The treatment of tank wastes

The treatment of tank waste

In the Leblanc process nearly all the sulphur of the salt-cake remains in the "tank waste" or residue from the lixiviation of the black-ash. The average composition of this waste is shown in the following tables : -
 

                 COMPOSITION OF TANK WASTE

        COMPOSITION OF WEATHERED TANK WASTE

"Then fresh waste is thrown on the dump, the changes produced by weathering cause great nuisance. The air is contaminated by the hydrogen sulphide and sulphur dioxide liberated, and the soluble polysulphides of calcium and sodium formed are dissolved by rainwater making the objectionable "yellow liquors," which run into streams and sewers.
In fresh waste the sulphur is chiefly in the form of sulphide and thiosulphate of calcium, but in weathered material these have been converted by oxidation into sulphate and sulphite, which in themselves cause no trouble except by their bulk.
The simplest method of disposing of waste is to send it out to sea and dump it, if the works are so situated that this is convenient; or, if this is impossible, to spread it evenly and beat it down hard to prevent as far as possible the infiltration of rain. But since the sulphur thus lost every year represents an enormous money value, many attempts have been made to recover it in an available farm. Of the numerous processes proposed only three need be considered here.
In Mond's process the waste was treated directly in the lixiviating tanks by blowing air or chimney gases through the wet mass. This oxidized the waste according to the following reactions:- 1) 2 CaS + 2 Hp = Ca(SH)2 + Ca(OH)2'
2) Ca(SH)2 + 40= CaS20a + H20.
But the hydration and oxidation processes were slow, and after a time it was necessary to lixiviate the mass, blow in air, and again lixiviate .. By several lixiviations the calcium sulphydrate and thiosulphate were dissolved, forming "yellow liquors." To recover the sulphur these were treated while still hot with dilute hydrochloric acid, the following reactions * taking place : -
3) CaS20a + 2 HCl = CaCl2 + H20 + S02 + S.
4) Ca(SH)2 + 2 HCl = CaC12+ 2 H2S.
In the presence of the calcium chloride solution the two gases, sulphur dioxide and hydrogen sulphide, react upon each other, forming water and free sulphur: -
5) 2 H2S + S02 = 2 H20 + 3 S.
The hydration and oxidation process was so controlled that the proportion of thiosulphate to sulphydrate yieldec1 one molecule of '"Mactear proposed to use the same reactions for the treatment of the drainage from old waste heaps, which were creating a nuisance. sulplur dioxide to two molecules of hydrogen sulphide. When properly worked very little escape of hydrogen sulphide occurred. The precipitated sulphur was filtered from the solution of calcium chloride which went to waste. The sulphur was then refined. This process recovered about 60 per cent of the total sulphur, but it consumed a great deal of hydrochloric acid, which now has considerable value, and some sulphur was lost owing to the formation of sulphate and sulphite of calcium, which, being insoluble, were left in the residue after lixiviation. The process is not now in use. Shaffner and Helbig's * process depends upon the reaction between magnesium chloride and calcium sulphide in a boiling solution :-
1) CaS + 1\IgC12+ I-I20 = CaCl2 + :M:gO+ H2S.
2) J\IgO + CaCl2 + CO2 = CaC03 + l\IgCl2.
The second reaction was employed to recover the magnesium chloride, but the calcium carbonate formed was too impure for use in the black-ash furnace. The hydrogen sulphide set free was pure, and could he utilized by burning it with air, and conveying the resulting sulphur dioxide into the lead chambers of the sulphuric acid plant; or the sulphide could he decomposed with sulphur dioxide, according to the method given on p. 83, reactiol1 (5). Lime-kiln gases were used for the carbon dioxide in reaction (2). This process was not a commercial success.
The Chance-Claus process t appears to be the only successful method of recoveril1g sulphur on a large scale, and even this has not fully realized the original expectations of its promoters. The reactions of the process were proposed by Gossage in 183ï, but· although he worked on the idea for thirty years, and spent a large fortune in experimenting, he failed to make it a success. The following are the reactions involved :-

1) 2CaS + H20 + CO2 = CaC03 + Ca(SHh
2) Ca(SH)2 + H20 + CO2 = CaC03 + 2 H2S.
3) CaS~+ H2S =.Git (S:çI)2'

A pure carbon dioxide containing at least 30 per cent CO2 is necessary; this can only be cheaply obtained in a carefully regulated special form of lime kiln. The tank waste is diluted with water and put il1to one of a series of seven cast-iron cylinders, so arranged that one may be emptied and recharged, while the others are in uninterrupted operation. The freshly fiUed cylinder is made the last of the  series, while the concentrated carbon dioxide from the lime kilns enter the cylinder containing the most nearly decomposed" waste." The hydrogen sulphide liberated is made to pass into the succeeding cylinders, where it reacts with the calcium sulphide to form calcium sulphydrate, according to reaction (3). This sulphydrate is then decomposed by the carbon dioxide, according to reaction (2). During the formation of the sulphydrate, very little else than nitrogen escapes from the last cylinder; but when the decomposition of the sulphydrate by the carbon dioxide begins in the last two or three cylinders, hydrogen sulphide begins to escape from the apparatus; when this gas is 30 per cent H2S, it is collected in a gasometer; when below 30 per cent, it is turned into the most recently filled cylinder, where reaction (3) takes place. The hydrogen sulphide collected in the gasometer, together with ail', is passed through the Claus sulphur kiln (Fig. 41), in which the reaction

       H2S + 0 = H20 + S
 
takes place. On the grate (A) is a layer of broken fire-brick covered with about 12 inches of ferric oxide.

The mixture of hydrogen sulphide and air is led into the kiln at (B), and made to pass through the ferric oxide (previously heated to a dull red); this causes the reaction to take place, and at the same time, the heat generated by the reaction is sufficient to keep the iron oxide at the proper temperature, after being once well started. Sulphur, nitrogen, and water vapor escape from the kiln. The sulphur vapor condenses in the chamber (D) as liquid sulphur, and in (E) as flowers of sulphur, while the steam and nitrogen, together with a small quantity of sulphur dioxide, pass on to a condensing tower, where they are brought into contact with water, to retain the last traces of sulphur dioxide. When working well, this process recovers about 85 per cent of the sulphur. According to Lunge, the form of the kiln has been recently modified, but the principle of the process is unchanged. The water in the storage gasometer is usually covered with a layer of petroleum oil, to prevent the absorption of the hydrogen sulphide by the water. The process is not very lucrative at present, owing to the low price of sulphur, but since it reduces the nuisance created by the alkali waste, a number of English firms employ it. In 1893, over 30 plants were in operation in England, and more than 35,000 tons of sulphur recovered.