liquour was purified by concentration with zinc dust at a temperature of 49"C and the results of this are shown in Figure 6. Where 10 g Zn/litre of solution is used, the solution contains 1% of the original lead concentration after 25 minutes cementation.

The purified, zinc-rich solution is then extracted using an electrowinning process.

The results of this electrowinning, using a magnesium cathode and a nickel or nickel plate anode at a spacing of 1.5 cms are as follows: Current Current % Density gfl Efficiency (Ampsft-2) 3.7 - 4. 1 36 33.85 4.1-7.7 67 33.85 7.7-13 58 67.7 13-20 78 67.7 20 - 29 98 67.7 29-32.7 96 67.7 On a commercial scale, the probable power requirement, assuming varying cell efficiencies and current densities would be of the order of 3000 KWh/ton zinc won, neglecting power losses due to power equipment.

In a practical design, to recover 0.44 tons zinc per hour, nine cells of dimensions 4ftx4ftx 10ft using 3x3 cathodes, with 38 cathodes per cell and 40 anodes, will be required.

The residual dust, which was about half the weight of the original, had a zinc level of the order of 8 to 10% zinc, and an iron content of about 40%. About 80% of the original zinc was recovered.

The residual dust could be further purified by roasting with sodium carbonate at a temperature of preferably above 700"C, more preferably above 7600C. At higher temperatures, roasting times are reduced but energy requirements are higher.

Roasting of the caustic leach residues at 860"C for 50 minutes, followed by caustic leaching of the roasted product gave zinc extractions in excess of 90 gn of the residual values.

The product obtained therefore had a negligible zinc level, of less than 1%.

The extraction stage illustrated in Figure 7 includes an additional pre-leach step which is recommended for the removal of lead from oxidised dusts such as produced by arc furnaces. Dividing the figure is a dotdash line above which are those steps associated with the optional dilute preleach. Optimum lead removal is achieved at an alkali metal hydroxide, in this case NaOH, concentration of 1 to 2M the leach being effected at, say, 520C for half an hour, depending upon the particle size of the dust.

Following filtration the residue is washed in water and roasted in a reducing atmosphere produced for example by roasting with coal or other carbonaceous material, at from 800" to 1 1000C, the reduced dust then being passed to the main, concentrated leach such as described in Examples 1 and 2, possibly after mixing with fresh blast furnace dust.

Pregnant alkali metal hydroxide solution is passed from the pre-leach, for recaustisation with lime and subsequently, the extraction of Pb with some Zn, by cementation with metallic Zn. A cementation product of approximately 80% Pb, 20% Zn will be obtained. The purified alkali metal hydroxide solution is then concentrated by evaporation and combined with the pregnant solution from the concentrated leach for further cementation with metallic zinc. After cementation, Zn is extracted by electrowinning and the purified concentrated alkali metal hydroxide solution recycled to the concentrated leach.

The metallic zinc required for the cementation steps may be added in the form of any waste material containing metallic zinc, the process thereby affording the further facility of reclaiming Zn from waste material such as metal finishing waste.

The process according to the present invention preferably is carried out on a single site, that is to say, the zinc recovery plant is linked with the ferrous metal production plant in which the flue dusts are generated. For economic reasons, however, it may be necessary to build a zinc recovery plant at some central location accessible to a number of plants supplying iron-bearing flue dusts, the treated dust being recycled to one or all of the supply plants.

Figure 8 is a flow diagram of a steel production process incorporating an extraction stage such as illustrated in Figure 7. Blast furnace and converter dusts are separated from the flue gases, from which useful heat can be recovered, and fed to the zinc recovery plant, possibly via the blast furnace charge sintering furnace. A pelletising machine is provided to form the treated iron dust into pellets which are then recycled to the blast furnace.

The process described above not only avoids cleaning costs and the problem of disposal of the dust and hence pollution but, in addition, produces saleable by-products and enables the utilisation of otherwise wasted iron.

WHAT WE CLAIM IS: 1. A process for the production of ferrous metals or alloys thereof in which ironbearing flue dust containing Zn and optionally other heavy metals is generated in a furnace, and including an extraction stage which comprises the steps of: separating the iron-bearing dust from the furnace flue gases; leaching the dust in an alkali metal hydroxide solution having a concentration greater than SM, and a pulp density of 1:1.5 to 1:8 at a temperature in the range of 80"C to the boiling point of the solution, to dissolve Zn and other heavy metals present; separating the pregnant alkali metal hydroxide solution from the residue; washing the residue to remove the alkali metal hydroxide and returning the iron dust residue to the furnace; purifying and clarifying the pregnant alkali metal hydroxide solution: recovering the Zn and other heavy metals present to regenerate the alkali metal hydroxide solution; and recycling the alkali metal hydroxide solution for re-use in the leaching step.

