In tartaric acid works the lees is partly pressed and partly used without being pressed, so long as the deposit of the wine continues; for working during the greater part of the ycnr the lees is very strongly pressed and dried.
(i) Working with Wet Lccs.—Both the pressed and impressed lees is first used for making brandy, of which it yields 1 to 4 per cent. and the resulting schnapps—so-ealled lager brandy "—is much esteemed by some people. As this lager brandy is considerably dearer than the spirit from other sources (potatoes, maize, &c.) it is the custom to add to the lees before distillation, at least as much spirit ss it is expected to derive from the lees as lager brandy, and the properties of tho lager brandy are so strong that this may be done with impunity. If, however, the lager brandy by itself be rectified, a pure spirit is the result, which is better adapted than any other to liqueur nlaking, and is much used for that purpose.
After the distillation is complete, the lees is run into a large vat of about 2000 to 3500 gallons capacity, and is thinned with water by means of a pump. About 50 cwt. of lees are put into the vat, which is tlun almost filled with water, and to this about 1 cwt. of muriatic acid is added. The ngituting machinery is then set in motion, and the liquor heated by steam nearly up to the boiling point. When this is attained the steam and the agitator are suspended, and the contents of the vat are left for some time to settle. By far the greater portion of the liquid thin becomes clear ; this is drawn off by a syphon into a second vat of suitable proportions, and then mixed with powdered chalk, and constantly agitated till neutralized to a weak acid reaction. Through the chloride of lime formed all tho tartaric acid will be deposited. Then the liquid is put iuto a third vat, where the tartrate of lime is itself deposited, and afterwards washed. The slimy deposit from the first mentioned vat is pressed by steam or compressed air through a filter press, such as is used in sugar works, and afterwards the tartaric acid contained in it is recovered. The press residue can be used for Frankfort black, or for potash making.
(ii.) With Dry Lees.—The pressed lees is cut into pieces the size of a fist and dried in the air, and then serves for material in autumn and winter when no wet lees is to be had. Before making it into tartrate of lime it is generally ground to powder in a meal mill, about which there is no difficulty. From 18 to 25 cwt. of this ground lees are put into a vat holding 2000 to 3500 gallons, the is filled up with water, 50 to 100 lb. of crude muriatic acid are added, and the mixture is stirred up and heated by steam till it nearly boils. When this point is reached, the agitator is stopped, the mix ture is allowed to settle, and is then treated as in the former method.
The tartrate of lime procured from lees is distinctly recognizable from that obtained from tartars ; it is cleaner, whiter, and easier to wash ; the tartaric acid made from the lime tartrate thus produced is considerably more crystallizable, while the gypsum resulting from the decomposition with sul phuric acid is also much whiter, and separates out much more easily than that yielded in the treatment of tartars.
The treatment of the residues for making Rochelle salts, &c., is analogous to that adopted with lees.
It is not necessary that the manufacture of the tartrate of lime and the tartaric acid be carried on at the same place ; on the contrary, the tartrate of lime is often separated first, on account of the cost of transport of the bulky lees. In this case, the tartrate is pressed and dried, because it ferments easily when preserved in a wet state.
2. Extracting the Tartaric Acid from the Tartrate of Lime.—To get the acid out of the tartrate of lime, the latter is treated with sulphuric acid. Theoretically, to decompose 9.4 parts of tartrate of lime, 4.9 parts of monohydrated sulphuric acid aro necessary, but practically more is needed. This is because tartaric acid crystallizes much better in a solution containing strong mineral acids, while small quantities of non-decomposed tartrate of lime, or of sulphate of potash resulting from an im perfect washing of the tartrate of lime, greatly impede the crystallization. When the tartrate of lime is freshly prepared, sometimes as much concentrated sulphuric acid may be added as is equal to the amount of chalk used at the commencement of the operation. The tartrate of lime is gradually mixed with the sulphuric acid in a suitable vessel, and enough water is added to produce a semi-liquid paste ; it is heated to 75° (167° F.) by steam, and agitated by suitable apparatus. The mass foams considerably at first, but this soon ceases, when a sample may he taken for treatment with a solution of chloride of calcium, at about 38° Tw. (23° B.) The volume precipitated will suffice, after a little experience, to show when enough acid has been added. It is important that it should always be a little iu excess. The whole mixture is then put through filters of wood lined with lead, containing straw and felt. The filtered solution is heated by steam in leaden vessels, and will deposit still a little gypsum. When the liquor is concentrated, the temperature must not rise above 70° to 75° (158° to 167° F.), as the sulphuric acid would then exercise a carbonizing influence on the tartaric acid.
When the liquor reaches about 75° Tw. (40° B.) it is put to crystallize in large tanks of wood, lined with lead, or in earthenware pots. The crystallization proceeds three times as rapidly in the latter as in the former, but the crystals are very small, which is of no importance, however, if they be still coloured. The mother liquors are then evaporated a second and a third time. The crystals obtained aro passed through the turbine, re-dissolved, the solution decolorized and filtered through wicker-work, a little sulphuric acid added, the solution evaporated down to about 63° to 75° Tw. (35° to 40° B.), and crystallized out in leaden vessels. Very fine needle-shaped crystals are thus obtained. These aro passed through the turbine, dried and sifted. They always contain a little lead and sulphuric acid. For pharmaceutic purposes they are re-dissolved without addition of sulphuric acid, evaporated to 63° Tw. (35° B.), and crystallized in earthenware jars. It then suffices to dry the crystals in the air. These crystals are distinguishable in form and size from the others, the hemihedral faces are less developed, and they contain less lead and sulphuric acid. As to the residue of the sifting, this is reduced to powder, and appears in commerce as " prepared " tartaric acid. This is generally the most impure.
3. Refining the Tartar.—Tho crude tartar is pulverized in large vessels with water and hydro chloric acid, and dissoved by boiling ; bone black is added, and the liquid is filtered under pressure. This yields a tartar which, after crystallization, may be considered as pure. Excess of bone black must be avoided, for otherwise the crystals will be grey. .
Much commercial unsifted tartar is adulterated with sand, or with dry lees, and many tartars contain 10 per cent. of tartrate of lime.