ing pipe, to the second mixing box, where it is again agitated and aerated, after which it passes to the second froth box. A commercial scale plant consists of about eight mixing boxes and corresponding eight froth boxes arranged alternately in series. The circulation of the pulp is illustrated diagrammatically in figure 2.

FIG. 2.-SHOWING OPERATING DETAILS OF THE FROTH FLOTATION PLANT
FOR THE SEPARATION OF COAL.

The first froth boxes recover the highest grade coal, and the later boxes the poorer grade material. The remainder is discharged from the bottom of the last froth box. Hence, by a suitable arrangement of receiving launders, one or more products may be made as follows:

1. If the froths from all eight boxes are received by one launder, two products will be made :

(a) Clean product of high grade.

(b) Remainder of no value.

2. If the froths from the first boxes are received by one launder,

and the froths from the later boxes by another launder,

three products will be made:

(a) Clean product of high grade.

(b) Middle product.

(c) Remainder of no value.

The clean coal froth, as removed from the flotation plant, contains about equal weights of coal and water. A method of dewatering this froth has been developed by Messrs. Minerals Separation, Limited, and, as it has been successfully applied on a large scale in England and France, the patentees have kindly supplied the following notes:

"In relation to the dewatering of coal concentrates, the method to be recommended depends on whether it is desirable to make the concentrates into durable briquettes to be subsequently used as fuel, or merely to prepare the concentrates for subsequent coking. In any case, the treatment depends on the fact that, when the coal particles have been superficially coated with a thin layer of tar or the like, their surfaces are no longer wettable by water. As a consequence, the water entangled amongst the particles can be readily pressed out, leaving a product which is relatively free from water.

In the briquetting process, soft pitch in a molten condition, or a molten mixture of tar and hard pitch, is sprayed into the concentrate pulp, which may contain equal weights of coal and water, the pulp being agitated meanwhile. As a consequence the particles become superficially coated with a layer of binder and, on raising the temperature of the pulp to about 60°C-100°C., an intense flocculation is produced. If the flocculated pulp is pressed in a mould provided with means for the water to escape, a hard durable briquette, containing less than 10 per cent. of water, is produced. If the briquette is allowed to stand for a few hours, the content of water drops to 4-5 per cent.

If the coal concentrates are to be dewatered in preparation for subsequent cok. ing, the coating of the coal particles and then flocculation can be effected in cold circuit by the addition of tar, preferably in the form of an emulsion. The flocculated coal can be dried by pressing in moulds or by drainage on a porous belt; or the water can be pressed out between two porous belts which run between rollers."

For laboratory flotation tests, results of which are given later in this paper, a machine consisting of one mixing box and one froth box, and which is therefore discontinuous in operation, is used. The conditions are essentially similar to those of the commercial plant. Results approximating to those of the perfect separation have been obtained on many Indian coals, and equally good results could, presumably, be obtained in large scale operation.

The possibility of commercially cleaning Indian coals is not limited therefore, as hitherto, by practical difficulties, but by the financial aspect of the matter. The scope of froth flotation is determined, not by its ultimate capabilities, but by the constitution of the coals available for treatment, and by the effects of supply and demand.

CLEANING OF INDIAN COALS FOR COKING.

The nature of the coal, whether coking or non-coking, determines the uses for which it is suitable. The purposes for which the coal can be used determine the advantages to be gained by cleaning. The treatment of coal for coking presents the greatest advantages.

Coking coals are those of which the better constituents are strongly coking, and the inferior constituents are weakly coking or non-coking. In the first-class coking coals the proportions of the constituents are such that the coal gives a satisfactory coke. In the second class coking coals the proportions of the constituents are such that the coal gives a weak coke of high ash content. Some of the more inferior coking coals are of such quality that they may be regarded as commercially non-coking. The difference is merely one of a difference in the proportions of the constituents. Flotation can concentrate the better constituents, vitrain and clarain, and eliminate the higher-ash non-coking ones, durain and fusain. Hence the proportion of clean product, of a quality suitable for coking, which can be separated, depends only on the quality of the seam and its heterogeneous nature, i.e. its cleanability. Flotation clean products from second class coking coals have given cokes of excellent quality, and one of the largest iron and steel works in India has expressed the opinion that they are as good as can be desired.

Flotation, or any other process, cannot separate a coking product. from a coal which does not contain any coking constituents India has large reserves of sub-bituminous non-coking coals. Their high moisture contents are an indication of the fact that they are constituted of vegetable products much less altered than is the case in the coking coals. The constituents of these sub-bituminous coals are all non-coking, and for this reason the coals are incapable of being cleaned to yield coking products.

CLEANING OF INDIAN COALS FOR BRIQUETTING.

In general the most inferior constituents of the Indian coals, although of fairly high ash content, have calorific values of 5,000 to 7,000 B. Th. U. For this reason cleaning of the Indian coals cannot appreciably increase their calorific values, and, unless the conditions are exceptionally favourable, cleaning for briquetting will not be profitable.

RESULTS OF FLOTATION TESTS.

The better constituents are floated first, and in the later boxes of the plant the froths are of increasingly inferior qualities. Hence the percentage of clean product from any particular coal determines the quality of the product. Results are set out to show this variation, and the quality of the remainder is also indicated. For the sake of simplicity only two products are given, but the question of whether two or three products should be made must be decided in particular cases when the specific requirements can be taken into consideration. The coals tested may be divided into five classes :

I. Coking coals of Jharia, Bokaro and Barakar.
II. Non-coking coals of the Raniganj district.

III. Coking Tertiary coals of Assam.

IV. Non-coking coals of the Central Provinces, Bihar and Orissa.
V. Non-coking Tertiary coals of the Punjab and the North-

West Frontier Province.

Coals which have not yet been tested will probably fall into one of these five classes.

I. COKING COALS OF JHARIA, BOKARO AND BARAKAR.

These are coking coals from Jharia, Giridih, Bokaro, and the Barakar area of the Raniganj field.

Typical analyses are as follows:

Results obtained from face samples, representative of run-of

mine coal, are set out below:

Ref. No. 236. "17 Seam," Jharia area. Ash 12.7 per cent.