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May, 1948

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Garnets

The Garnet group consists of several species of trisilicates, with the general formula 3RO,R₂O₃ 3SiO₂. Alamandite, 3FeO Al₂O₃ 3SiO₂, is the commonest variety and is red to black in colour, Andradite, 3CaO, Fe₂O₃ 3SiO₂ is often black, green or yellow, Grossularite, 3 CaO Al₂O₃ 3SiO₂, is white yellow or green, Uvarovite, 3CaO Cr₂O₃3SiO₂ is emerald green, Pyrope, 3MgO Al₂O₃ 3SiO₂, red to black, and Spessartite, 3MnO Al₂O₃3SiO₂, is deep hyacinth or brownish red.

These minerals have a specific gravity ranging from 3.5 to 4.2 and a hardness of 6 to 7.5 or even slightly higher and are found in a great variety of rocks. They are well - known semi-precious gem stones, but their chief use is as abrasives.

In the treatment of garnetiferous rock, the main object is to eliminate the other constituents with the production of as little fines as possible. Jigs of the Harz and James types are commonly used but dry concentrators have also been employed in New Hampshire and North Carolina to remove Mica, Felspar and Quartz. When Garnets occur in Kyanite schists or other moderately high specific gravity rocks, it may be desirable to employ magnetic separation.

Flotation is also feasible for fine sizes using either anionic and cationic collectors. One of the best combinations is said to be the new 801 and 825 reagents of the American Cyanamid Co., both of which are used in conjunction with fuel oil and a higher alcohol as frother, in a circuit rendered acid to a pH value of about 3.5 with sulphuric acid. Conditioning should be carried out in an exceptionally thick pulp, which may be thinned out before actual flotation.

At the Barton Mines Corporation, situated at North Creek, N.Y., sink and float is now employed as a rougher operation, this addition to the plant being put into operation in 1941. Mining started about 55 years ago and the first workings were in the oxidised zone where the ore was crumbly and the Garnets easily separated. In 1924 a mill employing gravity concentration, including jigs and tables was placed in operation. The ore consists of Alamandite in a metamorphosed rock, Hornblende being the chief accessory mineral. Other minerals present include Felspars, Pyroxene, Biotite and minor amounts of Magnetite, Pyrite and Ilmenite.

In the flowsheet of the new plant, the minus 1¼ in. material from a cone crusher is screened to remove minus 10 mesh and the oversize, which represents about 70 per cent, of the original feed, is treated in the Heavy Media cone. The balance (minus 10 mesh) is deslimed and concentrated in jigs. In the cone, the specific gravity of the medium is 3.17 at the top and 3.22 at the bottom. As the Garnet here has a gravity of 3.9 to 4.1 and the Hornblende just over 3.0, a fairly good separation is made.

Both the float and sink products are drained and washed in the usual manner whilst the float is split at in. during this operation in order to make a course material for road ballast. The sink product is likewise screened on 7/16 in. and 10 mesh during washing so that the plus 7/16 in. material can be crushed in rolls and returned to the washing screen. The concentrate, i.e., the minus in. plus 10 mesh material, is finally cleaned in jigs as shown in the flowsheet Fig. 1.

A ferrosilicon medium is employed and is cleaned in the usual manner using Crockett type magnetic separators.

In the past year, a flotation plant has been added to deal with material too fine for jigging and it is understood that an anionic reagent of the "800 series is being employed.

Graphite

Graphite is a soft form of carbon which is black, greasy, resistant to most chemicals, a good conductor of heat and electricity and a good lubricant, with a specific gravity of 2.1 to 2.3 and a hardness of one on Mohs scale. Amorphous Graphite, sometimes called black-lead is a less valuable form.

Graphite is used for crucibles, retorts, refractories, foundry facings, lubricants, etc. The higher grades are derived from Ceylon, Korea and Mexico and lower grades from Central Europe, and the United States.

Treatment varies very considerably; in Ceylon, the Graphite is hand-picked, screened and cobbed but lower grade deposits containing fine flake material must be concentrated mechanically.

The important factor in any treatment is that the Graphite must be freed without destroying the flake or without producing excessive fines. It has been said that it is one of the easiest minerals to concentrate into a rough product but one of the hardest to clean finally.

Both table concentration and flotation are commonly employed and since Graphite tends to coat the waste, cleaning is often ineffective or at best, difficult to control. Final cleaning is therefore often achieved by grinding in rolls or buhrstone mills followed by screening to remove the harder impurities.

Flotation has been applied successfully to material as coarse as 10 mesh, being used commercially in Norway, Sweden and the United States.

The usual collector is kerosene with pine oil or a higher alcohol as the frother whilst creosote may be employed as a froth stiffener and sodium silicate or hydrofluoric acid added to assist in making a clean concentrate. In spite of many attempts, however, the production of very high grade flake Graphite by normal methods is not very successful and high priced products may be purified by dissolving any silica in hydrofluoric acid or fusing it with soda ash and subsequently extracting the silicate in water.

At the American Graphite Company's plant, situated at Ticonderoga, New York, a 87 per cent. recovery is made from quartz schist ore and 100 tons per day is treated.

The ore is crushed in a jaw crusher followed by rolls, then ground in a conical mill and treated by flotation in four sets of 8 ft. by 2 ft. Callow cells. Once-cleaned concentrate is screened at 100 mesh and the oversize reground in a conical pebble mill before two further cleaning stages. The concentrate from this operation is again screened on 100 mesh and the oversize reground before final cleaning in more Callow cells. The ultimate concentrate is settled to 40 per cent. in water and dried in a rotary drier, operating in conjunction with an air classifier. In this way two grades of Graphite are made, one carrying 86.5 per cent. C and the high grade flake, 91 per cent. C from an ore containing some 5.5 per cent. carbon. The flowsheet is shown in Fig. 2.

