Fig. 44. - Colour mixing machine with automatic device for filling and emptying (Hoechst system).

Plate XVIII.

The provision of air for a dye works is therefore a considerable item of expenditure, and must be carefully estimated. A very satisfactory type of compressor and vacuum pump is that shown on Plate XVI, made by the Schweiz Lokomotiv und Maschinenfabrik in Winterthur (Witte's system, cf. also, p. 189).

The cost of the water used must also be carefully determined by the engineer by accurate measurement, as very large quantities of water are employed, particularly as cooling water for the condensers.

Duties of the Works Chemist. - The work of the supervising chemist is possibly the most interesting in the whole of the industry, as it is impossible to control chemical reactions merely by giving an order, but their course must be accurately followed the whole time and any deviations corrected. The chemist must be au courant with the whole process, and must know every stage of the manufacture in full detail. In this connection attention may be called to the remarks made on Benzo Fast Blue (pp. 144 - 5).

Manufacture. - The ordering of the necessary raw products is done by means of requisition forms which are usually sent to the Material Stores on the previous day, or occasionally to some other department of the works. The chemicals are brought to the shed on the evening before they are required so that everything will be ready when the manufacture is begun. The chemist is responsible for the products until the moment that they issue in the dry form from the shed. As many colours are sensitive to heat, and therefore require careful heating, their drying must always be supervised by the chemist so that he will always be able to give an account of the effect of the drying upon the strength and shade of a dye. Such cases have been dealt with in connection with Methylene Green and Azo Yellow.

Standard Dye-House. - The finished colour is sent directly from the drying shed to the dye-house, where a small sample is dyed out against the Type or Standard. The figures obtained are sent immediately to the management, the costing department, and the chemist concerned in the matter, so that all may be kept continuously informed. Frequently a dyeing test is carried out with a small sample of the colour taken directly from the filter-press, so that any faults may be recognized at that stage.

Drying. - In recent years vacuum drying chests have been coming more and more into use, as it has been shown that the steam consumption is less and the strength of the product greater. Plate XVI shows a modern vacuum dryer as used with various modifications. Stable intermediate products such as sodium β-naphthalene sul-phonate, and simple azo colours, can be dried simply on steam plates, or may even be dehydrated in tunnel-kilns on the counter current system, though here also vacuum drying is becoming more general owing to the saving both of time and space. The Badische Anilin- und Sodafabrik, for instance, make use of about 500 vacuum dryers, and have, so far as possible, given up the older system of drying.

In order to dry a product rapidly it must be ground up at least once during the drying. As much dust is formed during this breaking up of the press-cake, many drying sheds are provided with dust extractors.

A modern improvement is to condense the vapours from the drying chests so that the pumps do not suffer so much from the action of acid or alkaline vapours.

Standardization. - When a certain number of works batches have been dried, they are ground up and made up to a standard or type strength. The grinding and mixing is usually carried out in a special mixing department which is under the control of the dye-house. (This dye-house has no connection with the scientific and commercial propaganda dye-house, which is intended to serve quite another purpose.)

Grinding. - Nowadays the colours are ground up in modern centrifugal mills, such as that shown diagrammatically on Plate XVII. The capacity of such a machine exceeds that of the older edge-runner mill or ball-mill by some 10-50 times, whilst at the same time the particles are ground smaller. Many complaints of inadequate solubility of a product are to be ascribed to incorrect grinding, as, in the older types of apparatus, the substances were pressed together, thus producing almost shaly tablets of great hardness which dissolve only with difficulty.

Whenever possible the approximate necessary quantity of diluent is ground up with the dye so as to diminish the length of time necessary for the mixing. The concentrated colour is mixed with the standardizing material (Glauber salt, common salt, soda, or dextrin), and the mixture is run into the mill. The disintegrator illustrated has an automatic sieve and also magnets for the removal of iron particles, which are always present in the materials. The dye is broken up whilst in motion by the specially shaped grinding pins, and is whirled round and round until it passes through the sieve (Fig. 39). Owing to the centrifugal effect, much air is sucked in, which must be allowed to pass out from the apparatus again. Filter-bags (G, Plate XVII) in the form of piping permit the escape of the air, but keep back all the dust. The main portion of the powder is retained in the air chamber (F), the stream of air striking the walls tangentially. If very soft material is being disintegrated, such as β-naphthol or naphthalene, it is better not to have the sieve as it is readily stopped up. The ground products are carried by means of worm-conveyors direct into the mixing troughs, where they are well mixed up for several hours. Plate XVII (Fig. 42) shows a modern mixer which can be filled or emptied automatically by means of a reversible worm-feed. This type of mixing apparatus is made for dealing with quantities up to 4 tons, and is gradually replacing the older, uneconomical mixers, particularly when very large quantities are being dealt with. Simpler mixers are also made use of which are provided with compressed air and vacuum, like grain-silos. Certain dyes must be pulverized outside the grinding shed owing either to a danger of fire (picramic acid dyes) or owing to their unpleasant properties {e.g. Bengal Blue or Naphthol Blue, p. 173). As soon as the strength and shade has been passed by the dye-house as correct, the dye is sent off to the packing-house, from whence it is handed over to the Sales Department. The management, costing department, and works chemist are all informed of any matters of special interest such as good or bad yields or shades. The responsibility of the works chemist finishes with the delivery of his products whether dyes or intermediates.

Fig. 39. - Diagram of a "Perplex" disintegrator.

B. Feed-pipe. 1. Fixed grinding pins. 2. Rotating pins (1200-2000 rotations per minute). 3. Sieve.