GB2089335A - Removal of mercury from industrial effluent - Google Patents

Abstract

Mercury is removed from an industrial effluent by treating the effluent with a sulphide or hydrosulphite of an alkali metal in the presence of a flocculant at a pH of from 7 to 12 so as to precipitate an alkaline sludge containing precipitated mercuric sulphide and flocculated material, treating the alkaline sludge with effluent to redissolve the flocculated material but not the mercuric sulphide, and removing the mercuric sulphide.

Description

SPECIFICATION Removal of mercury from industrial effluent This invention relates to a process for removing mercury from industrial effluent or sewage containing chemically bonded mercury.

It is well known how important is the problem of pollution by mercury. Extremely serious cases of poisoning, some of them fatal, have occurred, and large quantities of mercury has been found in fish caught in areas heavily polluted by mercury-containing effluent.

The seriousness of the problem has led the governments of the most important industrial countries to regulate the content of mercury, both in metallic and compound form, of industrial effluent, especially those in liquid form. In Italy, the maximum allowed limit is 5 micrograms per litre.

One of the most serious sources of pollution by mercury are processes for the electrolytic production of chlorine using mercury cells, and catalytic processes which employ mercury salts as catalysts for organic syntheses, for example the production of acetaldehyde and vinyl chloride from acetylene.

The industries concerned have dedicated strenuous efforts towards the solution of the problem and several methods have been suggested for reducing the mercury content of the industrial effluent to the specified limits.

Thus, for metallic mercury, special filtration such filtration with activated charcoal, or the formation of metallic amalgams, has been used, whereas, for combined mercury, precipitation, adsorption of ion-exhange resins, reduction to elemental mercury and other chemical and electrochemical methods have been suggested.

Among the precipitation methods, the one which is predominantly used is that which results in the formation of mercuric sulphide. This method is generally carried out by treating the effluent polluted by mercury compounds with solutions of sodium sulphide or hydrosulphite, at a pH of about 8, by flocculating the resulting colloidal precipitate with ferric chloride or other suitable flocculant, and by filtration, upon decantation, of the sludge formed. This procedure has, however, some drawbacks. Thus in order that a quantitative precipitation of mercury may be approached as close as possible, a large excess of sodium sulphide is used. However, sodium sulphide is prone to hydrolysis with consequent formation of hydrogen sulphide, which is highly toxic in itself.

Moreover, the use of ferric chloride as the flocculant results in the formation of large quantities of iron hydroxide, an extremely bulky precipitate which is difficult to collect on filters and which, in addition to resulting in the consumption of large quantities of ferric chloride, has the further and more serious problem of the need to treat large quantities of sludges having a low mercury content and at the same time an extremely high water content (up to 60% or even 70%).

Even the use of nonionic flocculants, although it overcomes the problem, of the consumption of large quantities of ferric chloride, does not solve the difficulties inherent in filtering and subsequently treating the sludge.

At present, the filtration of colloidal precipitates is usually carried out in a rotary filter equipped with a "Dicalite" liner which is both a supporting member and a filtration aid, the precipitate being removed by scraping the "Dicalite" liner with a doctor blade. The "Dicalite" liner prevents damage to the filter screen during removal of the precipitate, but stripping of the "Dicalite" always occurs so that a continuous consumption thereof takes place during filtration.

According to the present invention, there is provided a process for removing mercury from an industrial effluent containing mercury, which comprises treating the effluent with a sulphide or hydrosulphite of an alkali metal in the presence of a flocculant and at a pH of from 7 to 12 so as to precipitate an alkaline sludge comprising precipitated mercuric sulphide and flocculated material, treating the alkaline sludge with said effluent so as to redissolve the flocculated material but not the precipitated mercuric sulphide, and removing the precipitated mercuric sulphide.

It has now been found that the drawbacks described above can be overcome, at least so some extent, by the process according to the invention, which process, in preferred embodiments thereof, comprises the step of subjecting, upon decantation, the alkaline sludge resulting from the precipitation with sodium sulphide, to the action of a small amount (from 10% to 20% of the total) of the effluent from which mercury is to be removed. It has been found that this treatment makes it possible to redissolve the amorphous flocculated substances to leave, as a residue, the mercuric sulphide precipitate which can be easily collected on a conventional filterpress, or, at any rate, without any filter-aids such as "Dicalite", and with a minimum residual moisture.The mercuric sulphide thus obtained, furthermore, is usually free of coarse impurities, so that it can directly be used to obtain elemental mercury by heating. The invention will now be illustrated by the following Example, which describes the removal of mercury compounds contained in an effluent resulting from the catalytic synthesis of vinyl chloride from acetylene.

The process described in the Example was carried out, initially, in laboratory-sized apparatus, and, subsequently, in commercial-sized units. The single Figure of the accompanying drawing is a block diagram illustrating the process. In the drawing, 1 is a stream of calcium hydroxide suspension, 2 is an aqueous solution of flocculant, 3 is a stream of the effluent from which mercury is to be removed, 4 is an aqueous solution of sodium sulphide, 5 is an equalizer, i.e. a reaction vessel, which is equipped with a stirrer and in which a pH of 7 to 9 is maintained, 6 is a settling/ flocculating tank, 7 is a silica filter, 8 is an activated charcoal filter through which flows the effluent still containing traces of mercuric sulphide, 9 is the effluent from which mercury has been removed, 10 is an alkaline sludge, 11 is a dissolver to which a portion of the effluent stream 3 is passed, 14 is a fiiter-press or other direct filtration system, 12 is a recycled stream of the effluent and 13 is the acidic and moist sludge obtained by filtration.

EXAMPLE The effluent from the vinyl chloride synthesizing plant was acidic (i.e. had a pH of about 1.8) and contained mercury compounds in an amount which was nearly 10 milligrams per litre. In order to comply with Italian legal requirements, the mercury content should be reduced to not more than 5 micrograms per litre.

In the laboratory tests, the effluent was treated in the equalizer 5, with stirring, with lime milk (an aqueous slurry of lime) to make it alkaline, i.e. to give it a pH of about 10. Concurrently, there were added 5ml per litre of an aqueous solution of a polyacrylamide-based nonionic flocculant (Prodefloc N2M, a Trade Mark of Prodeco Company) having a concentration of 1 gram per litre.

Subsequently, an aqueous solution of sodium sulphide, having a concentration of 5 grams per litre, was added in an amount which was three times the stiochiometric amount of mercury, stirring being continued for approximately one hour. Stirring was then discontinued to enable the precipitate to settle in the tank 6.

After standing for 3 hours, the clear supernatant effluent was withdrawn and filtered through the activated charcoal filter 8, whereby the residual mercuric sulphide was separated. The mercuric sulphide, which was present in the settled effluent in an amount slightly below 100 micrograms per litre, was reduced by filtration to an amount of from 2 to 5 micrograms per litre.

Meanwhile, the alkaline sludge was transferred from the tank 6 to the dissolver 11 wherein a portion of the effluent to be treated was added as such (that is at a pH of about 1.8) until the pH of the mixture attained a value of from 2.2 to 2.5. At this stage, about 80% of the sludge had dissolved, whereas there as left, in undissolved form as a heavy precipitate, only mercury sulphides and possibly sulphides of metals of the first and s#econd Groups of the Periodic Table.

The precipitate was collected in the filter 14 under vacuum, on a filtering cloth, without any need to use "Dicalite" as a filtering aid.

The aqueous filtrate 12, which still contained mercury compounds, was recycled to the equalizer 5.

The precipitate collected in the filter 14, which in quantity was about 0.1 gram per litre of effluent treated, has a content of about 10 % of mercury, in terms of elemental metal.

Tests on an industrial scale were carried out following the same order of steps adopted in the laboratory. There were used metallic vessels of larger size instead of glass apparatus. The dimensions of the industrial apparatus were about 300 cubic metres for the equalizer 5 (i.e. the vessel in which the effluent was made alkaline), about 120 cubic metres for the settling/ flocculating tank 6, about 3 cubic metres for the activated charcoal filter 8, and about 3 cubic metres for the alkaline sludge dissolver 11.Also, in the industrial procedure, the neutralizing lime milk was fed in a metering device until a pH of 8 to 9 was attained.The alkaline solution was passed to the settline/flocculating tank 6 to which was also fed sodium sulphide (in an amount of 3 times the stoichiometric amount required for precipitating mercuric sulphide so as to precipitate also other metals possibly present) and to which the nonionic flocculant had already been passed.

It was ascertained that the consumption of flocculant in the industrial run was one half of that of the laboratory procedure.

Any residual excess of sodium suiphide was adsorbed by the activated charcoal filter 8. Upon settling, the flocculated sludge was sent to the dissolver 11, whereas the clear effluent was sent to the activated charcoal filter 8.

The effluent leaving the settling/flocculating tank 6 had a mercury content of from 50 to 200 micrograms per litre. Upon adsorption on the activated charcoal filter 8, the average mercury content of the effluent leaving the filter was reduced to a value of from 2 to 5 micrograms per litre. The silica filter 7 prevents any floating flocculant from clogging the activated charcoal filter 8.

