METHOD FOR THE PRECIPITATION OF ARSENIC AND HEAVY METALS FROM ACIDIC PROCESS WATER

Abstract

The invention relates to a method for the precipitation of arsenic and heavy metals from acidic, in particular sulphuric acid, process water (12), containing both arsenic and heavy metals, comprising a precipitation method phase (II) with a precipitation stage (D) in which arsenic and at least one primary heavy metal are precipitated together, wherein a sulphide precipitating agent (20) is added to the process water (12) such that arsenic is precipitated as arsenic sulphide and the at least one primary heavy metal is precipitated as metal sulphide. The precipitation method phase (II) comprises a conditioning stage (C) which is carried out before the precipitation stage (D) and in which a conditioning agent (16) is added to the acidic process water (12), which has an effect on the character, in particular the filtering properties, at least of the precipitated arsenic sulphide.

Claims

1. A method for precipitating arsenic and heavy metal from acidic process water containing both arsenic and heavy metal, the method comprising: a precipitation method section with a precipitation stage in which arsenic and at least one primary heavy metal are jointly precipitated by the addition to the process water of a sulfide precipitation reagent causing precipitation of arsenic in the form of arsenic sulfide and of the at least one primary heavy metal in the form of metal sulfide, wherein the precipitation method section comprises a conditioning stage, which is implemented before the precipitation stage and in which the acidic process water is admixed with a conditioning agent, which affects the filtration properties, at least of the precipitated arsenic sulfide.

2. The method of claim 1, wherein the conditioning agent is hydrogen peroxide (H.sub.2O.sub.2) or ozone (O.sub.3).

3. The method of claim 1, wherein the conditioning agent is added substoichiometrically, stoichiometrically or superstoichiometrically relative to the arsenic content of the process water.

4. The method of claim 3, wherein the conditioning agent is added in a ratio of 0.5:1, in a ratio of 1:1 or in a ratio of 1.5:1 relative to the arsenic content of the process water.

5. The method of claim 1, wherein, before the conditioning stage in an analysis stage, the process water is analyzed at least for the arsenic content.

6. The method of claim 1, wherein the acidic process water is sulfuric-acid process water.

Description

[0014] An exemplary embodiment of the method of the invention is elucidated below with figures, in which:

[0015] FIG. 1 shows a method scheme;

[0016] FIG. 2 shows photos of results of the method implemented on the laboratory scale.

[0017] In the scheme, two pumps are designated 2 and 4, and conveying lines are illustrated by arrows whose direction indicates the respective conveying direction. The conveying lines have not been labeled individually.

[0018] In a pretreatment method section denoted by I, there is a pretreatment in which a washing acid 6 obtained in the aforementioned flue gas treatment is first prepared for the separation of arsenic and copper. For example, undissolved particles of arsenic trioxide and dust particles carried in particular by the washing acid 6 can be precipitated using precipitation aids of the kind known per se, and separated off. For this purpose the washing acid 6, in a deposition or filter stage A, is passed via a feedline to a filter unit 8. The solids deposited are transferred into a collecting container 10, from where they are passed on to a disposal facility. The filtrate obtained now forms the process water 12 which is to be freed from arsenic and heavy metals, principally from copper. In an analysis stage B, the composition of the process water 12 is determined at least in relation to the arsenic content and also, in the case of the present exemplary embodiment, in respect of the copper content and/or the concentration of sulfuric acid. Process waters or washing acids of the kind contemplated here typically have a sulfuric acid content of between 1% and 35%, and contain between 3 g/L and 18 g/L of arsenic. The copper content is generally situated at orders of magnitude between 0.1 g/L and 12 g/L.

[0019] The method section I for pretreatment may comprise not only the filter stage A but also further treatment stages or treatment steps, but this is of no further interest here.

[0020] In the case of the present exemplary embodiment, copper defines the primary heavy metal. The process water 12 freed from dust is strongly acidic and has a pH=0. The process water 12 is then fed to a precipitation method section II, in which arsenic and copper are precipitated together and optionally with other heavy meals present. In this precipitation method section II, the process water 12 is first pumped into a conditioning reactor 14, where it is admixed, in a conditioning step C with stirring, with a conditioning agent 16 which affects the nature at least of the precipitated arsenic sulfide. In the present exemplary embodiment, the conditioning agent 16 added is hydrogen peroxide H.sub.2O.sub.2 or alternatively ozone O.sub.3. As indicated above, the conditioning agent 16 is added substoichiometrically, stoichiometrically or superstoichiometrically relative to the arsenic content of the process water 12.

[0021] It is possible where appropriate to do without the method section I and a corresponding pretreatment. In that case the process water 12 corresponds to the washing acid 6; this acid is then passed directly into the treatment reactor 14.

