FI61300C - FOERFARANDE FOER AOTERVINNING AV KVICKSILVER - Google Patents

Description

RSfiPI M (i,) PUBLICATION n

WA lj 'n) UTLÄGGNI NOSSKRIFT O I O U U

4§2SS »C Patent issued IS 07 1032 ^ Patent oeddelat ^ v ^ (S1) Kv.lk3 / lm.CI.3 C 01 G 13/00 FINLAND — FINLAND (21) P * t« nttlhak «mu <- Pitmcmeknlng 1701/7 ^

(22) H »k« ml * pttvl - An * 6kntngtdt | OU.O6.7U

* * (23) AlkupUvt - GlltlfhMadag OU.06.7U

(11) Tullut | ulklMkal - Bllvlt offwitlig 06.12.7 U

Patent · and rckirterihallituc .... a.,,. ,. „, _ '. . . , (44) Nihtivlkalpanon | a kuul.) Ulkal «un pvm. - οη ho

Patent · och regicterstyrelaen 'Anrtkan utltgd och utl.ikrlftM puMlc «-» d · Ui (32) (33) (31) Privilege claimed —Begird priorltet 0 5.06.7 3

English-England. (GB) 26731/73 (71) Imperial Chemical Industries Limited, Imperial Chemical House, Millbank London S.W.l, England-England (GB) (72) Alec Donald Clitherow, Buncorn, Cheshire, Philip Richard Edwards,

The present invention relates to a process for the recovery of mercury from chlorine-containing effluent of a mercury cathode electrolysis cell and / or to a cell room wash. Runcorn, Cheshire, England-England (GB) (7U)

British Patent Specification GB 1 339 261 discloses a process for removing mercury from a chlorine-containing waste brine solution from a mercury cathode electrolysis cell by adding sulphide or hydrogen sulphide ions to the brine to give a sufficient amount of brine redox potential preferably more negative than +0.25 volts, precipitating mercury sulfide; and removing the precipitated mercury sulfide by passing the brine down through a filter comprising at least two layers of immediately overlapping granular filtration material, thereby reducing the average particle size of the granular material in successive overlapping layers and increasing the specific gravity of the granular material downward from the top of the filter.

As described in the above-mentioned publication, 2,61300 of the precipitate is usually removed from the filter by back-washing with a liquid, usually water, and preferably also by purging with a gas, usually air. The gas is preferably blown upwards to remove solids adhering to the filter materials and then the liquid is passed upwards through the bed to fluidize it and remove the precipitate therefrom. After the precipitate is removed, the liquid flow rate is generally reduced in stages until the layer has settled substantially back to its original shape.

The solids in the backwash liquid are removed by any suitable means, e.g. by settling or filtration. Metallic mercury can be recovered from the solids thus obtained by any suitable means, for example by heating, for example with iron or lime, or by dissolving by treatment with a chlorine or hypochlorite solution or an alkali metal sulphide solution and reducing the resulting solution electrolytically or with a reducing agent, for example sodium amalgam.

One disadvantage of the above method is that, in practice, large volumes of backwashing liquid are required to remove the mercury sulfide precipitate from the Mixed media filter. This in turn results in increased settling and / or filtration costs to separate the mercury sulfide prior to performing further mercury recovery treatments. The additional filtration step is generally complicated by the need to add filtration aids (e.g., silica / alumina mixture) to facilitate filtration, and in the case of leach-based recovery methods, it is generally necessary to perform a third filtration step to separate the filtration aid from the recovered mercury. The recovery of mercury by treating the backwash fluid itself is impossible due to the large volumes of backwash fluid associated with it. A method has now been invented which allows the mercury sulphide precipitate to be recovered in soluble form directly from the medium mixture filter itself, thus avoiding the disadvantages associated with the recovery of mercury from backwash fluids.

The present invention thus relates to a process for recovering mercury from a chlorine-containing waste salt solution of a mercury cell and / or from washing waters of a mercury cell space, which process adds sufficient sulfide or hydrogen sulfide ions to the brine to give a redox potential of the brine greater than the .45 volts, whereupon the mercury sulfide precipitates, and the precipitated mercury sulfide is removed by passing the brine down through a filter comprising at least two layers of immediately overlapping granular filter material, thereby reducing the average particle size of the granular material in successive overlapping layers and , characterized in that the precipitated mercury sulphide is removed from the filter by treating the filter with a liquid which dissolves the mercury sulphide and removing the mercury-rich solution from the filter.

The liquid used to dissolve the mercury sulfide from the media filter may be any liquid in which the mercury sulfide is soluble, provided that said liquid does not adversely affect the filtration properties of the filter. A suitable liquid is, for example, an aqueous solution of an alkali metal sulfide or hydrogen sulfide, or a solution of chlorine gas in an aqueous medium, but it is preferable to use a source of hypochlorite ions for this purpose.

The source of hypochlorite ions is suitably an aqueous solution of an alkali metal hypochlorite, for example a sodium hypochlorite solution. A solution of sodium hypochlorite in brine or water is a particularly suitable source of hypochlorite ions.

The mercury sulphide precipitate can be conveniently removed from the medium mixture filter by filling the filtration layer with hypochlorite / brine, allowing the solution to stand in contact with the layer for a suitable length of time, for example 1-30 minutes, and then draining the clear solution. The residual solution contained in the filtration layer can be suitably removed, for example, by filling the layer with brine and draining said brine. In the event that other solids remain in the filtration layer, these can be removed by rinsing with air and then performing a backwash and then discarding the wash water.

