SYNOPSIS

Benefication Coal

Benefication Coal

Various methods for the beneficiation of minus 0,5 mm coal are examined briefly, and tests are described on a cyclone of 150 mm diameter operating with magnetite as the medium.

It was found that, for sharp separations, at least 50 per cent of the magnetite should be finer than 10/xm. At that size, the ash content of the product was about 7 per cent at yields of 59 to 64 per cent, and the loss of medium was reasonable (about I kg per ton of fines treated).

SAMEVATTING

Verskillende metodes vir die veredeling van steenkool kleiner as 0,5 mm word kortliks ondersoek en toetse met ‘n sikloon met *n diameter van 150 mm wat met magnetiet as medium werk, word beskryf.

Daar is gevind dat vir duidelike skeidings minstens 50 per sent van die magnetiet fyner as 10/xm moet wees. Met daardie grootte was die asinhoud van die produk ongeveer 7 persent met opbrengste van 59 tot 64 persent, en die mediumverlies redelik (ongeveer I kg per ton fynkool behandel).

Introduction

The need to utilize energy resources judiciously is of global concern, and because coal is South Africa’s main source of energy, the total utilization of coal (i.e., its mining, beneficiation, transportation, and usage) must be optimized.

Owing to the difficulty of cleaning minus 0,5 mm coal, the present practice is either to dump it or to add it to steam coal, despite the fact that these fines constitute a potential source of high-quality coal. Provided that the minus 0,5 mm coal can be upgraded economically to a coal of low ash content (about 7 per cent), it could be used as a blend coking coal.

Quality of Fine Coal

The results of washability tests on minus 0,5 mm coal indicate that an average yield of about 50 per cent and an ash content of 7 per cent can be obtained. Typical washability results for fines from Witbank No. 2 Seam are given in Table I, The minus 0,075 mm fraction amounted to 15 per cent of the minus 0,5 mm material and had an ash content of 25 per cent.

Table I indicates a theoretical yield of 58 per cent at an ash content of 7 per cent. If the organic efficiency is as low as 69 per cent, a practical yield of at least 40 per cent can be expected. It is conservatively estimated that about 3 million tons of this minus 0,5 mm coal are generated in South Africa annually. If these fines were prepared at the rather poor efficiency mentioned, 1,2 million tons of high-quality coal would be potentially available to industry per annum. Even at the low price of R15 per ton, this coal is worth R18 million per annum.

Difficulty of Cleaning

The washability results in Table I indicate that, at the relative density required to produce a product with an ash content of 7 per cent, the near-density material § would amount to about 80 per cent. Coal from Witbank No. 2 Seam averages roughly 50 per cent near-density material, but this can range from about 20 per cent to as high as 80 per cent.

In 1928, Bird1 related the degree of separating difficulty to the near-density content of coal, stating that, if this content exceeds 25 per cent, the separation becomes formidable. Clearly, this is indicative of the extent of the coal-preparation problem in South Africa.

During the 1976 Symposium on Coal Research in South Africa, Horsfall aptly described the beneficiation of fine coal in South Africa as follows: ‘With fine coal drawn from seams having fairly easy washing characteristics, all available methods work; but with the more difficult coals, only dense medium processes will, in a single stage, give an efficiency of the order required’.

As far back as 1949, Van der Walt2 indicated that reasonably sharp separations of minus 0,5 mm material could be effected in a dense-medium cyclone, and in 1977 Fourie and Erasmus3 indicated that an ecart probable value of 0,06 or better must be selected as the ideal in South Africa for units to be employed in the production of coal with an ash content of 7 per cent.

Other Beneficiation Methods

Froth flotation is probably the best-established method of cleaning minus 0,5 mm fines and is in extensive use abroad. In South Africa however, froth flotation was confined until recently to the upgrading of coking-coal fines. Although these fines are classified, according to South African standards, as being amenable to froth flotation, the process is far from straightforward. Studies directed towards a search for more selective reagents included various frothers4, like Aerofroth 65, Aerofroth 73, Aerofroth 77, pentanol, tri-ethoxy-butane (TEB), crude tar acids, and eucalyptus oil, in conjunction with paraffin as collector. Eucalyptus oil appeared a very promising reagent, even when used on its own. It was also found effective for certain weakly coking coals.