The slude fed to the dissolver 11 was treated with the raw effluent. The ratio of effluent to alkaline sludge depends upon the pH of the effluent and the solids content of the sludge, and usually ranges from a minimum of 10 to a maximum of 20. After dissolution, the acidified sludge was allowed to settle, and a clear effluent was removed by siphoning. The effluent was recycled to the equalizing vessel 5. The acidic sludge from the dissolver 11 was subjected to filtration in the filter-press 14, a compact filter cake, which can easily be removed from the filter cloth thereby being obtained.

For comparison, mercury was removed from the same effluent, without treating the alkaline sludge with the effluent. The most conspicuous difference was the necessity of emptying the settling tank 6 daily and of filtering the alkaline sludge as the latter attained an excessive bulk, whereas, in the case of the process of the invention, it was sufficient, though not strictly compulsory, to effect the filtration weekly.

Regeneration of the activated charcoal filter was carried out in situ after more than 45 days use by leaching with a 3% solution of hydrochloric acid containing a corrosion inhibitor.

In the following Tables there are reported data relating to a number of tests carried out according to the procedure of the invention and, by way of comparison, data relating to tests carried out without sludge treatment. The amounts of mercury contained in the effluent to be treated and the effluent exiting the activated charcoal filter are reported, as well as the compositions of the sluges obtained.

TABLE 1 Content of mercury in effluent in micrograms per litre per litre Test Entering treat- Exiting floccu- Exiting active NO ment plant lator charcoal filter 1 ~ 1 5,900 65 4.0 2 8,400 80 4.6 3 6,500 40 3.6 4 10,600 35 2.2 5 4,000 83 6.0 6 1,600 20 3.0 7 2,100 39 3.6 8 1,100 78 2.1 10 3,600 37 3.2 11 3,400 83 2.9 12 8,000 95 4.0

TABLE 2 Composition of sludges after filtration (% by weight) Analysis on wet samples (% by weight) Test Amount N0 Kg/m3 Moisture Mercury Iron Calcium 1 0.05 50 8.90 4.56 5.28 2 0.06 55 9.5 3.85 4.85 t ~ 3 0.048 51 8.85 4.2 5.3 Co 4 0.051 53 10.5 3.6 5.0 13(") 0.38 72 3.33 3.26 6.24 14(') 0.311 74 1.75 2.85 5.8' 15( ) 0.4 68 2.5 3.3 4.2 16( ) 0.35 60 3.2 2.80 5.0 5 O.056 52 9.2 4.25 5.35 6 0.04 48 10.1 4.00 4.95 7 0.048 50 8.9 3.25 5.00 8 0.060 53 9.9 3.80 5.20 9 0.050 50 8.95 3.90 5.40 10 0.042 52 8.4 4.15 4.8 11 11 0.055 51 9.2 3.95 5.3 C 0.051 49 10.3 ~ c 12 0.051 49 10.3 4.30 4.9 17(") 0.311 71 2.2 2.9 5.5 18( ) 0.330 70 1.8 3.2 4.9 19( ) 0.295 68 2.3 2.85 5.8 20( ) 0.300 65 3.2 2.7 6.2 21(") 0.350 69 3.5 3.5 4.5 22( ) 0.310 64 2.9 3.3 5.9 23( ) 0.298 60 3.35 2.95 5.0 24( ) 0.308 65 2.85 3.00 4.8 (,)The tests so marked relate to precipitate of mercury as sulphide without subsequent treatment of sludge with a portion of the effluent

Claims (9)

1. A process for removing mercury from an industrial effluent containing mercury, which comprises treating the effluent with a sulphide or hydrosulphite of an alkali metal in the presence of a flocculant and at a pH of from 7 to 12 so as to precipitate an alkaline sludge comprising precipitated mercuric sulphide and flocculated material, treating the alkaline sludge with said effluent so as to redissolve the flocculated material but not the precipitated mercuric sulphide, and removing the precipitated mercuric sulphide.

2. A process according to claim 1, wherein the precipitated mercuric sulphide is removed by filtration and the filtrate is recycled to the stage of treatment of the effluent with a sulphide or hydrosulphite of an alkali metal.

3. A process according to claim 1 or 2, wherein the effluent is treated with sodium sulphide.

4. A procss according to any of claims 1 to 3, wherein the alkaline sludge, before treatment thereof with said effluent, is allowed to settle and resulting supernatant liquor is removed therefrom.

5. A process according to any of claims 1 to 4, wherein the alkaline sludge is treated with from 10 to 20% of said effluent.

6. A process according to any of claims 1 to 5, wherein the effluent is acidic.

7. A process according to claim 6, wherein the effluent is treated with an aqueous slurry of lime to render it alkaline.

8. A process according to any of claims 1 to 7, wherein the flocculant is ferric chloride or a nonionic flocculant.

9. A process for removing mercury from an industrial effluent containing mercury, substantially as hereinbefore described with reference to the accompanying drawing.