Kyanite

Kyanite, Sillimanite and Andalusite all possess the same composition, Al₂O₃ SiO₂, but crystallise in different systems and possess different physical properties although they revert to Sillimanite when heated to between 1,350 and 1,400 deg. C.

Kyanite is a common mineral occurring in gneisses and schists. It is mined in the United States, India and elsewhere. In India, it is simply hand selected but gravity concentration has been employed in some other places at it possesses a specific gravity of 3.5 to 3.7. Unfortunately, however, it breaks into "slivers which tend to be washed to the tailing. Pneumatic table treatment has been used at the Celo mine, situated at Burnsville, North Carolina, where the final concentrate was treated by magnetic separators to remove Garnets, Biotite and other iron-bearing impurities.

The ore carries 15 to 20 per cent. Kyanite, 10 to 15 per cent. Garnets, 25 to 30 per cent. Quartz, 35 to 40 per cent. Mica and 2 per cent. sulphides and was treated at a rate of 175 tons per day yielding an 85 per cent. concentrate with a recovery of 80 per cent. Recently, however, a flotation plant has been installed to which reference is made below, with improved recovery and grade. When flotation is employed, desliming is usually essential whilst a scrubbing with sodium hydroxide is frequently some assistance before desliming.

Anionic reagents are employed as collectors using sodium silicate or a phosphate as a depressant together with soda ash or sodium hydroxide. Sulphuric acid may be useful in cleaning and "aerosol" wetting agents are said to assist the action of collectors. Tarlton recommends a short pre-treatment of a deslimed pulp with sodium hydroxide. At the Kyanite Products mine, the ore is washed, crushed to 1/8 in. in three stages, then ground in rod mills to 35-mesh, deslimed and treated by flotation with two stages of cleaning. The flotation concentrate is finally dewatered, dried to 1 per cent. moisture and a little ferrugineous material removed by a five-roll Dings high intensity magnetic separator. The flowsheet is shown in Fig. 3. At the Celo mines, the sulphide as well as Biotite is first removed using aerofloat and laurylamine hydrochloride, then the Kyanite is floated with oleic acid, using calgon as a modifier in the new flowsheet.

Cleaning is done with further additions of calgon when the final concentrate carries 97 per cent. Kyanite. Flotation feed is ground to 65-mesh and a recovery of over 90 per cent. is possible from an ore carrying 15 to 20 per cent.

In certain cases the Kyanite concentrate is finely ground or calcined to Sillimanite before being marketed.

Mica

The micas of commerce consist of Muscovite and Phlogopite, as Biotite is of little value. Muscovite or white mica, K₂O 3Al₂O₃ 6SiO₂ 2 H₂O, is the better for insulators but Phlogopite or amber mica (approx. AlMg₃KH,Si₃O₂) is definitely more resistant to heat.

The electrical industry takes most of the sheet mica but smaller quantities go into stove windows, goggles and the like, whilst ground mica is used mainly in roofing felts, wallpaper manufacture, as a filler in paints, rubber and plastics and for decorative purposes.

Sheet mica is prepared by hand, being cobbed, split into sheets or " rifted trimmed into various sizes, graded, classified into grades depending on colour, flexibility, sheen and the presence of inclusions and finally split to the desired thickness.

Very small flake or ground mica is either derived from waste of splitting or produced as a by-product in clay mining. Separation from granular silicates is usually carried out by flotation either employing an anionic or cationic collector. When fatty acids are used, the mica may be activated with lead nitrate or alkali earth compounds, whilst alkaline resinates have been recommended as the best collectors. When cationic reagents are employed, fairly short chain amines or starvation quantities of the longer chain members are preferred, together with sulphuric acid and aluminium sulphate to depress granular silicates. Supplementary frothers may be pine oil, cresylic acid, whilst fuel oil or kerosene often assist the collecting action of the amine.

When the mica is to be used as a filler, as well as for some other purposes, it requires grinding but this is a difficult operation owing to the elasticity and toughness of the flakes. Wet ground mica is worth several times more than the dry ground variety as it possesses much better appearance as well as more

slip and chaser mills of the Chilean type arc employed with wooden die rings and rollers. Dry grinding is carried out either in hammer or attrition mills whilst some new installations employ micronisers. After grinding in any of the foregoing ways, the mica must be screened into various grades to suit market requirements.

At the English Mica Company's plant in North Carolina, various grades are produced, including Biotite, and both wet and dry grinding is employed. In the wet process, grinding is a hatch operation using chaser mills. The mica is added, damped slightly and ground for eight hours, after which the mill is discharged, the sand is separated by settling and returned to the mill until all the mica is freed. The overflow mica is settled, filter pressed and dried in steam-jacketed driers before being graded by dry screening.

Dry grinding is carried out in a microniser, operating with high pressure steam at 550 deg. F. This machine consists of a shallow circular chamber wherein a circulating charge of mica is acted upon by numerous steam jets, issuing through orifices arranged tangentially. The high pressure steam cuts across the flow of mica by setting up intense interaction which reduces the mica in size but maintains its flake-like shape. The steam leaves the unit from the centre and classification is effected by this action.

At the Southern Mica Company's plant, a mica schist is mined in an open pit, sluiced to a trommel where the minus 1 in. plus in. material only is retained. This is crushed in rolls, rescreened at in., the oversize being crushed in rolls once more and rescreened, so that only plus in. material is saved. This is dried, screened, oversize being sold as coarse flake, whilst undersize is ground dry and resized, eliminating the very fine sand. Some of the coarse flake mica is also ground in a Raymond pulveriser and graded into different sizes.

For a description of mica recovery plants in the clay industry, the reader is referred to Part 1 of this article, which was published in the March issue.

Drawings and Photographs accompanying the article