PROCESS FOR TREATING MERCURY-CONTAINING SLUDGE TO RECOVER MERCURY THEREFROM

(21) Application No, 54645/71 (22) Filed 24 Nov. 1971

(31) Convention Application No. 103692 (32) Filed 26 Nov. 1970 in

(33) Japan (JA)

(44) Complete Specification published 25 Sept. 1974

(51) International Classification B01D 15/04//C22D 1/04

(52) Index at acceptance

CIA G17 R

C7B A1C

(72) Inventor YOSHIKAZU KUMIHIGASHI

(54) PROCESS FOR THE RECOVERY OF MERCURY CONTAINED IN A SLUDGE DISCHARGED IN THE COURSE

OF ELECTROLYSIS OF AN AQUEOUS ALKALI METAL SALT

SOLUTION

(71) I, YOSHIKAZU KUMIHIGASHI,

a citizen of Japan, of 630-3, Han-cho,

Minoo-shi, Osaka-fu, Japan, do hereby de-

clare the invention, for which I pray that

a patent may be granted to me, and the

method by which it is to be performed, to

be particularly described in and by the

following statement: -

The present invention relates to a process

for the recovery of mercury contained in a

sludge formed in the course of electrolysis

of an aqueous alkali metal salt solution.

Since mercury is very harmful to the health

of human beings, various recovery processes

have been tried to prevent mercury escaping

with the waste products of processes in which

mercury is used.

The electrolysis of an aqueous alkali metal

salt solution in a mercury cell is one in-

dustrial process which uses mercury in a

large quantity. In such a process mercury

may be present in a fairly high concentration

in a sludge formed during the step of dissolv-

ing raw alkali metal salt in a dilute brine dis-

charged from an electrolytic cell and remov-

ing by precipitation magnesium, calcium and

heavy metal ions and other impurities whidi

originate in the raw alkali metal salt and

which have adverse effect on the electrolysis

reaction. Therefore, many attempts have

been made to try to provide a satisfactory

process for removing mercury from such

sludges. For instance, one prior process for

the recovery of mercury from the sludge

comprises dissolving the sludge formed as a

result of the step of purifying the aqueous

alkali metal salt solution in a strong acid and

contacting the resulting solution with an

anion exchange resin. However, even after

this treatment there still remains a harmful

amount of mercury in the solution, by virtue

of the adsorption equilibrium of the anion

exchange resin, so that it is not permissible

to discharge solution directly into rivers or

seas. In addition, on the commercial scale this process has the economic disadvantage that is needs a large quantity of equipment due to the low adsorfcency of the known anion exchange resin.

The present invention has been made from a consideration of the problems associated with known processes for recovering mercury from a sludge discharged from the step of electrolysis of an aqueous alkali metal salt in a mercury cell.

In accordance with the present invention there is provided a process for the recovery of mercury from a sludge discharged from the step of electrolysis of an aqueous alkali metal salt in a mercury cell, which comprises dissolving the sludge in a mineral acid and contacting the resulting solution of the sludge at a pH in the range of 3.0 to 7.0 with a chelate forming resin containing in its functional groups sulphur or oxygen chemically linked to nitrogen to cause adsorption of mercury by the resin.

In the sludge discharged from the mercury process for electrolysis of an aqueous alkali metal salt, there are usually contained magnesium hydroxide, calcium carbonate, various heavy metals as well as mercury in various forms strongly adsorbed by such solids. Accordingly, it is essential in the present invention to dissolve the sludge in a mineral acid. The acid may be hydrochloric acid, sulfuric acid or the like, though hydrochloric acid is preferred. Solution of the sludge in the mineral acid may be attained in any known manner, and it is not critical whether the sludge is added to the mineral add or the mineral add to the sludge. The solution may be carried out in two stages. While the sludge may be dissolved in the mineral acid just as it is, it is sometimes advantageous in the practice of the process of the present invention on a commercial scale to filter the sludge to form a cake and then dissolve the resulting cake in the mineral acid thereby minimizing the amount of the mineral acid needed. In genera!, the mineral acid preferably is r=ed in an amount of one to two- times the weight of the solid in the sludge. Of course, there are cases where a sludge cannot be completely dissolved in a mineral acid. In such case, the insoluble matters are separated by, e.g., filtration, sedimentation or centrifugal separation, and the eluate only is contacted with the chelate-forming resin.