Other collectors5*6, such as kerosene, dieselcne, and turpentine, were investigated, but the outcome was not entirely satisfactory. It is possible that more careful studies would reveal more effective reagents, and this is being pursued.

In contrast, the flotation of non-coking coals proved disappointing in the extreme. Their poor floatability is attributed to the small amount of ‘bright’ coal they contain.

As a means of cleaning fine coal, water-only cyclones appear attractive for two reasons: they offer likely capital-cost advantages, and they can easily be integrated in existing dense-medium washing plants. However, it has been found7*8 that the accuracy of separation is not as a rule sufficiently high to enable this type of separator to rank as an alternative to any of the established dense-medium processes for small coal. A degree of cleaning is undoubtedly achieved, but the ecart probable is about 0,25.

Tests have indicated that the water-only cyclone operates both as a density separator and as a classifier. This results in the cutpoints of various size fractions of the feed being distributed over a wide range of relative densities, and the overall cutpoint is noticeably higher if a considerable amount of fine material is present. The separating effect in the minus 150 /xm fraction is almost negligible, and that on the coarser fractions is merely an ash reduction; for example, the ash content can be reduced from 17,6 to 15,4 percent at a yield of 75,2 per cent, while the theoretical yield according to the washa-bility characteristics is as high as 94,0 per cent at the same ash content.

Tests on the compound water cyclone9 or tricone produced promising results. For example, a raw coal smaller than 0,5 mm with an ash content of 18,2 per cent was washed to give a product with an ash content of 12,8 per cent. The ash content of the plus 150/xm size fraction was found to be 9,4 per cent, and that of the minus 150 ftm fraction was still as high as 16,7 per cent. Appropriate desCiros can be applied, but that leaves the fraction betwoen 150 and 75/xm almost untreated. It is therefore considered unlikely that the hydrocyclone will find wide application in the benefieiation of minus 0,5 mm coal.

Concentrating tables are widely used in the U.S.A. but thoy are not yet in commercial use in South Africa. Research10, however, has indicated that the low capacity of those units can be a major drawback: a quarter-size table cannot handle much more than 0,5 t/h if coal of 7 per cent ash is to be produced from coal smaller than 1 mm. Ecart probable values were about 0,08 with serious ‘tails’ increasing the ash content, and the organic efficiencies were as low as 72 per cent.

Various combinations of flotation, water-only cyclones, and tables were investigated, but a product of about 7 per cent ash could be obtained only when the table was used as the second-stage unit. However, this configuration has the disadvantage of low capacity.

Work Done Elsewhere

In the U.S.A., Deurbrouch11 has shown the suitability of the dense-medium cyclone for washing down to 0,075 mm. Sokaski and Geer 12 have made similar reports, and Mengelers and his co-workers13’14 have produced several papers on this subject.

In 1957, the Dutch State Mines and Coppee designed and built two dense-medium cyclone plants in Belgium, at Tetre and Witerslag, which treated a feed smaller than 10 mm with effective cleaning to about 0,15 mm. Similar plants followed in the U.S.A., but, although encouraging, the separations were still not of the required efficiency and sharpness15.

Unfortunately, most South African coals have difficult washability characteristics. Recourse had therefore to be made to the dense-medium process for the cleaning of minus 0,5 mm coal, but the process had first to be refined to suit the circumstances.

Pilot Plant

With this objective, the Fuel Research Institute in 1976 erected a dense-medium cyclone pilot plant for the beneficiation of minus 0,5 mm coal, and commissioned it during the first quarter of 1977. Fig. 1 is a simplified diagram showing the flow of the solids and the medium in the pilot plant.

Raw coal nominally smaller than 0,5 mm is deslimed in a cyclone and dewatered on a screen. The fraction between 0,5 and 0,075 mm is mixed with correct dense medium, and is pumped to a separating cyclone 150 mm in diameter. Magnetite is recovered from the washed coal by a bank of magnetic separators, the cleaned coal being thickened in a cyclone and finally dewatered on a screen. The discard from the separating cyclone is treated in a similar way. Each bank of magnetic separators comprises a rougher unit feeding to a cleaner unit, the underflow from both reporting to a scavenger unit.