Separation of mercury from mercury sulfide can be accomplished by treating one or more times with a hypochlorite solution and performing one or more wash treatments in conjunction therewith. It is preferred to use an excess of hypochlorite relative to the amount of mercury in the bed (calculated as Hg), for example an excess of 10-100%. Satisfactory removal of mercury from the bed (about 99% of mercury) can be accomplished, for example, by a single treatment with sodium hypochlorite / saline containing 100% excess sodium hypochlorite relative to the mercury in the bed, followed by two washings.

The mercury in the liquid obtained by treating the bed with hypochlorite can be recovered, for example, by passing the liquid without further treatment to a mercury cathode electrolysis cell, where the mercury is recovered by electrolytic reduction in the cell. Alternatively, chemical methods can be used for recovery.

The waste salt solution obtained from the mercury cathode electrolysis cell and / or the washing waters of the mercury cell space often contain an insoluble substance containing mercury as well as dissolved mercury. If the mercury is in an insoluble form, it must usually first be converted to a soluble form, for example by reacting it with chlorine or hypochlorite ions.

It is preferred that the waste salt solution and / or the wash water of the mercury cell space be treated to remove an insoluble substance, e.g. graphite, before adding sulphide or hydrogen sulphide ions. This can be done in any suitable way, for example by filtration.

When treating the waste salt solution of the mercury cathode electrolysis cell and / or the wash water of the mercury cell space, it is desirable to first remove as much chlorine as possible, for example by blowing air or treating with sodium sulfite solution before precipitating the mercury sulfide. The chlorine removal can be supplemented by any suitable method, for example by the addition of sulphide or hydrogen sulphide ions.

Alkali metal sulfide or hydrogen sulfide is generally used as the source of sulfide or hydrosulfide ions to completely remove the chlorine described above and to precipitate mercury sulfide. Sodium hydrogen sulfide is generally preferred.

It is preferred to precipitate mercury sulfide when the redox potential of the brine with respect to the saturated KCl calomel electrode is more than +0.25 volts.

Although the invention is generally applicable to the treatment of acidic saline, the pH is not critical and it is possible to treat a saline having a pH between, for example, 1-11. However, it is preferred that the pH be maintained between 5-7 during precipitation.

The materials that can be used to form the media mixture filter 61300 can be any suitable material as long as only in at least two immediately overlapping layers does the average particle size in successive layers decrease and the specific gravity increases from the top of the filter downward.

Examples of suitable materials are anthracite, coal, activated carbon, graphite, quartz, flint, silica, magnesium oxide, alumina, garnet and sand.

The invention is illustrated in the following example, where the abbreviation "ppm" means parts per million by weight / volume.

Example 3 A waste-brine solution from a 0.4 m / h mercury cathode electrolysis cell containing 14.0 ppm Hg was filtered through a pressure filtration plate to remove graphite and then partially chlorinated by continuous treatment with sodium sulfide solution so that the potential of the partially chlorinated solution remained +0.90 volts measured with a platinum electrode compared to a saturated KCl calomel electrode. Complete chlorine removal and mercury sulfide blending were performed simultaneously in a second vessel by continuously treating the partially chlorinated solution with sodium hydrogen sulfide solution to reduce the potential to +0.15 volts. The precipitated mercury sulfide was then filtered by passing the brine through a 15.2 cm diameter medium mixture layer comprising a 20.3 cm thick top anthracite layer (particle size -7 to +7 BSS), 35.6 cm thick, a middle layer of silica (18-25 BSS) and a 76.2 cm thick alumina base layer (approximately 36 BBS), supported by a 22.9 cm thick layer of coarse alumina and a 15.2 cm thick with a layer of round stones. Filtration was performed until 54.7 g of Hg was loaded on the bed. The liquid was then drained from the filtration layer, and the filtered liquid was replaced with 16 liters of saturated brine containing sodium hypochlorite having a normality of 0.27 with respect to chlorine. This is roughly 100% excess hypochlorite calculated from the reaction equation: 4NaClO + HgS = HgCl2 + 2NaCl + Na2SO4 ·

The layer was completely immersed in hypochlorite / saline for 1/2 hour. The solution was then allowed to drain and collected and the layer was washed with three successive portions of liquid, all of which were collected. The distribution of mercury in the extracted solution 61300 and in the wash water is shown in the following table.

Fraction Mercury In each case Concentration in phase by phase (ppmfw / v / v). amount of mercury removed (g) amount of mercury (%).

Extract 3160 47.4 86/7 1st wash 460 6/4 11.8 2nd wash 26.8 0.4 0.8 3rd wash 2.0 0.03 0.05

Claims (6)

7 61300 A method for recovering mercury from a chlorine-containing waste salt solution of a mercury cell and / or mercury cell space wash water, comprising adding sulfide or hydrogen sulfide ions to the brine in an amount such that a redox potential of the brine is greater than that of the saturated KCl calomel electrode. wherein the mercury sulfide precipitates, and the precipitated mercury sulfide is removed by passing the brine down through a filter comprising at least two immediately overlapping layers of granular filtration material, thereby reducing the average particle size of the granular material particles in successive overlapping layers and reducing the specific gravity of the granular material; precipitated mercury sulfide from the filter by treating the filter with a liquid that dissolves the mercury sulfide and removing the mercury-rich solution thus obtained from the filter. Process according to Claim 1, characterized in that the precipitated mercury sulphide is removed from the filter by treating the filter with a source of hypochlorite ions. Process according to Claim 2, characterized in that the source of hypochlorite ions is an aqueous solution of alkali metal hypochlorite. Process according to Claim 3, characterized in that the source of hypochlorite is an aqueous solution of sodium hypochlorite. Process according to Claim 4, characterized in that the source of hypochlorite ions is a solution of sodium hypochlorite in brine or water. Process according to one of Claims 2 to 5, characterized in that an excess of 10 to 100% of hypochlorite relative to the amount of mercury sulphide in the bed (calculated as Hg) is used.