CHAPTER IX.

GENERAL CHEMICAL TRADES.

Distillation of Ammoniacal Liquors.

THE gas liquor fresh from the works 'scrubbers' contains:

in addition to the constituents of the water supplied for scrubbing purposes and variable tarry or oily substances. On adding a considerable quantity of lime to the liquor, and heating, the ammonia is displaced by it, and the acids of the above salts are left combined with the lime base.

As the residual liquor leaves the stills it is generally a thick brown liquid, supersaturated, at atmospheric temperatures, with lime or lime salts, covered with an oily film, and containing much lime in suspension.

Its objectionable constituents are the tarry matters, ferrocyanides, sulphides, suspended lime, and the free lime in solution.

A sample after settlement and filtration through sand filters contained the following:

Ferrocyanides, ferricyanides, and sulphocyanides were also present. It is evident, therefore, that simple settlement and filtration is insufficient. The first important step is, of course, the settlement of the suspended lime, and this is easily accomplished by means of ordinary precipitation tanks.

The free lime might also be neutralised and eliminated in the same tanks by means of precipitants such as copperas, aluminium sulphate, etc., but the quantity of lime present is so great that the copperas required for neutralisation is enormous and the sludge in proportion.

Dr Lunge, in his "Coal Tar and Ammonia," * describes the purification plant in use at the workmen's colonies in connection with Messrs Krupps' works, Essen, from particulars supplied by Dr Solomon, the chemist there. The clarifying plant is shown in fig. 105.

"The liquid first runs into the catchpool A, where the heavier substances suspended in the water and those floating on the top are retained. From there it flows into one or other of the feeding-vessels, B B; two of these are provided, so that one may be always in operation. They contain a small over-shot waterwheel, the prolonged axle of which carries on either side a cross with baling-buckets and a stirrer for each of the tanks, separately holding milk of lime and solution of copperas. This water-wheel is turned by the inflowing waste liquor itself, and will, of course, turn more or less

* "Coal Tar and Ammonia," by George Lunge, Ph.D., Professor of Technical Chemistry in the Federal Polytechnicum, Zurich. London: Gurney & Jackson.

quickly according to the supply of such liquor; hence the baling buckets, fixed at the sides of the wheels, will take up more or less of the chemicals, exactly in accordance with the quantity of liquor to be purified. Thus the supply of chemicals is self-regulating, and only requires the filling up of the reservoirs from time to time with milk of lime and solution of copperas." The number and size of the baling-buckets must be determined once for all by practical trials, as well as the concentration of the chemicals. The milk of lime taken up by the buckets fixed to one side of the wheel is discharged into the waste liquor just below the wheel; but the copperas solution taken up by the buckets on the other side of the wheel is conveyed in a special conduit into the reservoirs D D. The liquor, mixed with a sufficient quantity of lime, flows by a channel into one or other of the tauks C C (there are two of these, so that one can be cleaned out without interrupting the work of the other). It runs on in the direction of the arrows, and the zigzag partitions arranged in the tanks cause a large portion of the suspended matters, along with those precipitated by the lime, to settle down. The liquor, still saturated with lime, is run by means of a spreading-shoot into one of the four separate tanks, D1, D2, D3, D4, where it meets with the copperas solution coming from B1 or B2. This causes a thick, flocculent, dark green precipitate to be formed, consisting of ferrous hydrate and calcium sulphate, which quickly settles down as the liquor travels on, and carries down all the finely divided matter still in suspension. (I have proved by experiment that nearly all the tarry matters found in waste ammonia liquors are carried down here, and the liquor issues nearly, or even quite, devoid of colour and smell.) The flow is here also broken by zigzag partitions and checks; but in order to better retain the flocculent precipitate there are also peat filters placed in the way of the liquor, in lieu of ordinary checks. A small agitator, a, causes an intimate mixture of the liquor and of the copperas solution. The necessary moving power may be either derived directly from the wheels in B B, or else from a separate wheel driven by the clarified water; but the latter is only possible where there is a sufficient head of water at disposal.

"Since, in order to completely retain all the flocculent precipitate, the tanks DD would have to be made of an inconvenient length, another arrangement is provided for that purpose. This is the set of catchpools EEEE, from which the completely clarified liquor runs away through F. "In some cases (especially with sewage) it will be necessary to restore to the outflowing liquid the oxygen taken away from it by the ferrous hydroxide. If the outflowing liquor is used for driving a water-wheel, as shown in fig. 77 at b, there will be sufficient contact with atmospheric oxygen as the liquor is running over the wheel. In other cases such contact can be caused by special means (probably unnecessary in our case).

"The mud collecting in the tanks CC and DD is from time to time

removed by taking out the checks and opening the discharge-valves. It then flows, with sufficient fall, into the mud-tank G, and is pumped up from here by a dredger or other suitable means, in order to drain in special filtering-basins. It is possible to keep the lime precipitated in C and the iron precipitated in D separate, and to utilise the former for agricultural

purposes.

"The milk of lime and the copperas solution ought not to be made with foul liquor, but with clear water. For this purpose that coming from F may be conveyed into the tank H, and can be pumped up from here as it is required.

"The plant shown here serves for daily purifying from 2000 to 3000 cubic metres (say, tons) of concentrated sewage; it costs about £750, inclusive of mud-filters, and requires only one man for superintendence, as everything is self-regulating."

The effluent from this treatment, however, is found to contain much organic matter as well as much mineral matter in solution; in fact, it is rarely less than 1000 parts per 100,000.

Practically the same treatment was in vogue at Messrs Metcalf & Co., Ltd., Church, for some years the tanks being supplemented by sand filters (fig. 106).

The best method of treatment yet seen by the author is :—

1. The injection into the liquors of, and thorough agitation by, chimney
gases. The carbonic acid dissipates any sulphides present and
throws down some portion of the lime which forms a fine precipitate
and enmeshes a considerable amount of matter frothed' out of
solution. The vigorous agitation of saturated solutions nearly
always results in a solid scum, especially if they be alkaline.
2. Subsidence.

3. The conduction of the clarified liquors to precipitation tanks and
treatment with basic alumina ferric and a little ferric iron solution.
A dense precipitate of aluminium hydrate and lime sulphate is
formed, accompanied by much organic matter.

4. Aëration on the Mather & Platt system (see page 25), followed by subsidence.

The top liquor after this treatment is clear, colourless, neutral, and has a greatly reduced soluble content, that remaining being principally salts of lime and soda-though probably the bulk of the sulphocyanides are present also, and these are always considerable.

A much more complete and scientific method is that of H. W. Crowther, Patent 11,964, 1893. The gas liquor is treated with lime kiln or other gases containing carbonic acid, the precipitated calcium carbonate is allowed to subside, and the cleared separated liquid is agitated with nearly sufficient