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March, 1948

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Certain minerals, often termed the non-metallics, are used in the main, not as a source of any metal but in the natural state, and the dressing problem is normally to raise the grade to the requirements of the trade for which the mineral is required, or to eliminate small quantities of impurities which are objectionable. Furthermore, since many industrial minerals are of small value compared with the metallic minerals, the need for making a dean product frequently overshadows the importance of recovery.

Such non-metallic minerals include Asbestos, Barite or Barytes, Clay, Corundum, Diamond, Felspar, Fluorite, Garnet, Graphite, Kyanite, the Micas, Phosphate Rock and Apatite, Talc and Vermiculite.

Asbestos

Asbestos is the commercial name given to the fibrous variety of several minerals, but Chrysotile, 2H₂O 3MgO 2SiO₂, a fibrous form of serpentine, yields the bulk of the world's supply.

Anthrophyllite (Fe, Mg)O SiO₂, an amphibole, is brittle and possesses a low-tensile strength, but is more resistant to heat and acids, being used for special purposes, whilst Amosite, an iron-rich variety, possesses much longer fibres and may be readily spun. Crocidolite, NaFe (Si0₃)₂ FeO SiO₂ is worked in South Africa, and fibrous Tremolite, CaO 3MgO 4SiO₂, is produced in Italy, both being commercially termed asbestos.

The leading producing countries are Canada, the U.S.S.R. and Africa, in order of importance.

Extra long and first quality long fibre asbestos may be hand sorted and beaten out to separate the fibre. The usual treatment, however, consists of stage crushing and screening, the fibres being removed after each reduction during the screening. Primary crushing is carried out in the usual manner, using jaw or gyratory crushers, followed by "reduction" gyratory crushers or hammer mills for further reduction, the rock being dried at some point during the coarse crushing operation and the fibres being removed by means of fish-tail suction pipes fixed over shaking tables, with plane surfaces of steel or over screens, whilst the sand and waste passes through. All crude fibres from these aspirators are further screened and cleaned, often using trommels fitted with light wooden paddles which rotate inside the cylinder to agitate the fibres.

Barytes

Both Barytes (or Barite, BaCO₃) and Witherite (BaSO₄) are used extensively for a wide variety of purposes. The former is a white mineral, occasionally stained a reddish colour or darkened by carbonaceous material, with a sp. gr. of 4.5, and is commonly found as the gangue mineral in lead and zinc ores, this type of occurrence being usual in Britain and Germany, In the United State, however, almost the entire production is derived from replacements in dolomite and limestone.

Witherite is less common but is more valuable as it carries a higher percentage of barium, and, being soluble in acids, is in greater demand for the manufacture of certain barium salts. Its specific gravity is slightly lower than that of barytes at 4.29 to 4.35.

Barytes is employed as an extender in paint, a filler in rubber, asbestos, paper, etc. as a protection against X-rays, in fluxes for brass, in drilling muds, in sealing wax, and as a medium in De Vooys process for coal cleaning.

A very considerable quantity is used in the manufacture of paint due to its covering power and inert nature, and should not contain less than 93 per cent. BaSO4, but better grades often contain over 95 per cent. Lithopone, a mixture of precipitated barium sulphate and zinc sulphide, is also used extensively in paint manufacture and is prepared by mixing barium sulphide with zinc sulphate.

Witherite is largely employed in the case hardening of steel, in glazes, in the glass industry, in certain refractories and for water softening. Metallic barium also finds uses in alloys with aluminium and in bearing metals, whilst amongst the compounds, barium peroxide is employed in the manufacture of hydrogen peroxide, barium chloride in the dye industry, and barium hydroxide in beet-sugar refining.

The treatment of Barytes and Witherite is usually restricted to hand picking, log-washing and concentration by jigging and tabling. Flotation, both with soap and cationic reagents, has been readily effected in the laboratory, but commercial plants have not been installed to any great extent.

As the specific gravity differential is not great in most ores, it is obviously important to employ close sizing if good results are to be obtained by gravity, whilst sink and float should also be practicable.

