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PRODUCTION METHODS FOR 15 PER CENTFERROSILlCON

The cheapest and commonest production methodis the melting of steel scrap, quartz, and reducing agentin a submerged-arc furnace. The three raw materials areweighed, mixed well, and fed into the furnace.' At theMetalloys plant of Amcor, a 7,5 MVA furnace is usedfor the production of IS per cent ferrosilicon. Thecharacteristics of this furnace are as follows:Shell diameter 6400 mmHearth diameter 4000 mmElectrode diameter 890 mmSecondary voltage 90 to 110 VResistance 0,85 mflReactance 1,0 mfl

The power consumption 'including auxiliaries' for Itonne of IS per cent ferrosilicon amounts to 2100 kWh.The manufacturing process has several specific prob-

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Figure 1Flowsheet for the production offerrosilicon

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by M. SCIARONE* (presented by Mr Sciarone)

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SYNOPSISThe heavy-medium separation process is gaining in importance. So that a closer and more effective control of theprocess can be achieved, higher demands are being made on the medium. The paper reviews the production processesfor ferrosilicon of 15 per cent silicon, and makes special mention of the application of the medium, the importance ofraw materials, and relative manufacturing costs.

INTRODUCTIONFerrosilicon for heavy-medium separation is an alloy

with a silicon content between 14 and 16 per cent. Theprocess of mineral beneficiation by heavy-medium sep­aration involves the separation of mineral particles byvirtue of their difference in specific gravity. The mediumconsists of a suspension of ferrosilicon powder (15 percent silicon content) in water. The density of the mediumis between that of the sink and that of the float material.

Ferrosilicon (15 per cent silicon) is very well suited toheavy-medium separation because of its high corrosionresistance and its good magnetic properties. Because theferrosilicon particles are small, the specific surface areais large, and, as corrosion is a surface phenomenon, ahigh corrosion resistance is needed. For the recovery offerrosilicon from the separated float and sink material,magnetic separation is the easiest and cheapest way if themedium has good magnetic properties.

*Metalloys Limited, South Africa.

The Production of Ferrosilicon Powder forHeavy-medium Separation

lems, some of which are mentioned below.(I) The handling of large amounts of steel scrap, which

does not flow easily, is difficult.(2) The low resistance affects the operation adversely if

sufficient caution is not exercised.(3) The silicon content of the alloy must be kept to

between 14 and 16 per cent. This problem may leadto considerable amounts of off-grade product, whichcannot be used for any purpose other than remeltingin the furnace.

The furnace is usually tapped every two hours. Thestandard practice for the handling of the tapped metalwas to cast the liquid ferrosilicon in sandbeds in layers ofabout 50 mm, cool it, break it up into lumps, crush it intwo stages, and feed it to a ball mill for grinding. Thiscumbersome practice was abandoned when the 7,5 MVAfurnace was commissioned, because the large tonnage offerrosilicon that had to be handled made it economicallyunattractive. Instead, a granulating plant was built froman Iscor design. In the granulation process for liquidferrosilicon, the melt is solidified by a high-pressurestream of water into small particles. The complete flow­sheet is shown in Figure I. The granulation process forferrosilicon has several advantages over the 'old' handl­ing method, the first being that the material is in a sizerange between 2 and 0,05 mm, which makes subsequentgrinding of the material much easier than when crushedmaterial is used. The second advantage of this rapid­quenching technique is that the material cannot segregateduring solidifying, and consequently a homogeneousmaterial is obtained.

The final powder is obtained by the grinding of thegranulated 15 per cent ferrosilicon in a ball mill. Pow­ders of different fineness can be produced by adjustmentof the air-classifying equipment. A magnified picture ofmilled ferrosilicon is shown in Figure 2. The mostimportant variable of the milled ferrosilicon is its corro­sion resistance, which, according to Robinson and DuPlessis1 , can be increased by additions of chromiumand/or other metals. However, when chromium wasadded on a trial basis, the results were inconclusive, andfurther testwork is being carried out. Another method forthe production of 15 per cent ferrosilicon is the meltingof high-grade ferrosilicon and the dilution of this melt

with steel scrap until the melt has a silicon content of 15per cent. The melt from an induction furnace has aslightly different chemical composition from that of the15 per cent ferrosilicon produced in a submerged-arcfurnace. This difference in chemical composition makesit possible for spherical material to be obtained from theatomization of the melt with steam. The flowsheet of theatomizing process is shown in Figure 3.

