GB2117004A

GB2117004A - Electrolytic recovery of lead

Info

Publication number: GB2117004A
Application number: GB08305289A
Authority: GB
Inventors: Raymond Hugh Morris; Leslie Edward Wylde
Original assignee: Associated Octel Co Ltd
Current assignee: Innospec Ltd
Priority date: 1982-03-17
Filing date: 1983-02-25
Publication date: 1983-10-05
Also published as: GB8305289D0

Abstract

Lead is recovered from lead slag and other lead-containing materials, particularly motor vehicle exhaust gas filters by leaching with hot caustic alkali followed by electrolysis of the leach liquor to recover the lead.

Description

SPECIFICATION Electrolytic recovery of lead This invention relates to the recovery of lead from lead slags and other lead-containing materials, particularly, but not exclusively, lead-contaminated exhaust gas filters that have been used to extract lead from exhaust gases from internal combustion engines using leaded fuels.

Environmental legislation and general public concern over lead pollution of the atmosphere, particularly from motor vehicles using leaded fuels, have intensified the search for an effective exhaust gas filter for motor vehicles and in recent years many proposals have been made. One line of research has been directed to the development of exhaust gas filters comprising a iayer of alumina, optionally containing metal salts such as carbonates, phosphates and borates, deposited on a support and especially a wire wool support of stainless steel. Typical patents in this field include U.K. patents 1 271 710, 1 349 887 and 1 498 130 and U.S. patents 3 227 659,3 231 520 and 3 495 950.

The use of such filters, however, in turn involves other problems, e.g. in the disposal of the used filters, and clearly it would be of enormous economic value if such filters could be regenerated or at least treated for the recovery of the lead extracted and the more expensive components of the filters, for example, the stainless steel cases.

One possible method is to treat the spent filters with warm dilute nitric or other acid to remove the lead compounds and then to precipitate the dissolved lead as a pure, separable lead compound such as the sulphate. An aqueous acid effluent would result which would require neutralisation and treatment for the removal of incidentally dissolved alumina and impregnant (e.g. phosphate) before discharge to the environment. Alternatively, as lead oxides are amphoteric, lead compounds could be extracted from the spent filters by warm caustic solutions, e.g. of sodium hydroxide. Again incidental solution of some alumina and impregnant would occur. This could then be acidified with sulphuric acid for the separation of lead as the sulphate leaving an aqueous acid waste requiring neutralisation and treatment before discharge.

Disadvantages of these example methods are that relatively little acid or alkali can be utilised before the extracting liquid becomes saturated with lead compounds and thereafter the efficiency of extraction is unacceptably low. Saturation values are about 3% m/v Pb in 1.5 M nitric acid at 60 C and 6% m/v Pb in 10 M sodium hydroxide at 600C.

Thus considerable amounts of acid and alkali would be required per unit of lead extracted efficiently.

It would clearly be beneficial if the dissolved lead could be removed in some way from the extractant with concomitant regeneration of the reagent so that it could be used to extract a far greater amount of lead before requiring treatment because of an excessive build up of dissolved alumina and impregnant.

This can be achieved by caustic extraction of lead compounds followed by electrodeposition of metallic lead from the caustic solution as described and discussed hereafter.

In accordance with the present invention, it has been found that the lead can be economically recovered from such materials by a combined process of leaching with hot concentrated alkali, and electrolysis of the alkaline leach liquor. In the case of exhaust gas filters of the above described type, the process alkali leach can, by suitable adjustment of the treatment conditions, also be used to soften and loosen the catalytic alumina deposit thereby enabling separation of the support, if this is so desired.

Typically, in accordance with this invention, the used exhaust gas filters, or other lead-containing material, e.g. lead slag, are leached with alkali, preferably an alkali metal hydroxide e.g. caustic soda, at a concentration in the range 2.0 M to 1 6 M, preferably from 5-1 5 M and at a temperature of from 300C to 4000 C, preferably at about 900 C. Preliminary testing has indicated that high leach temperatures not only accelerate the dissolution of the lead compounds from the filter but also provide leach solutions of high lead concentration which are beneficial in the subsequent electrolytic treatment in that, with higher lead concentrations in the electrolyte, the metallic lead deposits on the cathode are of a dendritic, crystalline nature and are therefore more easily recoverable than the slime deposits which occur at lower concentrations.

Electrolysis of the alkaline leach solution is carried out at temperatures in the range 1 ooh to 1000C, preferably 50 to 800C using metal electrodes e.g. stainless steel, mild steel, brass, nickel or lead, or graphite electrodes. Present experimental data indicates graphite, lead and nickel as the preferred cathode materials, more especially nickel.