[0022] After a corresponding residence time in the treatment reactor 14, the process water now conditioned, referred to by 12a, is transferred into a precipitation reactor 18 of a precipitation stage D. A sulfide precipitation reagent 20 is added therein to the conditioned process water 12a, with stirring. The sulfide precipitation reagent 20 employed in practice is inorganic sulfide, such as sodium hydrogen sulfide NaHS, for example. Also contemplated, however, are other sulfide precipitation reagents, such as disodium sulfide, Na.sub.2S, for example. It is possible as well to use hydrogen sulfide, H.sub.2S, which in turn may also be generated by means of hydrogen sulfide-producing bacteria, as is known per se. The sulfide precipitation reagent 20 is added to the conditioned process water 12a at a temperature of about 40° C. to 50° C.

[0023] In the precipitation reactor 18 there is a joint precipitation of arsenic sulfide and copper sulfide. Sulfides of the other heavy metals present may also undergo precipitation, but the liquid phase of the mixture 22 then present also contains dissolved cadmium and mercury in particular after the precipitation stage D.

[0024] The mixture 22 then present in the precipitation reactor 18 is now passed to a deposition section III, where it passes through one or more separation stages. Illustrated representatively in the figure is a separation stage E, in which the precipitation products present are separated off from the mixture 22 by means of a filter unit 24. In the case of the present exemplary embodiment, the mixture 22 is passed through a filter cloth 26, to give a filter cake 28 and a filtrate 30. In practice the mixture 22 is sedimented beforehand. As a result of the conditioning with the conditioning agent 16 prior to the precipitation procedure, the sedimentation time for the precipitation product can be reduced by up to 50% in comparison to the precipitation product obtained, which is obtained without the conditioning stage C. Conversely, the volume of the precipitation product obtained is reduced by up to more than 60%.

[0025] It is possible overall, as a result of the improved filtration properties of the precipitation product, to achieve a significant increase, possibly of more than two fold, in the service life of the filter unit 24 and especially of the filter cloth 26.

[0026] The filtrate 30 additionally contains at least the aforementioned cadmium and mercury, and is passed to a further treatment IV of the kind which is known per se, and so will not be addressed any further.

[0027] The filter cake 28 is collected and can be subsequently supplied—in a manner likewise known per se—to a disposal section V, and disposed of. As elucidated above, the filter cake 28 is generally incinerated.

[0028] In laboratory experiments, the method described above showed significant effects on the sedimentation and filtration properties of the precipitation products: in the case of a wastewater A having an arsenic concentration of 7.5 g/L, a copper concentration of 0.3 g/L and a sulfate concentration of 350 g/L, a total of 10 g/L of NaHS (effective) were added. With this it was possible, for the liquid fraction of the mixture 22 and/or for the filtrate 30, to reduce the arsenic concentration to below 50 mg/L and the copper concentration to below 1 mg/L. Even with a substochiometric addition of hydrogen peroxide H.sub.2O.sub.2, with a molar ratio of 0.5 to arsenic, it was possible to achieve a significantly larger and compact flake, a transparent clear phase, and a sludge volume lower by at least 20%, in comparison to precipitation without the prior addition of hydrogen peroxide H.sub.2O.sub.2.

[0029] In the case of a superstoichiometric addition of hydrogen peroxide H.sub.2O.sub.2, with a molar ratio of 2 to arsenic, it was possible to obtain an even more compact precipitated sludge, the volume of which is about 40% lower in comparison to precipitation without prior addition of hydrogen peroxide H.sub.2O.sub.2.

[0030] This is illustrated by the table in FIG. 2. As can be seen in column 2 therein, sulfide precipitation without prior addition of hydrogen peroxide H.sub.2O.sub.2 results in a cloudiness without a clear phase. There is poor sedimentation of the precipitation products or none at all; the filter cake which remains is slimy. Column 3 shows the result of the substoichiometric addition of hydrogen peroxide H.sub.2O.sub.2, whereby the precipitation products sediment well and there is a clear phase and a heavy sludge formed, which has good filtration properties, producing a compact filter cake that exhibits good detachment behavior from the filter. Column 4 of FIG. 2 demonstrates that with the superstoichiometric addition of hydrogen peroxide H.sub.2O.sub.2 there is, in particular, an even better sedimentation behavior.

[0031] In the case of a wastewater B with an arsenic concentration of 10 g/L, a copper concentration of 2 g/L and a sulfate concentration of 40 g/L, a total of 12 g/L of NaHS (active) were added. With this it was possible, for the liquid fraction of the mixture 22 and/or for the filtrate 30, to reduce the arsenic concentration to below 4 mg/L and the copper concentration to below 0.5 mg/L. Here again, hydrogen peroxide H.sub.2O.sub.2 was added substoichiometrically in a molar ratio of 0.5 to arsenic. Flake formation was likewise very positively influenced, and, in comparison to precipitation without prior addition of hydrogen peroxide H.sub.2O.sub.2, the settling rate was approximately halved and a sludge volume lower by at least 25% was attained.