Indeed, a separation in a high density "quicksand" is actually being carried out at Malehurst Mine, Shropshire, and in the El Portal plant, in the States.

Frequently, sulphide minerals such as Galena, Sphalerite and Pyrite occur in association with Barytes and are separated either by gravity or by flotation. When Galena alone is present, it may be possible to eliminate a very high percentage solely by gravity means, the lead-rich concentrate being subsequently cleaned-up by flotation if necessary, but Sphalerite and Pyrite with specific gravities nearer that of Barytes can only be separated by flotation or possibly magnetic separation. When, as is generally the case, the quantity to be eliminated is small, the best procedure is to grind and float a rougher gravity concentrate.

Another common contaminant is iron oxide, and this is removed by magnetic separation using high-intensity deflection separators or by roasting and screening as the barytes tends to decrepitate and so forming a concentration in the fines, whilst the iron oxides and any silica remain in the coarser fraction.

The separation of fluorspar is also necessary in some instances, and may be accomplished in one of three ways: (1) using Sink and Float, (2) jigging with a closely-sized feed, and (3) employing flotation.

The first method is now being employed on a fluorspar mine in Derbyshire, the second is reasonably efficient provided the feed to jigs is closely sized or classified, but if the dissemination is fine, a combination of gravity and flotation may be most desirable.

Barytes is fairly readily floated from siliceous gangues, but few commercial plants are in operation, probably because, in most cases, adequate recoveries and grade of concentrate can be produced by simple gravity treatment.

A wide variety of collectors of the anionic type are suitable for siliceous ores, but the presence of Calcite or Fluorite makes the separation more difficult. Hammann suggested citric acid as a depressor for both these minerals as well as for silica, whilst Coghill and Clemmer found that Fluorspar and Calcite were not appreciably floated if sodium silicate were employed along with Emulsol X-1 (a sodium salt of a complex sulphated fatty alcohol). Clemmer and O'Meara also found that a silica gel restrained the flotation of Calcite and Fluorspar without retarding the Barytes.

They also reported that a combination of sodium silicate, copper sulphate and sodium dichromate, with a sodium oleate-oleic acid collector, yielded a 97 per cent. BaSO₄ concentrate, with a 82 per cent. extraction on a mixed ore. For Calcite alone, quebracho is an effective depressor, whilst the grade of concentrate can often be improved by adding some dichromate to the cleaner circuit. Numerous collectors include Tall oil, naphthenic acid, calcium lauryl sulphate, Emulsol X-1, Emulsol X-2, and Emulsol K-1301, Alphasol, Shell 795, Cyanamid reagent 825, as well as oleic acid and Reagent 708. Both barium and lead salts are reported as activators, whilst Aerosol wetting agents have been recommended as promoter-assisting reagents.

Pine oil, cresylic acid and alcohol reagents, such as B-23, are suitable frothers. Oleic acid is not a good collector to employ however, when the concentrate is to be used for oil-well mud because of its resistance to wetting, but Shell 795 is said to yield a more wet table material.

Barytes may also be floated with amine reagents when the recovery is aided by adding sulphate ions. On the other hand, barium chloride reduces the floatability and many silicate minerals may be floated from the Barytes with such a depressor, an amine and about 8 lb. of hydrochloric acid per ton.

When Fluorspar is present in any quantity, it may be necessary to use repeated cleaning and a combination of reagents; a good example of the treatment of such an ore is to be seen in the test work done by the Department of Mines, Ottawa, on an ore from Lake Ainslie district of Novia Scotia.

In this operation, the ore was ground in closed circuit, with a 48-mesh screen, the undersize being treated on a table, to remove as much of the barytes as possible, the table tailing being deslimed and treated by flotation, first to recover the barytes and secondly to recover the fluorspar.

The reagents used consisted of sodium silicate, barium chloride, citric acid and Duponal L.S. to the barytes conditioning tank, Duponal L.S. to No. 3 rougher cell and sodium silicate to the No. 1 cleaner cell.

For the Fluorite flotation, quebracho and oleic acid was added to the conditioning tank and further oleic acid to the flotation cells, whilst quebracho was introduced to the first cleaner cell.