Tho following are some of the characteristics of the separating cyclone:

  • Coal feedrate :5 t/h
  • Pulp feedrate :35 000 1/h
  • Operating pressure range: 85 to 150 kPa
  • Diameter : 150 mm

Pilot-plant Test

Fairly coarse magnetite was used in the beginning as shown in Table II.

Although the separations were promising (Table III), fairly high losses of magnetite (about 2 to 3 kg per ton of feed coal) were recorded. These losses relate to magnetite that adhered to the products, and exclude any losses in the effluent, which were not measured.

In an attempt to improve the separating performance, much finer magnetite was employed, the size grading shown in Table IV being typical. The plant efficiency improved dramatically, but, as the very fine magnetite was gradually lost from the circuit, the separations deteriorated. The results of Table V are typical of that stage of the investigation.

In Tests 2 and 3, the very fine magnetite was still present, whereas in Test 1 it had been lost from the circuit, indicating that the fineness of the medium is of prime importance for efficient separations.

A batch of extra-fine magnetite with only 2 per cent oversize material at 27 /xm was obtained and used, resulting in very sharp separations. However, the magnetite losses were beyond control, and it was decided that this aspect should be investigated further. A method of freeing the clean coal and discard from the adhering magnetite and of recovering the very fine magnetite from the effluents by a Magnadisc system proved to be an effective solution to the problem.

The stage had now been reached where sharp and efficient separations could be obtained at a wide range of relative densities (1,40 to 1,90) and at an acceptable loss of medium. However, all the tests had been conducted over short test periods (about an hour’s duration), and continuous runs of, say, eight-hour shifts for several days were considered essential to prove the suitability of the process for commercial operation.

It was also decided that more efficiency parameters should be included in the studies. The drawback of the 6cart probable concept is that it takes no cognizance of the asymmetry of the partition curve, and the ‘tails’ are also frequently ignored. For abetter understanding of the performance of the unit, it was decided that the following parameters should be evaluated in all future tests:

  • partition density and equal errors outpoint,
  • ecart probable values at partition coefficients of 25/75, 90/10, and 95/5,
  • organic efficiency,
  • misplaced material in the product and in the discard.

It was further decided that, before a continuous run was attempted, a systematic study should be made of the optimum magnetite fineness for the process.

As a first step, ready-ground very fine magnetite commonly supplied to the market was used as a medium. With this magnetite the difference between the overflow and the underflow densities of the separating cyclone were found to be about 0,9. Under these circumstances, poor performances were recorded.

By the use of progressively finer magnetite, the performance improved, and after considerable testwork it was decided that the size gradings shown in Table VI are ideal.

A bulk sample of minus 0,5 mm fines from Witbank No. 2 Seam weighing several hundred tons was obtained, and the plant went into continuous operation for some seventy hours. The results of four tests carried out during this period are given in Table VII, showing that excellent performance was obtained at a low consumption of medium.

Conclusion

It can be concluded that the ready-ground magnetite as supplied to industry at present is not fine enough for the beneficiation of minus 0,5 mm coal by dense-medium cyclone. For sharp separations, at least 50 per cent of the magnetite should be finer than 10 /xm. However, if the magnetite is milled too fine, the losses may become excessive.

With magnetite milled to the required specification, the losses are reasonable, at an average of about 1 kg per ton of coal treated. An average of 0,7kg per ton of coal treated for adhering magnetite is probably as low as one can expect. The loss in the plant effluent, which averaged less than 0,3 kg per ton of coal treated, appear exceptionally good if it is remembered that it consists of magnetite substantially below 7,5 pm in size.

In view of the excellent separations and low losses of medium achieved over the continuous test run, it was concluded that the process was ready for commercial installation. The first plant using this process is currently under construction at Greenside Colliery. It is being built as an integrated extension to the existing plant on No. 2 Seam. Provision is being made to produce both low-ash coal and middling from a feed that is nominally between 0,5 and 0,075 mm at a maximum feed rate of 45 t/h.

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