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February, 1937

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A short time ago we had the privilege of visiting the ore testing station established at Billingham-on-Tees just over six years ago. Certain equipment was then provided for ore testing and ore research, and recently it was decided to augment the facilities by the installation of an ore-dressing plant for continuous milling. In this way laboratory tests with small samples of ores can be confirmed, when it is desirable, on a larger sample of ore. For a laboratory investigation not less than two cwt. of ore are required for a large scale test a minimum of two tons of ore is required. It is now possible to thoroughly investigate ore treatment problems of all kinds in this testing station and, it may be mentioned, this service is available free of charge. Such research work is strictly confidential to the parties concerned, all samples being known by numbers which are allocated to them on arrival at Billingham. No expense or liability is incurred by the senders of the ore other than the cost of freight on the ore samples from the mine to Billingham, and the service is largely worked through the associated companies of I.C.I. abroad and their agents.

Such a service is most helpful to mining engineers and mine operators ; for their benefit a questionnaire is drafted on which full details of the actual problem or problems can be stated. As an example of this, the source of water should be stated and a sample or a complete analysis forwarded; any marked seasonal changes in weather should be mentioned and described. It is to the advantage of both sides to give the fullest information, in order to enable the metallurgists at Billingham to carry out the investigations expeditiously. In addition to the investigation of problems from existing mines, the testing of ores from properties in the course of development is also undertaken. Various methods of treatment are thoroughly examined in order to determine the most satisfactory flowsheet and a report incorporating the results is sent direct to the owners. Strict priority is observed, the average time elapsing from the receipt of the samples till the results are established is five to six weeks.

There are four main laboratories and a general account of the work carried out in each is given below.

Small scale laboratory

In this preliminary investigation laboratory, great importance is attached to the mineralogical examination of the ore, which is carried out in considerable detail. Rock cutting and polishing equipment is available for the preparation of thin sections for examination in reflected and polarised light in order to assist in the determination of the rock-forming minerals. Microscopic examinations and chemical tests are extensively employed in the examination of ores, concentrates, tailings, and other products for the purpose of establishing the exact mode of association of the minerals. The following examples illustrate the valuable assistance which the microscopic examination affords in confirming the conclusions deduced from assays and experiments

In the flotation of a lead/zinc ore containing some copper and silver, the two last mentioned minerals are preferably recovered in the lead concentrate. In the actual tests, assays of the products showed that most of the copper and silver reported in the zinc concentrate and, in spite of the use of various combinations of reagents, it was not possible to improve the recovery of copper and silver in the lead concentrate. Microscopic examination of the concentrates showed that the copper and silver occurred as minute inclusions of tetrahedrite in the sphalerite, and this would have been liberated only by extremely fine grinding beyond the limits of economical working. Thus the microscope had revealed the cause of the high copper/silver content in the zinc concentrate and, moreover, had shown it was impossible in any event to accomplish the release of the tetrahedrite with economical grinding.

Another sample was received in a finely ground condition; it was required to separate lead which, from examination, appeared to exist in the form of galena, from the remainder of the mineral which consisted largely of cassiterite. A number of flotation tests were carried out, but it was impossible to reduce the lead content of the tin concentrate to the required limit. Microscopic examination of the concentrate revealed the fact that the lead existed as pyromorphite and mimetite these minerals are not floated by reagents which promote the flotation of sulphides.

Cyanidation, water concentration, straking, amalgamation, flotation, sizing tests, etc., are carried out in this small scale laboratory. The ore is crushed in a jaw crusher, followed by rolls, the discharge from which is ground in either pebble or rod mills to the degrees of fineness which have been found to be necessary by preliminary grinding tests and the microscopic examination of the products from sizing tests.

The cyanidation tests are conducted in either rotating bottles or agitated vessels. The period of agitation, amounts of reagents used, etc., are varied in order to determine the most suitable set of conditions for an y particular ore. In other words, the correct period of contact of the solution with the ore, and the most economical amounts of cyanide and lime are established. In addition to cyanidation, filtration and settlement tests are conducted to ascertain whether difficulties in these operations are likely to be experienced in practice. Numerous difficulties are encountered; it is, however, beyond the scope of an article to give adequate details of the methods adopted in dealing with such refractory ores.

Sizing tests

Particle size is a most important study in ore dressing since the reduction of ore to an appropriate size for treatment often forms the greatest single item of cost in the whole process of recovery. Sizing tests are carried out by means of a Rotap sieve shaking machine in which the sieves, either I.M.M. standard or Tyler series screens according to the country of origin of the ore, are shaken mechanically. Apart from the saving in time, this machine ensures consistent and reliable results as the personal equation is eliminated.

Elutriation equipment is available for dealing with particles whose size is below that which can be determined with any degree of accuracy by sieving. Such particle sizing by elutriation is to be preferred to microscopical sizing as it produces actual fractions which may be studied and assayed.

Flotation

Small scale flotation tests are carried out in either sub-aeration or pneumatic machines depending on the type of ore or flotation product which is being treated. In order to expedite investigations, three sub-aeration and one pneumatic machines may be operated simultaneously. Compressed air and vacuum, in addition to the usual laboratory services are constantly available.

Assaying laboratory

A special laboratory has been equipped for assaying. Where so many samples of different ores are under investigation at any one time, it is, of course, essential to observe the strictest precautions in order to avoid contamination of samples. For this reason a specially constructed room has been built; all mixing and sampling is undertaken by experienced assayers. The fusions are conducted in a gas-fired circular furnace which accommodates 24 crucibles simultaneously. Two gas-fired muffle furnaces are provided for cupellation, roasting, etc.

Chemical laboratory

Another laboratory is devoted entirely to the chemical examination of ores and products. Every ore which is submitted is subjected to a complete analysis in order to determine the proportions of the various minerals present in the ore. In addition to such complete analyses of ores, analyses of mine waters, flotation and gravity concentration products, cyanide solutions, continuous plant products such as pregnant solutions, zinc-box precipitates, etc., are continually required.

Continuous plant

This plant has been erected in a building designed and built specifically for the purpose. Storage bins, crushing, and grinding equipment are isolated from the remainder of the building in order to avoid any dust, produced during crushing and grinding operations, from contaminating the rest of the plant. Particular attention has been given to the arrangement of the machinery; every item is easily accessible to enable the frequent adjustments necessary in testing work to be carried out immediately. All the equipment has been so designed that it may be dismantled and cleaned at the end of each run with the minimum of inconvenience and delay. The equipment comprises plant for crushing, grinding, elevating, pumping, cyaniding, precipation, filtration, thickening, amalgamation, straking, flotation, and water concentration. All the individual items can be connected by means of flexible hoses ; this enables any type of flowsheet to be investigated.

The procedure adopted when a sample of ore is received is as follows :– The ore is first of all reduced to minus 1 in. size (if composed of large and small pieces), thoroughly mixed, coned and quartered down in order to obtain a representative sample of 20 to 30 lb. weight for the initial laboratory investigations. After this sample has been taken out, the remainder is fed to the jaw crusher, set to approximately in., and thence to the rolls. This gives a product of roughly minus five mesh and is taken by elevator to a storage bin in the main section of the building. Ore from this bin is released by means of a plate and knife type control feeder and fed to the ball mill which can be operated either in open or closed circuit with a Dorr classifier ; the oversize is returned to the ball mill. If the ore contains coarse gold, the ball mill discharge can be passed over amalgamation plates or corduroy strakes. Otherwise it is pumped to settling tanks where the surplus water is decanted. The thickened pulp is then available for treatment by flotation, water concentration or cyanidation. For instance, if the latter process is necessary in the case of a gold-bearing ore, the thickened pulp is transferred to a tank which has a capacity of one ton and is fitted with a mechanical agitator and air lift. The cyanide solution is added and the period of agitation may vary from 5 to 50 or 60 hours, depending entirely on the ore being treated. A filter press separates the gold bearing solution from the now relatively valueless rock and the solution is passed through zinc boxes in order to precipitate the gold. This zinc box precipitate is treated with sulphuric acid to remove the excess zinc and the residue, after being dried, is smelted with fluxes determined according to the impurities present. Further instances, all of extreme interest, could be given but sufficient has been said to indicate the value of the service available in ore treatment problems.

We have pleasure in acknowledging the courtesy of Imperial Chemical Industries Limited, in granting us facilities to visit their ore testing station.

Drawings and Photographs accompanying the article