In this operation, the sodium silicate acted as a dispersant, barium chloride as a froth modifier in conjunction with citric acid, in order to control the voluminous froth and increase the density of the froth with no apparent adverse affect on the floatability of the Barytes. Duponal was the collector for Barytes, which does not require a long conditioning time prior to flotation but is a very strong frothing agent relative to its collecting power, and, consequently, it is necessary to modify the froth with the barium chloride and citric acid. In the Fluorite flotation, quebracho is used to depress Calcite.

One of the few commercial flotation plants is that at Salair, U.S.S.R., which handles the tailing from a zinc flotation plant containing 60-65 per cent. BaSO₄ and 25 to 30 per cent. silica. About one to 1.5 kg. of naphthenic acid, and 0.3 kg. waterglass are used per ton, enough benzine being added to prevent thickening and poor dissemination of reagents in the pulp. The Barytes is stated to float equally well in an acid or alkaline circuit.

In order to obtain a first grade white, it is usually necessary to bleach the ground concentrate with hot dilute sulphuric acid in lead-lined tanks. In some cases, better colour is obtained by using hydrochloric acid, by adding sodium chloride to the sulphuric acid or by using dilute nitric acid, whilst in one instance the barytes is roasted with sodium phosphate in order to convert the iron present to ferric phosphate, which is colourless.

When acid "bleaching" is employed, it is either done by leaching coarse material or by agitation of the finer material. After this bleaching is complete, the barytes is washed and dried.

A good example of straightforward gravity concentration of Witherite is to be found in the plant of the Settlingstone mine, where the ore is hand picked after being broken to 6 in., then crushed to 1 in., the products being sized and concentrated on jigs and tables, as shown in Fig. 1.

Another purely gravity plant is to be seen in that of the Devonshire Baryta Co. Ltd. Here the ore body consists of Barytes associated with shale and a little Pyrite, Galena and traces of Sphalerite and is first sized in a trommel, the oversize crushed to about 1 in., then sized in trommels. The first trommel is fitted with a 1 in. screen, the oversize being passed to rolls which are closed with the trommel, whilst the undersize is fed to a second trommel. In this second unit the material is sized into five products namely, minus 1 in. plus ½ in., plus 3/8 in., plus ¼ in., plus 1/8 in. and minus 1/8 in. The four coarse sizes pass to modified Harz jigs, designed on the mine, in which are incorporated special gate discharges for the concentrate, as well as elevators to lift the concentrate from the hutch and deliver it to the loading bins. The under 1/8 in. material is de-watered and de-slimed in a pyramidical tank, then classified into seven products by specially designed hydraulic classifiers, each product being treated on a table via a de-watering box. The tables, seven Wilfley and one James, make four products, a lead rich concentrate which is cleaned on a further table, a concentrate of Barytes which is de-watered in spiral de-watering classifiers before passing to the loading bins, a middling which is retabled and a tailing which is rejected. The flowsheet is given in Fig. 2.

In Germany, most of the plants employ simple gravity concentration, although at Meggen, in Westphalia, the fines, representing about 10 per cent. of the output, are being treated experimentally by flotation.

A typical example of a German plant is that of the Deutsche Baryt-industrie at Bad Lauterberg.

Here the mine ore is crushed at the mine, being then transported in eight-ton side-tipping wagons to the mill, where it is first screened at 12 mm., the oversize passing to a Krupp ball mill, the discharge of which joins the under 12 mm. material and is split into five sizes from 12 to 1.2 mms. by a series of trommels. Each size is jigged in Harz type machines, using concentration on the sieve for the larger fractions, the concentrates being removed by a pipe and pen type discharge, whilst jigging through the sieve is employed on the finer fractions, using about 1 in. thickness of steel shot ragging.

All material under 1.2 mm. is classified, the first spigots feeding jigs and the last five feeding tables, whilst the overflow is thickened and likewise tabled. Slimes which overflow this last spitzkasten passes to a large thickener of the Dorr type, and its underflow is settled in ponds and sold for coal washing.