The pH of the solution of the sludge in mineral add must be in the range of 3.0 to 7.0 at the time it is contacted with the chelate-forming resin. At a pH exceeding 7, i.e., in the alkaline range, there remains in the solution a large amount of a precipitate because of the low solubility of die sludge which makes it difficult to completely remove solids by adsorption on bringing the solution into contact with the chelate-forming resin. In general, the lower is the pH of the solution, the higher is the solubility of mercury contained in the sludge and the more fully the elution of the mercury is attained. However, to attain effective adsorption of mercury by the chelate-forming resin, the pH of the solution of the sludge should not fall below 3.0. The preferred pH range is from 3,5 to 5.5. While the required pH range is attainable concurrently with solution of the sludge in the mineral acid by the proper choice of the proportion of the sludge and mineral acid, it is preferred to dissolve the sludge in a mineral acid at a lower pH to effect complete solution of the sludge and thereafter to adjust the pH into the required range by the addition to the solution of an alkali, e.g., a caustic alkali or alkali carbonate.

After the solution of sludge in mineral acid has been obtained and, if necessary its pH adjusted, it is next brought into contact with the chelate-forming resin. The chelate forming resins used in the present invention are those containing chemically linked sulfur and nitrogen atoms in functional groups or those containing chemically linked oxygen and nitrogen atoms in function groups. Chelate forming resin are, in general, known as resins containing two or more kinds of functional groups and having a capacity of adsorbing a particular ion through chelate linkage. In the sludge solution in mineral acid mercury exists in various forms and various ions. It is a quite surprising phenomenon that despite the coexistence of such various ions, mercury is selectively adsorbed and eliminated upon contact of the solution with the chelate resin used in the present invention.

With respect to the mechanism of the adsorption of mercury by the resin. I believe that absorption of mercury ion takes place through the chelate linkage formed by the aid of the functional groups, and a certain physical adsorption of metallic mercury. However, I do not wish to be bound by this theory.

As chelate forming resins having a capacity of adsorbing heavy metals there are known those having functional groups of the S-Ν type, Ν-O type, O-O type and other types, and condensates of pyrogallic acid and formaldehyde and of resorcinol and formaldehyde. In the process of the present invention chelate-forming resins of the S-Ν or O-Ν types are used. Representative of the chelate forming resins which may be used in the present invention are those containing such functional groups as, e.g., thioureadithiocarbamate- or iminodiacetate groups. The skeleton resin may be a copolymer of an aromatic vinyl compound, such as styrene or vinyltoluene, with a crosslinking agent, such as divinylbenzene, or condensate of resorcinol with formaldehyde. Such chelate forming resins are supplied, e.g., under a trade name of Chelate Absorption Resin ΝΑ by Hokuetsu Tanso Kebushiki Kaisha and Diaion CR-10 by Mitsubishi Kasei Kogyo Kabushiki Kaisha.

The preferred contact time between the solution of sludge in mineral acid and the chelate-forming resin in the process of the present invention varies depending on tire adsorbing capacity and adsorbing rate for mercury of the chelate-forming resin used, the manner in which the contact is effected, and on other factors. In cases where the contact is attained by passing the solution through a packing layer of the chelate-forming resin, it is preferred to pass tire solution at a liquid space velocity of 5 to 20 litres/resin-litre/hr., more desirably 6 to 10 litres/resin-litre/hr. At a liquid space velocity above the upper limit of the above range, the adsorption of contained mercury is liable to be insufficient whilst, on the other hand, a liquid space velocity below the lower limit of the above stated range is disadvantageous from the viewpoint of processing capacity of the unit. The contact of the solution of sludge in mineral acid with chelate-forming resin in accordance with the present invention may be conducted in any suitable manner. In general, in the practice of the process of the present invention on a commercial scale it is most advantageous to carry out the contact by passing the solution through a column packed with the chelate-forming resin. The solution may be introduced into the column at the top of the column. Alternatively, it may be introduced at the bottom of the column while the treated solution is withdrawn from the top of the column.

The process of the present invention is of particular value to those industries in which, sodium chloride, potassium chloride or other alkali metal chlorides are electrolyzed in an aqueous solution in a mercury process. As will be seen from the examples which follow,