2. A process according to claim 1 wherein the concentration of the alkali metal hydroxide solution in the main leaching step is from 8 to 16M.

3. A process according to claim 2 wherein the said concentration is from 10 to 14M.

4. A process according to any one of claims 1 to 3, in which separation of the pregnant alkali metal hydroxide solution from the residue is at least partly effected by filtration at a temperature in excess of 60"C.

5. A process according to any one of claims 1 to 4, wherein purification and clarification of the pregnant alkali metal hydroxide solution is effected by cementation with metallic Zn.

6. A process according td any one of claims 1 to 5, wherein before returning the iron dust residue to the furnace, the residue is roasted with sodium carbonate at a temperature above 700"C, and again leached in a caustic solution.

7. A process according to any one of claims I to 6, comprising pre-leaching the dust in a dilute alkali metal hydroxide solution.

8. A process according to claim 7 and comprising separating the pregnant alkali metal hydroxide solution produced by preleaching from the residue, and reducing the residue by roasting prior to the main leaching step.

9. A process according to claim 8 wherein the residue is roasted in a reducing atmosphere.

10. A process according to claim 8 or claim 9 wherein the residue is roasted with carbonaceous material at from 800 to 1100"C.

11. A process according to any one of claims 1 to 10 in which recovery of the Zn and/or other heavy metals to regenerate the solution, is by electrowinning.

12. A process according to any one of claims 1 to 11, wherein the iron dust residue is formed into pellets before being returned to the furnace.

13. A process for the production of ferrous metals or alloys in which ironbearing flue dust containing Zn and other heavy metals is generated in a furnace and including an extraction stage substantially as herein described with reference to the examples and the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. it may be necessary to build a zinc recovery plant at some central location accessible to a number of plants supplying iron-bearing flue dusts, the treated dust being recycled to one or all of the supply plants. Figure 8 is a flow diagram of a steel production process incorporating an extraction stage such as illustrated in Figure 7. Blast furnace and converter dusts are separated from the flue gases, from which useful heat can be recovered, and fed to the zinc recovery plant, possibly via the blast furnace charge sintering furnace. A pelletising machine is provided to form the treated iron dust into pellets which are then recycled to the blast furnace. The process described above not only avoids cleaning costs and the problem of disposal of the dust and hence pollution but, in addition, produces saleable by-products and enables the utilisation of otherwise wasted iron. WHAT WE CLAIM IS:

1. A process for the production of ferrous metals or alloys thereof in which ironbearing flue dust containing Zn and optionally other heavy metals is generated in a furnace, and including an extraction stage which comprises the steps of: separating the iron-bearing dust from the furnace flue gases; leaching the dust in an alkali metal hydroxide solution having a concentration greater than SM, and a pulp density of 1:1.5 to 1:8 at a temperature in the range of 80"C to the boiling point of the solution, to dissolve Zn and other heavy metals present; separating the pregnant alkali metal hydroxide solution from the residue; washing the residue to remove the alkali metal hydroxide and returning the iron dust residue to the furnace; purifying and clarifying the pregnant alkali metal hydroxide solution: recovering the Zn and other heavy metals present to regenerate the alkali metal hydroxide solution; and recycling the alkali metal hydroxide solution for re-use in the leaching step.

2. A process according to claim 1 wherein the concentration of the alkali metal hydroxide solution in the main leaching step is from 8 to 16M.

3. A process according to claim 2 wherein the said concentration is from 10 to 14M.

4. A process according to any one of claims 1 to 3, in which separation of the pregnant alkali metal hydroxide solution from the residue is at least partly effected by filtration at a temperature in excess of 60"C.

5. A process according to any one of claims 1 to 4, wherein purification and clarification of the pregnant alkali metal hydroxide solution is effected by cementation with metallic Zn.

6. A process according td any one of claims 1 to 5, wherein before returning the iron dust residue to the furnace, the residue is roasted with sodium carbonate at a temperature above 700"C, and again leached in a caustic solution.

7. A process according to any one of claims I to 6, comprising pre-leaching the dust in a dilute alkali metal hydroxide solution.

8. A process according to claim 7 and comprising separating the pregnant alkali metal hydroxide solution produced by preleaching from the residue, and reducing the residue by roasting prior to the main leaching step.

9. A process according to claim 8 wherein the residue is roasted in a reducing atmosphere.

10. A process according to claim 8 or claim 9 wherein the residue is roasted with carbonaceous material at from 800 to 1100"C.

11. A process according to any one of claims 1 to 10 in which recovery of the Zn and/or other heavy metals to regenerate the solution, is by electrowinning.

12. A process according to any one of claims 1 to 11, wherein the iron dust residue is formed into pellets before being returned to the furnace.

13. A process for the production of ferrous metals or alloys in which ironbearing flue dust containing Zn and other heavy metals is generated in a furnace and including an extraction stage substantially as herein described with reference to the examples and the accompanying drawings.