This product represents some 10 per cent. of the output and carries from 85 to 90 per cent. BaSO₄.

Bleaching is also carried out, using two methods, the first for coarse material and the second for ground barytes. In the first method, the jig concentrate is leached with sulphuric acid at 90 deg. C., the temperature being maintained with steam for two hours. The acid is then withdrawn for re-use and the Barytes washed by decantation.

Fine table material is ground, then agitated in acid (20 per cent, by volume) at the same temperature and washed in a similar manner to the coarse material.

Both products are ground, ordinary grade being supplied by drying the bleached mineral from the leaching tanks and dry grinding in stone mills.

The superfine grade is usually ground wet in silicalined ball mills, using 10 mm. porcelain balls as grinding media, the mill being of the batch type. After some six hours' grinding, the mills are discharged, the pulp settled and filtered in a rotary vacuum filter, the cake, still containing 20 per cent. moisture, being packed in barrels.

A dry grade is made by drying the slurry as delivered at the filter on a film drier, which is steam heated, but the highest grade is all sold in a wet state.

At the Malehurst mill, the ore contains clay, Calcite, Quartz and sometimes a little copper sulphide. It is washed on a grizzley and the oversize crushed in a jaw crusher, then further crushed in rolls to some in. This material is then sized into five products in trommels, the three coarse products being treated in jigs and the two fine products in Malehurst paddle classifiers after being de-slimed. In these special classifiers, the process of hindered settling is repeated many times and a fine concentrate is discharged from the head of the classifiers.

The only middling product comes from the jig treating plus 7/16 in. material, which is recrushed and retreated. The remainder of the jig and classifier concentrate is conveyed to storage hoppers, from whence it is automatically fed to a continuous rotary gas-fired furnace. Here pure Barytes decrepitates to a white powder, leaving the siliceous material unbroken, so that after cooling on a special conveyor, screening separates a No. 1 grade as undersize, whilst No. 2 quality is made by grinding the oversize in a Raymond mill.

The dressing plant of the Silverband Mine, owned by Messrs. Laporte and situated at Milburn Grange, Westmorland, is somewhat similar in layout, the coarse material being jigged, while the minus 1/8-in, material is treated in Malehurst classifiers, specially modified Akins classifiers and log washers so arranged that a degree of gravitational separation is achieved.

Another straightforward gravity plant (flowsheet No. 3) is that of the New Riverside Ochre Co., Cartersville, Ga., U.S.A., having a capacity of 100 tons per hr.

Here the ore is washed and some Barytes hand selected as lump ore, the remainder being screened, classified and the products treated on jigs and tables. In the jigs, a hutch product is drawn off, which is cleaned on Plat-O tables, as well as an over screen product which is ready for shipment.

Table concentrate (as well as jig concentrate for some markets) is reduced to 12 mesh in a hammer mill operating in closed circuit with a 12-mesh screen and treated on two Stearns high-intensity magnetic separators to remove iron oxides, the final concentrate carrying only 0.7 per cent. Fe.

A rather unusual flowsheet (No. 4) is found in the El Portal plant of the National Lead Co. Gravity concentration is used only on the undersize from a 7/8-in, screen, using Harz jigs, the oversize being ground in ball-mill with the jig concentrate, using no ball load, the coarse ore serving in its stead.

It is claimed that selective grinding takes place and that the mill acts as a gravity concentrator, making a better separation than is possible in jigs or tables. As the gangue is siliceous, it is considerably harder than the Barytes, and in consequence is not ground as fine. Furthermore, the pulp density is maintained about 3.4, so that a heavy media separation using the stirred-bed principle virtually takes place inside the mill, the lighter gangue floating and therefore suffering little reduction in size. The outlet of the mill is fitted with a trommel, having 3/16 in. and 30-mesh screens, where a considerable percentage of the waste is discarded.

A bowl classifier also operates in closed circuit with the mill, but both undersize and overflow are screened for further removal of waste. Another interesting point is that the fine Barytes is heated to 80 deg. F. in order to assist thickening before being finally dried in a film drier.

Drawings and Photographs accompanying the article