GB1600022A

GB1600022A - Recovery of heavy metals from flue dusts

Info

Publication number: GB1600022A
Application number: GB31564/77A
Authority: GB
Original assignee: Cardiff University College; Cardiff University
Current assignee: Cardiff University
Priority date: 1978-05-30
Filing date: 1978-05-30
Publication date: 1981-10-14

Description

(54) THE RECOVERY OF HEAVY METALS FROM FLUE DUSTS (71) We, UNIVERSITY COLLEGE CARDII:F, a British University, of C.U.I.C., University College, P.O. Box 78, Cardiff, CFl 1XL, 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: This invention concerns the recovery of heavy metals from flue dusts to which copending British Patent Specification No.

1,568,362 relates.

According to claim 1 of the Specification a process for the recovery of zinc and other heavy metals from iron bearing flue dusts generated in a furnace comprises the steps of separating the ironbearing dust from the furance flue gases; leaching the dust in an alkali metal hydroxide solution having a concentration greater than SM, a pulp density of 1:1.5 to 1:8 and a temperature in the range 80"C to the boiling point of the solution, to dissolve Zn and other heavy metals; 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 to regenerate the alkali metal hydroxide solution; and recycling the alkali metal hydroxide solution for re-use in the leaching step.

In many large scale iron and steelmaking operations the hot off gases are subjected to both wet and dry cleaning processes and the dust feedstock to a recovery plant will include dust from both sources.

What we now propose is a dust recovery or extraction process according to claim 1 of British Patent Specification No. 1,568,362, wherein magnetic materials are removed from the dust feedstock by magnetic separation, prior to leaching and preferably, in a magnetic field having an intensity of up of 12000 Gauss.

The feedstock is generally in the form of slurries having solids contents typically in the range 0.1 to 25% by weight. Since the dewatering of these slurries is essential before processing, the incorporation of magnetic separation into the system offers several processing advantages.

The arisings from an integrated steelworks can consist of dusts and slurries from blast furnaces, basic oxygen steelmaking, electric are furnaces, cast house dusts, mill scale and other arisings. The total arisings can be over 100,000 tpa (tonslannum), of which the most highly contaminated proportion from the arc furnances, is of the order of 10%. In this type of feedstock the zinc and lead level is typically 1-3%, the soluble proportion of which has to be removed to be suitable for ground disposal or recycling.

The requirement for the recovery plant is to prevent the pollution that ground disposal of unprocessed material would create by recycling the best available iron oxide to the steel plant, and remove leachable non-ferrous materials to produce an environmentally acceptable inert waste for dumping of non-recycled material.

Since the quantities of solid available in the slurry form predominate, direct processing would lead to the introduction into the leaching operation of unacceptable quantities of water, even after filtration. Alternatively a drying operation would be required. The former would be expensive in terms of the chemical losses necessitated in maintaining a water balance and the latter in terms of energy and extra capital involved.

The introduction of magnetic separation will allow the recovery of the bulk of the feedstock, typically (50-75, which will contain up to 90% of the iron in the solids.

The smaller portion containing the low susceptibility non-ferrous material can then be leached for the recovery of these values. This whole process is preferably carried out under essentially neutral (pH7) slurry conditions, at or close to ambient temperature. Filtration is therefore facilitated and the rates are higher than those for leach liquor filtration.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a flow diagram of a dust recovery process in which the feedstock slurry is subjected to two stages of magnetic separation prior to leaching.

The arising from one integrated steelworks comprised: tpa blast furnace dust 60000 blast furnace slurry 50000 basic oxygen plant slurry 50000 electric steelmaking slurry 7000 Total 167000 The mean analysis of the arising as proposed for recovery purposes was: Fe (total) 44.58 CaO 5.61 SiO2 4.78 Awl2 03 1.38 MgO 1.01 MnO 0.99 ZnO 0.84 PbO 029 Na2O 0.23 K2O 0.24 When processed in successive 2000 and 8000 gauss fields a recycle stream of approximately 11900 tpa of the two magnetic underflows combined was found to contain 71.5% of the total feed and included 87.5% of the iron content.The analysis was: Fe (total) 54.66 CaO 4.37 SiO2 5.05 Awl2 03 1.29 MgO 1.06 MnO 1.06 ZnO 0.17 PbO 0.11 Na2O 0.22 K20 0.19 This reduces the leach circuit throughput from over 20 tph to less than 6 tph.

The flow process shown in the Figure is most suited for dusts such as electric arc furnace dust having a low carbon content.

For blast furnace dust having a much higher (in the region of 20%) carbon content the feedstock slurry is preferably mixed with a frothing agent and subjected to a preliminary floatation separation prior to magnetic separation, the separated carbon conveniently being dried and compacted into fuel brickettes.

An important advantage of this invention is that it enables a significant reduction (by the order of 75%) in the size and capital cost of the process plant required for the dust recovery process, as compared with what otherwise would be necessary. Further the dewatering filtration stage is facilitated, rates are improved, less equipment (i.e. smaller filtration area) is required than for the recovery of the same weight of solids from leach liquor. Also less expensive construction materials may be used since the operating conditions are less arduous.

The bulk of the iron contained in the feedstock can be recycled in an upgraded form and this fraction contains the essentially unleachable non-ferrous material in the feed. The leachable non-ferrous materials, from BOS and electric arc processing, is upgraded for recovery purposes.

WHAT WE CLAIM IS: 1. A recovery process according to any one of the claims of British Patent Specification No.

1,568,362 wherein magnetic materials are removed from the dust feedstock by magnetic separation prior to leaching.

2. A process according to claim 1 wherein the feedstock is in the form of a slurry having a solids content of from 0.1 to 25% by weight.

3. A process according to claim 1 or claim 2 wherein magnetic separation is effected under essentially neutral slurry conditions at or near ambient temperature.

4. A process according to any one of claims 1 to 3 wherein the feedstock slurry is mixed with a frothing agent and subjected to a preliminary floatation separation prior to magnetic separation.

5. A process according to any one of claims 1 to 4 wherein the magnetic materials are removed by two stages of magnetic separation.

6. A process according to claim 5 wherein separation is effected by passing the slurry through magnetic fields respectively having an intensity of 2000 and 8000 Gauss.

7. A recovery process according to any one of the preceding claims and substantially as hereinbefore described.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The arising from one integrated steelworks comprised: tpa blast furnace dust 60000 blast furnace slurry 50000 basic oxygen plant slurry 50000 electric steelmaking slurry 7000 Total 167000 The mean analysis of the arising as proposed for recovery purposes was: Fe (total) 44.58 CaO 5.61 SiO2 4.78 Awl2 03 1.38 MgO 1.01 MnO 0.99 ZnO 0.84 PbO 029 Na2O 0.23 K2O 0.24 When processed in successive 2000 and 8000 gauss fields a recycle stream of approximately 11900 tpa of the two magnetic underflows combined was found to contain 71.5% of the total feed and included 87.5% of the iron content.The analysis was: Fe (total) 54.66 CaO 4.37 SiO2 5.05 Awl2 03 1.29 MgO 1.06 MnO 1.06 ZnO 0.17 PbO 0.11 Na2O 0.22 K20 0.19 This reduces the leach circuit throughput from over 20 tph to less than 6 tph. The flow process shown in the Figure is most suited for dusts such as electric arc furnace dust having a low carbon content. For blast furnace dust having a much higher (in the region of 20%) carbon content the feedstock slurry is preferably mixed with a frothing agent and subjected to a preliminary floatation separation prior to magnetic separation, the separated carbon conveniently being dried and compacted into fuel brickettes. An important advantage of this invention is that it enables a significant reduction (by the order of 75%) in the size and capital cost of the process plant required for the dust recovery process, as compared with what otherwise would be necessary. Further the dewatering filtration stage is facilitated, rates are improved, less equipment (i.e. smaller filtration area) is required than for the recovery of the same weight of solids from leach liquor. Also less expensive construction materials may be used since the operating conditions are less arduous. The bulk of the iron contained in the feedstock can be recycled in an upgraded form and this fraction contains the essentially unleachable non-ferrous material in the feed. The leachable non-ferrous materials, from BOS and electric arc processing, is upgraded for recovery purposes. WHAT WE CLAIM IS: 1. A recovery process according to any one of the claims of British Patent Specification No.