Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing single metal
Patent
1995-02-10
1996-08-27
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing single metal
205604, 75725, 75743, 75961, 423101, 423109, C22B 1924
Patent
active
055498116
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for decontaminating soils polluted with metals.
Protection of the environment has become a continuous preoccupation for a number of years and many studies have been conducted to solve the problems related to pollution.
Pollution of soils with metallic elements, caused in various ways, entails not only the presence of said elements in a concentration that is too high in relation to the accepted standards, but which can also entail a pollution of the aquifer strata and of watercourses.
It is therefore important to have the ability of treating these polluted soils so as to decrease this pollution by restoring the concentrations of metallic elements to levels which conform to the accepted standards.
However, the only decontamination methods available at present are of pyrolytic type or of hydrometallurgical type using an acidic route, and therefore they result particularly in the formation of wastes which are impossible to recycle, requiring storage, or in the release of toxic gases.
PRIOR ART
Nevertheless, a process for hydrometallurgical treatment by a basic route employing zinc, of the sludge and dust from iron and steel manufacture is known, for example from Document BE-A-894,733, which consists in the combined use of three successive operating stages, and in this order:
leaching of the sludge with soda (therefore a basic medium);
cementation by adding Zn metal to the leaching solution to precipitate and recover the metals which are more electronegative than zinc and to collect a solution rich in dissolved zinc;
electrolysis of this solution to recover the dissolved zinc by cathodic reduction.
However, this method, designed for the treatment of materials which are sufficiently rich in zinc to begin with (traditionally approximately from 5 to 50% by weight in the case of steel plants dust) is unsuitable for the treatment of substances with a low zinc content or even free from zinc, as is generally the case with the polluted soils with which the invention is primarily concerned.
The same observation applies to the treatment, similar to that outlined above, described in Document GB-A-1,568,362.
SUMMARY OF THE INVENTION
The aim of the invention is to arrive at a hydrometallurgical treatment with zinc, capable of efficiently decontaminating substances with a low or zero zinc content, such as polluted soils.
To this end, the subject of the invention is a hydrometallurgical process by a basic route using zinc for decontaminating soils polluted with metallic elements, these soils being slightly or not zinc-bearing, comprising a leaching stage so as to bring about the dissolution of the metallic elements to be removed, a cementation stage bringing about the deposition of said elements in metal form, performed by exchange with a zinc powder produced by a stage of electrolysis of the solution originating from the cementation stage, and according to which the zinc concentration of the solution subjected to the electrolysis stage is brought to a sufficient value (namely approximately at least 8 g/l) to satisfy the conditions required to obtain zinc powder in the electrolysis stage, by introducing elemental zinc in a soluble or dissolved form at any stage of the process, complementing the addition of metallic zinc performed in the cementation stage originating from the electrolysis stage.
In accordance with a preferred embodiment of the invention the complementary introduction of elemental zinc is performed at any stage between the output from the leaching stage and the input of the electrolysis stage in the form of a basic leachate of waste from iron and steel manufacture, especially of dust from electrical steel manufacture. Also preferably, said leachate is added to the main leachate originating from the treatment of the soils at the exit from the leaching stage, before the cementation stage.
One of the secondary characteristics of the invention is that the electrolysis of the solution is advantageously carried out on a magnesium cathode, preferably wit
REFERENCES:
patent: 3630669 (1971-12-01), Naito et al.
patent: 4588540 (1986-05-01), Kiefer et al.
patent: 4610722 (1986-09-01), Duyvesteyn et al.
patent: 4824482 (1989-04-01), Baldi
Charpentier Pierre E.
Rizet Laurent
Phasge Arun S,.
Unimetal - Societe Francaise des Aciers Longs
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