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floor space required for the machine, it will add to the height. Underneath each travelling-chain there is arranged a steam coil shown in the drawing.
The white lead to be dried is fed on to the top travelling-band at the front end of the machine by means of a special hopper, which will be referred to presently. It is carried slowly to the back end of the machine, when the band stops being filled with wet white lead. The next band is treated in the same way, and so on throughout the series until they are all charged. Then the steam is turned on, and passed through the steam coils for 24 hours, by which time the white lead will be quite dry. The bands are then set in motion and they carry the white lead to a shoot at the back end of the machine, down which it drops into the truck or on to a creeper to be conveyed to the grinding mills. The bands are then ready to be filled again. This operation of filling, drying, and emptying is done day after day.
The feeding or charging apparatus consists of a steamjacketted hopper placed on trunnions. Connected with this is a steam-jacketted pipe or shoot, fitted with an endless screw. With this feeding apparatus any of the travelling-bands can be easily and rapidly fed with lead, while the material does not require handling in any way.
Each of the travelling-bands is fitted with clutch gearing, so that each can be started or stopped independently of the others, while each set of steam coils is provided with separate inlet and outlet valves, so that one or two bands may be worked independently of the rest if so desired, while should a band break down or a steam coil begin to leak that particular portion of the machine may be stopped without affecting the rest.
Each band will hold about 15 cwts. of pressed damp white lead. A machine having 10 bands will dry about 40 tons of pressed white lead per week. The steam consumption, as compared with the old driers, is considerably less, which makes the cost of working much less, while the drying is uniform and regular, the entire apparatus being easily worked by one man, and is moreover easily accessible for repairs, which are, however, rarely wanted in practice.
In practice, pressed white lead will be sent into the machine to be dried, but it may be sent in in a semi-liquid condition, but in this case the output of dry white lead will necessarily be less, while the larger proportion of water will cause a larger consumption of steam to dry it. However wet the material
may be which is being dried, there is no condensation of steam on the bands, owing to each being provided with a separate set of steam coils, while all the water vapour which is driven off from the wet colour is removed by means of ventilators at the top of the machine.
The advantages of this machine are the non-handling of the material in the process of filling or drying and the consequent saving in labour. This has also a sanitary aspect when such poisonous materials as white lead are being dealt with. The danger in these arises from a handling to which they are subjected during the process of manufacture, such handling always resulting in the hands, clothes, &c., of the work-people becoming impregnated with it, sooner or later leading to lead-poisoning; if, then, the handling be reduced to a minimum, then this risk of poisoning is reduced to a minimum also, and this is what this new machine does in the operation of drying white lead.
In Fig. 34 is shown the plan of a drying stove devised by the Sutcliffe Engineering and Ventilating Co., of Manchester. It
Fig. 34.-Sutcliffe drying stove. takes the form of a brickwork chamber divided into three compartments by the partition walls, E, E. In the smallest of these is fitted a steam heating apparatus, B, consisting of a cylindrical boiler through which pass pipes from end to end. Into the space surrounding the pipes steam is sent. By means of a fan, A, air can be sent through the pipes, and, naturally, this becomes heated. In the other two compartments are fixed racks, D, D,
on which the pans or trays of colour to be dried can be placed. These racks stand away from the partition walls, and so leave a space which is in connection with a flue to carry away the air and water vapour with which it becomes charged. The hot air from the heating apparatus, B, passes along the passages, C,C,C,C,
Fig. 35.– Vacuum drying chamber. as indicated by the arrows,over the pans of colour, drying the latter in so doing, and then away by the exit flue. This stove is very efficient and not expensive to construct or maintain in operation.
For the purpose of drying such pigments as lakes, where the colour is liable to change if the heat applied be too great, there may be used the vacuum drying chamber shown in Fig. 35, and
made by Emil Passburg, of Berlin. In consists of an iron box in which are fitted a number of iron shelves, made hollow and containing pipes through which steam or hot water can be sent from the steam supply pipe shown at the right of the drawing. The chamber is in communication through the valve on the top of the chamber with an air pump, by means of which the air from the inside of the chamber along with the water vapour given off from the wet colour is continually drawn off. The heat maintained ranges from 95° F. to 120° F., and even at that low heat, which will not affect the tint or hue of any pigment, the drying is very rapid. The colour to be dried is placed in pans on the shelves. The charging of the chamber is easily done, and the working is clean. The temperature can be regulated by the valves on the steam pipes, and the extent of the vacuum, which can reach 26 to 28 inches of mercury, can be regulated by the working of the air pump. The chamber is made in various sizes, from one having 32 feet of heating surface to one having 854 feet.
In any stove the colours are best placed in earthenware pans of about 12 to 16 inches in diameter, and 3 to 6 inches in depth; smaller pans may be used, but it is not advisable to exceed the sizes just given. Pans made of galvanised iron have been used, but these are liable to rust and so lead to discolouration of the pigments dried in them; enamelled iron pans, which can now be bought at a reasonable figure, are well worth a trial as being lighter and less liable to break than earthenware pans.
PREPARING PIGMENTS OR COLOURS BY
Many colours-the chrome-yellows, Prussian blues, Brunswick greens, lakes, &c.—are prepared by a process of precipitation, the principle of which is that when two or more substances in the state of solution are mixed together a reaction sets in—what the chemist calls double decomposition occurs—and new products are formed; one of these being insoluble in the liquid used is thrown down or precipitated out of the solution, usually in the form of a fine powder. Thus when to a solution of nitrate of lead, one of chromate of potash is added, a yellow powder falls down; this on examination is found to be chromate of lead, while the liquor contains nitrate of potash in solution; thus there has been an exchange of constituents, the chromic acid has left the potash to form lead chromate, while the potash has combined with the nitric acid of the lead nitrate to form nitrate of potash ; the
chromate of lead forms a precipitate because it is insoluble in water.
Put into the form of a chemical equation this reaction is expressed as:
Pb2 NO, + K, CrO, Pb Cr 0 + 2K N 0g.
chromate. Another example of precipitation met with in colour making is that of zinc sulphide, from solutions of zinc chloride and sodium sulphide, which is represented in the following equation:
ZnCl2 + Naz S ZnS + 2 Na Cl.
sulphide. sulphide. Here, again, there has been an interchange of constituents, and the zinc sulphide being insoluble is thrown down as a precipitate.
As precipitation is a chemical reaction it always takes place in fixed and definite proportions; thus in the preparation of chromeyellow it is found that 331 parts of lead nitrate interact with 194 parts of potassium chromate, the result being that 323 parts of lead chromate are precipitated while 202 parts of potassium nitrate are left in solution; should the salts be mixed in any other proportion, then one or the other must be in excess, and this excess will be wasted; thus, suppose 150 lbs. of lead nitrate and 95 lbs. of potassium chromate are used; the latter quantity is not sufficient to precipitate all the lead from solution; consequently, the excess, which is 7 lbs., remains, and is practically wasted. The necessity of using equivalent proportions of the materials is a matter of importance as regards economy in making colours by precipitation.
Every case of precipitation is a case of double decomposition, so that the main product is always associated with bye products, which are sometimes worth recovering, or which may be utilised in other ways. Thus, in making chrome-yellow by the process mentioned above, potassium nitrate in solution is a bye-product; in places where fuel is cheap it might pay to boil down this solution and recover the salt. Then again, by paying attention to the bye-products, and their probable use in other ways, it is possible to effect economies in the production of colours. Thus, supposing lead sulphate is to be made, this can be done by precipitating a solution of lead acetate, with either sodium sul. phate or sulphuric acid, as shown in the following equations: