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3,674,428 United States Patent Office Patented July 4, 1972 2 mercury polysulfide to insoluble mercury sulfide, and the 3,674,428 MERCURY REMOVAL resultant precipitate recovered. The process is effective Warren E. Dean and Charles M. Dorsett, New Martins to such an extent that less than 1 part per million or pref ville, W. Va., assignors to PPG Industries, Inc., Pitts erably less than 50 parts per billion mercury is present burgh, Pa. in solution. Moreover, by the process of the invention SSWing. Filed Dec. 22, 1970, Ser. No. 100,833 there is no liberation of hydrogen sulfide into the atmos Int. C. C01g 1/12, 13/00 phere. In addition, it is not necessary to adjust the pH of U.S. C. 23-134 7 Claims an alkaline Solution, i.e., having a pH or 7 or above to an acid pH such as about 4 or 5 in order to decompose 10. the excess polysulfide, and then adjust the acid pH to a ABSTRACT OF THE DISCLOSURE highly alkaline solution preferably at least 9 in order to flocculate the insoluble mercury sulfide. Moreover, it has A process for removing mercury from an alkaline solu been found that an excess of sulfide ion of up to 500 per tion having a pH of at least 7 is provided comprising cent or more can be employed and the control required intermixing with said solution a sulfur compound in an 5 by the prior art processes obviated. Further, results equiv amount sufficient to provide sulfide ion in an amount alent or Superior to prior art methods can be achieved greater than the stoichiometric equivalent of mercury, Without the employment of flocculents and Settling agents. oxidizing the soluble mercury polysulfide and recovering The Solution to be treated can be entirely composed of the insoluble mercury sulfide. Water and mercury or it can comprise only a minor 20 -easam amount of water. It is only necessary that the mercury and sulfide ion be soluble or sparingly soluble in the This invention relates to a process for removing mer Solution and that the mercury sulfide formed be insoluble, cury from alkaline solutions and particularly removing and recoverable therefrom. mercury from mercury electrolytic cell effluent and waste The process of the invention is particularly efficacious streams containing mercury. 25 for alkaline brines from mercury electrolytic cells which Several methods are known for recovering mercury. comprise an aqueous solution of sodium chloride or For example, British Pat. 916,866 adds sulfide ions to a potassium chloride generally containing from 50 grams to chlorine-containing brine until the redox potential relative 100 grams per liter of sodium chloride or potassium chlo to the saturated KC-Calomel electrode is in the range ride. Effluents from said cells and cell operations (i.e., --0.85 volt to --0.93 volt, raising the pH in the range 30 Wash waters, Spills, seal waters, etc.), however, can con 7-9 by the addition of alkali and then continuing the in tain as little as 5 grams to as much as 300 grams per liter troduction of sulfide ions into the brine until the redox of Salt. The brines are normally complex solutions, how potential is in the range of 0 to -0.2 volt, flocculating ever, and contain in addition such elements as magnesium, the precipitate obtained, settling the precipitate and sep mercury, silicon, calcium, barium, strontium, zinc, iron, arating the substantially mercury-free brine. manganese, boron, aluminum, lead, tin, chromium, cop U.S. Pat. 2,860,952 (Bergeron et al.) adds at least a per, nickel, silver, vanadium, and germanium. In addi stoichiometric amount of sulfide to react with the soluble tion, dissolved chlorine or hypochlorite and chlorate ions mercury present in a slightly acidic brine and preferably are normally present. These brines are substantially sulfide a stoichiometric excess in the range of 20 to 100 percent, free as any sulfide ion reacts with the dissolved chlorine, adding thereto a soluble ferric compound and starch or 40 hypochlorite or chlorate ions. The term "substantially sul gum arabic, and adjusting the pH of the brine to between fide free' is intended to define any solution in which the 8 and 10 by the addition of sodium hydroxide. Thus, these materials which react with sulfide ion (e.g., hypochlorite references describe adding sulfide ion to the acidic brine ion) are present in excess, as well as solutions having no and then raising the pH to the alkaline side and optionally sulfide ion. adding flocculating agents in order to flocculate the mer 45 Although the invention is particularly suited for brines cury sulfide. The difficulty with adding sulfide to an and industrial effluents which are initially alkaline, such alkaline brine is noted in the British patent in that sulfur as are produced