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In the first case, sodium acetate is formed, and although this has cost money, yet it must be thrown away, because it cannot be economically recovered by the colour-maker. In the second case, the acetic acid formed may be utilised in producing a fresh stock of acetate of lead from metallic lead; in this case there are no waste products, and the manufacture of the lead sulphate is conducted most economically.
The character of the precipitate formed is modified in regard to its tint, its consistence, and in other ways by the conditions under which it is obtained. Thus, if barium sulphate be precipitated from cold solutions, it falls down as a very fine, rather light powder, which is difficult to filter; but if thrown down from hot solutions, it is not so fine, and can be filtered more readily. Again, in making chromes, the conditions under which the operation is performed have a wonderful influence on the result; a difference in the lead salt influences the tint, the nitrate giving a finer product than the acetate. The temperature also modifies the result considerably. Thus, the chrome which falls down from cold solutions is much paler and more voluminous than that obtained from hot solutions. The acid or neutral state of the solution also has some influence, while the presence of such bodies as alum or sulphate of soda has a material influence.
In making colours by precipitation, the following conditions affect the character of the resulting pigment:-(1) Strength of solution. Generally, weak solutions yield finer and more voluminous precipitates than strong ones. (2) Temperature. From cold solutions the product is usually finer and more voluminous than from hot solutions. (3) Proportion between the interchanging bodies.
This also has some influence on the result. Thus, in making chromes it is preferable to keep the lead in excess; if a soluble Prussian blue is required, then the potassium ferrocyanide must be in excess. Other examples might be given illustrative of this point. (4) The order of mixing is important. If the lead salt, in making chromes, were added to the bichromate of potash, the pigment obtained would not be so fine as in adding the bichromate of potash to the lead salt. In making soluble Prussian blue it is important to add the iron salt to the potassium salt, not vice versa. In making emerald green, the
acetic acid should be added to the copper before adding the arsenic preparation, if a good result is to be obtained. There is another feature in precipitation worth mentioning here. When a solution of a metallic salt is added to another solution containing two other salts, both capable of precipitating the first, it may happen that a kind of selective action may take place; the metallic salt will at first form a precipitate with one of the mixed salts only, and not until this action has ceased will it precipitate the other. An excellent example is the action of silver nitrate on a mixture of sodium chloride and potassium chromate; with the first it will give a white precipitate of silver chloride, with the second a dark red precipitate of silver chromate. When the two salts are mixed together, the silver nitrate will not precipitate the chromate until all the chloride has been thrown down, which point is shown by a change in colour of the precipitate from white to red. Another example is the precipitation of a mixture of sodium chromate and carbonate by zinc sulphate; in this case zinc carbonate is first thrown down. In precipitating a mixture of potassium bichromate and sulphuric acid with lead acetate, lead sulphate is thrown down before the lead chromate. This is a very interesting feature in precipitation, and is often taken advantage of by chemists for the separation of substances, one from the other; they know it as fractional precipitation.
The plant required for making colours by precipitation is comparatively simple. There are required, 1st, vessels wherein to dissolve the various ingredients used ; and 2nd, vessels in which the ingredients are mixed together, precipitation vessels, which are preferably of wood, as those of earthenware would be too easily broken, and could not be made so large as required. The two classes of tanks or tubs should be kept distinct; a dissolving tub should not be used for precipitating in. Then each colour that is made should have its own set of tubs. If a set of tubs were first used for making a chrome-yellow, and then for making a Prussian blue, the results in the second case would not be very satisfactory; the rule of a colour shop should be that every distinct colour has its own set of tubs. The reason for such a course is that it is practically impossible to clean the tubs so as to avoid the liability of the remnants of one batch spoiling the next batch.
