Pagina-afbeeldingen
PDF
ePub

as fast as it is given off from the wet colour. It should be borne in mind that the colour, just as it comes from the filters or

[graphic][subsumed][subsumed]

Fig. 31.-Drying stove for pigments.

presses, may contain from 25 to 50 per cent. of water; if, by any means, this water is prevented from escaping from the colour,

then the drying is retarded; or if it is prevented from readily escaping from the stove, it is liable to condense on the inside of the roof, and to fall down in drops on to the colour below. In some cases, e.g.,. chrome-yellows, these drops are apt to produce spots on, and discolouration of, the pigment which is being dried. The mc e freely the water vapour can escape into the atmosphere the less chance there is of such mishaps occurring. D is a door for filling the stove, and G, G skylights.

In dealing with barytes and china clay, special forms of drying stoves have been described.

Some pigments, like the two just mentioned, the oxide reds, burnt umbers, burnt siennas, ultramarinė, Guignet's green, are

Fig. 32.-Drying stove for pigments.

capable of standing a high temperature without being altered in shade; these may be dried in a stove heated to a high temperature, in which case the drying is done quickly. On the other hand, certain colours, such as the chromes, Prussian blue, emerald green, &c., must be dried slowly; for such colours the stove shown in Fig. 32 would be very useful. The two sides not represented in the drawing are of brick, and support the roof.

Stretching from side to side are a number of iron shelves just far enough apart to take an earthenware pan and leave a little space between it and the shelf above. These shelves do not stretch completely from back to front, but, as shown in the drawing, they are arranged to come alternately flush with the front and back, the side of the shelf nearest the front and back each shelf being turned up to form a flange. The front and back of the stove are made of a number of iron plates, which form a series of doors to the shelves, the top of the plates being bent over to catch on the flange of the shelf above, as shown in the drawing; it is not necessary that the doors should fit air-tight. A fan at the top of the stove creates a current of air through it, a chamber at the bottom is kept hot by steam pipes, or flue from a furnace; through this chamber passes all the air that is allowed to go into the stove; this hot air passing over and under the colours dries them, and, being hot, absorbs and carries away the water vapour liberated from the wet colours. This stove is effective and economical, and is so constructed that the pans of colour can be readily removed and the shelves quickly refilled.

A drying stove has been constructed in the following manner. A cylindrical vessel was constructed of iron plates of any convenient size. This was divided into three chambers by two perforated iron plates; in the central chamber, which is the largest, is placed the material to be dried; the bottom chamber is kept hot by means of steam pipes, and is provided with an opening to admit air. The upper chamber is fitted with an exhaust fan, so arranged as to draw the air out of the central chamber; the perforations in the plate dividing the central from the top chamber are larger than those in the plate dividing the bottom from the central chamber, the consequence being that the air is drawn away from the central chamber faster than it enters from the bottom hot air chamber, so that a partial vacuum is created in the central chamber which is beneficial to effective drying.

Messrs. W. J. Fraser & Co., Engineers, of 98 Commercial Road, London, E., make a drying machine (nominally for white lead, but which may be used for other pigments) which is shown in Fig. 33.

The construction of the machine will be seen from the drawing. It consists essentially of a strong iron box, rather narrow in proportion to its length and height, this box being supported on girders. In this casing is a number of rollers opposite one another at each end of the casing. Between each pair of rollers is arranged an endless steel travelling-band

carried by chains; in the drawing a portion of one side has been cut away, so as to show these travelling-bands.

The

[graphic][ocr errors]

number of these bands can be varied; the larger the number the greater will be the drying capacity of the machine; although an increased number of bands will not materially increase the

Fig. 33.-Drying chamber.

[ocr errors]

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

« VorigeDoorgaan »