in modern mercury electrolytic cells, the separates due to the reaction between hypochlorite and invention is equally applicable to any alkaline solution sulfide. Further soluble mercury polysulfides are formed containing mercury or even to acid solutions which can in alkaline solution when an excess of sulfide is added 50 be made alkaline by the addition of sodium hydroxide, so that the mercury is not completely recovered. The carbonate, or other alkaline materials. disadvantage with both of the aforesaid patented proce The sulfur compound employed can be any soluble or dures is that when an excess of sulfide is added to an acid sparingly soluble sulfur compound which will provide solution, hydrogen sulfide is formed and liberated into Sulfide ion in the solution. Exemplary of suitable com the atmosphere. 55 pounds are the alkali metal sulfides such as sodium sulfide, Although the methods taught by the references are use potassium sulfide, and lithium sulfide; the alkaline earth ful for acidic brines, many industrial effluents from mer metal sulfides such as magnesium sulfide, calcium sulfide, cury cells and other mercury-containing waste streams strontium sulfide, or beryllium sulfide; a hydrosulfied such are alkaline when discharged from the manufacturing as Sodium hydrosulfide or hydrogen sulfide can be bubbled operation. For these streams, it would not be efficacious 60 directly into the solution. Generally the sulfide is added as to lower the alkaline stream to a neutral or acidic pH in an aqueous solution but flake materials such as sodium order to add sulfide ions since it would then be necessary sulfide or sodium hydrosulfide can be used. to raise the pH back to the alkaline side in order to floc The term "mercury sulfide' as used herein means culate the formed mercury sulfide. mercuric sulfide, mercurous sulfide, or a mixture thereof. It has now been discovered that an aqueous solution 65 Normally the mercury sulfide is present as mercuric sul having a pH of at least 7, and preferably 9 or above, can fide and thus one mole of sulfide ion per mole of mercury be treated with an excess of sulfide ion over that required ion would provide a stoichiometric equivalent. to react or combine with the mercury present to form Inasmuch as the solution and particularly brines from insoluble mercury sulfide, and that said excess sulfide ion 70 a mercury electrolytic cell can contain a number of other which is reported to result in the formation of soluble materials which are reacted with sulfide ion such as the mercury polysulfide can be oxidized to decompose the aforesaid metals, elemental chlorine, hypochlorite, or 3,674,428 3. 4. chlorate ion, the sulfur compound must be added in an cell operation (800 and 3800 milliliters, respectively) con amount sufficient to combine or react with said materials taining 4,675 and 468 parts per billion mercury, respec and still have an excess of sulfide ion over that required tively, and having a pH of 12.8 and 12.9, respectively, to combine with the mercury present in solution. were treated at ambient temperature with air by passing The sulfide ion can be present in excess over the stoi air through the solutions by means of a fritted glass tube chiometric equivalent of mercury by as much as 500 per at a rate of 5 liters per minute for a period of 24 hours. cent, or more. It is not necessary, however, that the excess The samples were then filtered through a 0.3 micron milli be that high and because the excess sulfide present as pore filter and the soluble mercury in the filtrate deter mercury polysulfide must be oxidized to the insoluble mined. The results are reported in the following Table 2 mercury sulfide, it is normally preferable and quite simple O where it can be seen that in both cases the soluble mer to provide an excess of about 100 percent or less. An ex cury present in the filtrate was less than 50 parts per cess of at least 1 percent should be employed, however, in billion. order to insure complete reaction. The amount of sulfur compound added will also depend upon its particular solu bility in the solution. Thus, where the solution contains a TABLE 2.--THE DECOMPOSITION OF MERCURY POLY SULFIDE IN AQUEOUS SOLUTIONS FROM THE SETTLING large amount of organic materials and/or the sulfur com POND pound is only sparingly soluble in the solution, a large Air oxidation (24 hrs.) amount of sulfur compound may be necessary to obtain Soluble mercury (p.p.b.) the desired excess of sulfide ion. This can easily be de P. p.m. sulfide termined, however, and is no problem in brines and sub Prior to oxidation ion Before After stantially aqueous solutions when sodium sulfide, sodium 6 14,675 35 hydrosulfide or one of the other highly water sulfides is 2 2 468 34 employed.