Electrolysis is preferably carried out at current densities in the range 10 to 2000 amps/m2. In particular, it has been found, in accordance with a preferred aspect of the present invention, that, in order to obtain the preferred form of lead deposit on the cathode, i.e. as a dendritic crystalline material, a close relationship must be maintained between the operating temperature and the current density, and that for each temperature there is a clearly defined operating current limit for the formation of a crystalline lead deposit. This is illustrated by a series of experiments which have been carried out using as an electrolyte 25% NaOH solution saturated with lead.In this series of experiments electrolysis was carried out over a range of temperatures and a range of current densities with a note being made of the point at which the deposit changed from crystalline to slime for each temperature. The results show a linear relationship between the temperature and the log of the current density at which the deposit type changes as represented by the following equation: log (current density) = 0.0124 T + 1.87 where T is the temperature of the electrolyte in OC and the current density is in amps m-2 Using the electrolytic recovery process of this invention, lead recoveries of up to 98% based on the lead content of the leach solution have been achieved, at current efficiencies of greater than 90%.

The process has the further advantage that, by the use of electrolysis oniy, it is possible to recover the caustic liquor for immediate re-use to remove the lead compounds from a further filter. By alternating extraction and electrolysis steps it is readily possible to re-use the same solution for 10 times or more.

This provides a substantial economy in caustic soda but also has a further advantage in that the problems associated with the disposal of caustic soda when it is no longer useable in the process are much reduced.

The process of the invention is illustrated by the following example.

For this example a number of exhaust gas filters were used, being filters of the type comprising a stainless steel casing, a stainless steel wire wool catalyst support coated with alumina and impregnated with phosphate, and which had been used for the filtration and catalytic conversion of the exhaust gases from an internal combustion engine running on leaded fuel and which were therefore contaminated with substantial amounts of lead.

After being removed from the outer casing a first filter matrix was extracted in 1 2.5 molar sodium hydroxide solution at 600C for 24 hours. The long extraction time was to ensure efficient removal of the lead compounds from the matrix. In practice, a shorter extraction time is as efficient and would probably be used.

After extraction, the matrix was removed and analysed for residual lead. In all cases this was small.

The extract was then electrolysed using the following conditions: Electrode material Ni plate Cathode potential -1 v. (with reference to a standard calomel electrode) Current density 75 A m-2 Temperature 600C As electrolysis took place, dendritic metallic lead was formed on the cathode. When the current feil below a predetermined value, the electrodes were removed and the metallic lead recovered.

The deleaded electrolyte was then used to treat a second filter in the same manner, thereby to form a cyclical process which was repeated with a total of ten filters. The efficiency of lead recovery remained high throughout. In addition there was no significant interference from the build up of other components.

Typical results are indicated below: % of theoretical* lead content 99% 94% 98% of filters removed by extraction process % of lead in solution recovered 97% 99% 98% as metallic lead after electro lysis process Purity of metallic lead recovered 98% 94% 92% prior to further purification * i.e. based on the assumption that all the lead contained in the fuel is retained by the filter.

The results show that even under these non-optimised conditions, almost all the lead in the filters can be removed and recovered as metallic lead with a high purity.

After the final extraction the concentration of lead in the residual solution can be reduced to very low values by electrolysis prior to the safe disposal of the residues by conventional methods.

The combined extraction/electrolysis cycle described'has thus achieved a lead recovered to caustic used ratio at least 10 times greater than the simple extraction processes described above.

Claims

1. A method for the recovery of lead from lead-containing slags and other lead-containing materials which comprises leaching the slag or other lead-containing materials with hot caustic alkali having a molar concentration of from 2-1 6 M at a temperature of from 30 to 4000 C, and recovering the lead from the leach liquor by electrolysis at a temperature in the range 1 00C to 1000C.

2. A method according to claim 1 wherein said molar concentration is from 5 to 1 2.5 M.

3. A method according to claim 1 or 2, wherein the leach solution is aqueous sodium hydroxide.

4. A method according to claim 1, 2 or 3, wherein the lead slag is leached with said alkali at a temperature of about 900C.

5. A method according to any one of claims 1 4 wherein the leach liquor is electrolyzed at a temperature in the range 50 to 800C.

6. A method according to any one of the preceding claims, wherein the electrolysis is carried out using a lead, nickel or graphite cathode.

7. A method according to any one of the preceding claims, wherein the electrolysis is carried out at a current density of 10 to 2000 amperes/m2.

8. A method according to claim 7 wherein the leach solution is 25% NaOH and the current density satisfies the following equation: log (current density) = 0.0124 T + 1.87 where T is the temperature of the electrolyte in OC, and the current density is in amperes/m2.

9. A method according to any one of the preceding claims, wherein after electrolysis the deleaded electrolyte is reused as a leach solution to leach further lead slag or other lead-containing materials for the purpose of recovering the lead therefrom, which lead is recovered by a further electrolysis step.

1 0. A method according to any one of the preceding claims, as applied to the recovery of lead from exhaust gas filters used to filter exhaust gases from internal combustion engines using leaded fuel.

11. A method according to claim 10 wherein said filter is of the type comprising a stainless steel wire wool catalyst support coated with alumina and optionally impregnated with a metal salt as catalyst.