Fig. 36 shows a good arrangement of plant for preparing pigments by precipitation; a set of five tubs is shown; two of these, cc, are placed on the floor, and used for the actual precipi
tation; three, DDD, used for making solutions of the ingredients, and which may be smaller than C, are placed on a platform, P, above C. A steam pipe, S, with branch pipes, carry steam to all the tubs for the purpose of heating the contents, if that be necessary;
Fig. 36.-Plant for preparing pigments by precipitation. arrangements may be made for conveying water to these tubs. In the bottom of the tubs are plug holes or tap holes, which allow of the contents, when ready, flowing into C, through troughs. C has a number of holes, h, h, h, fitted with plugs, by the removal of which the supernatant liquors are easily run off after the pigment has settled. Or the liquors may be siphoned off. Although a set of five are shown in the drawing as being an economical number, yet three only are, as a rule, required to be used at one time for preparing a pigment, but
with five two batches of a pigment requiring the same materials may be more readily prepared. The size of these tubs must be proportioned to that of the quantity of pigment required to be turned out. A convenient size for the dissolving-tubs is 3 ft. 6 in. high, by 2 ft. 6 in. diameter; the capacity being about 110 gallons; the precipitating tub may be 8 ft. 6 in. high by 3 ft. 6 in. diameter, and will hold 320 gallons; batches of 7 to 10 cwts. of colour can easily be made in such tubs.
The usual method of procedure in making pigments by precipitation is as follows. The materials, after being weighed out, are placed in the dissolving-tubs with the requisite quantity of water; then, by means of the steam pipes, they are heated until complete solution has been effected. The liquors are now run into the precipitation tank, if they are to be used hot, or they may be allowed to cool before running into the precipitation tub. While running into the tub, it is desirable that the liquors be thoroughly mixed by stirring together. When all the liquors have been run into the precipitating tub, the mass is allowed to stand for the precipitate to settle; when this has occurred, the clear liquor is run off, fresh clean water run in, the precipitate stirred up and again allowed to settle out, and the water run off; if necessary, this washing is repeated once or twice ; finally, the precipitate is allowed to settle, the clear top wate: run off as much as possible, the precipitate thrown on to a filter for the rest of the water to drain away, and the still wet precipitate placed in the drying stove to dry.
In making some pigments, such as rose pink, the lake-pigments from the coal-tar colours, vermilionettes, &c., solid bodies such as barytes, whiting and orange-lead are added. This is best done by running one of the solutions into the precipitating tub, adding the barytes, &c., and stirring so as to get every particle of the solid thoroughly incorporated with the liquor; then the other solution is run in and the operation finished as described above. Unless care is taken to ensure the thorough mixing of the dry solid added with the liquors, the resulting pigment will have a speckled appearance.
In preparing pigments from such natural materials as logwood, cochineal, fustic, Brazil wood, &c., where the whole of the material is not dissolved, the actual colouring principle should be extracted by boiling in an apparatus so constructed as to permit the colouring principle being extracted and removed by boiling from the insoluble particles of wood, &c. Further details are unnecessary, as the extractors made are numerous and efficient.
A simple form would be a boiler fitted with a perforated false bottom, on which the dyewoods, &c., are thrown; a similar perforated plate is placed on the top of the mass, and the whole boiled with water; when the extraction is considered to be finished, the liquor is run off below the false bottom, and through a filter into storage tanks, in which it is kept until required for
Filtering, which follows precipitation, effects the separation of the pigment from the excess of water it contains. The simplest plan is to use a filter constructed of a sheet of calico loosely stretched over a skeleton framework of wood (see Fig. 37). The wet pigment from the precipitating tanks is thrown on to this sheet of felt; the water drains through, while the pigment remains on the top of the sheet.
On the whole, the process is an effectual one, and the filtration
Fig. 37.-Filter. is thorough; the speed and completeness depends upon the character of the precipitate and the quality of the filtering cloth. If the precipitate is powdery and somewhat granular in structure, then the filtering goes on fairly rapidly, and the liquor that drains away is fairly clear. On the other hand, there are precipitates, such as Prussian blue and blanc fixe, that are very fine, rather slow in filtering, and from which it is difficult to obtain a clear liquor.