Figure 2Milled jerrosilicon

The main purpose of this production method is to giveparticles that are more or less spherical (see Figure 4).Spherical .material has several advantages over non­spherical material. With spherical material, losses duringheavy-medium separation are lower as a result of re­duced adhesion losses, and the medium is washed off thesink and float product more easily. Losses of the mediumdue to corrosion are considerably lower with atomizedferrosilicon than with milled ferrosilicon because of thelow specific area of the atomized material. In addition,higher pulp densities can be obtained in heavy-mediumseparation with atomized ferrosilicon - a pulp densityof up to 3,85 kg/I, compared with 3, I kg/I for milledferrosil icon.

Solid particles of rounded shape and smooth surfacecan also be produced by the flame-spheroidizing of ir-

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Flowsheet jor the atomizing process·

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PROOUCT

OVERSIZE

Figure 4

Particles of silicon

regularly shaped, solid particles. The particles are passedthrough a high-temperature flame or plasma so that theyare melted at least on their surface. A problem with thismethod is that large particles do not melt completely ontheir surface and do not become round and smooth. Forthe flame-spheroidizing process, the fine ferrosiliconpowder normally used yields a fine spherical product.For coarse spherical material, another method had to befound. This involved water-atomizing of a melt of 15 percent ferrosilicon, but the particles produced in this wayare spherical only if the chemical composition is wellcontrolled. Many of these particles, however, are toocoarse for use in heavy-medium separation and have to bescreened out. This screened-out fraction is then milledand fina}ly flame-spheroidized.

The fraction of ferrosilicon particles from water­atomization that is suitable for heavy-medium separationis used together with the flame-spheroidized ferrosilicon.This combined material is of a superior quality to thesteam-atomized product in the shape and specific gravityof the coarse fraction and the size distribution of thewhole2 • It can be used up to a pulp density of 4,0 kg/lwithout the heavy losses that occur with the steam­atomized product. The process was developed by Iscorand was used at the Metalloys plant of Amcor. However,there were changes in the conditions at the particularore-beneficiation plant where the mixture of flame­spheroidized and water-granulated material was used,and lower pulp densities were permitted. Thus, the needfor this superior quality of ferrosilicon disappeared, andits higher manufacturing cost compared with that ofsteam-atomized ferrosilicon did not warrant its continuedproduction.

CONCLUSIONMilled 15 per cent ferrosilicon is the cheapest type of

ferrosilicon powder to produce. However, its quality is?ot constant, and its use in heavy-medium plants resultsm erratIc consumptions and difficult operation. Finer grad­mg of the ferrosilicon powder increases the milling cost,while the higher specific area of the finer powder causeshigher losses due to increased corrosion. The field ofapplication of milled ferrosilicon is limited to a densityof about 3, I kg/I.

Atomized ferrosilicon is about twice the price of mill­ed ferrosilicon, and its consumption is always lowerthan that of the milled material. The saving in consump-

59

tion by the use of steam-atomized ferrosilicon can be asmuch as 80 per cent or as little as 10 per cent. Onfy a trialrun will show what result will be obtained. The field ofapplication is wider, and pulp densities of up to 3,85 kg/Ican be obtained.

Flame-spheroidized ferrosilicon is as good as th~steam-atomized material. When cheap milled ferro­s!licon is used in the flame-spheroidizing process, thefmal cost of the spheroidized powder is about the same asthat of steam-atomized ferrosilicon. Water-atomized fer­rosilicon is very expensive, and its production is war­ranted only for special applications when steam-atomizedferrosilicon fails.

REFERENCESI. ROBINSON, F.P.A., and DU PLESSIS, DJ. Polarisation and

corrosion of ferrosilicon alloys for iron ore benefication media.Corrosion. vol. 22, no. 5. 1966. pp. 117-131.

2. HOFFMAN, D.l.N., and BEETON, T.B. Heavy separation media.V.S. PAT. 3,312,543, 1967.

DISCUSSIONMr A. Grant*:

What are the practical applications of 14/16 ferro­silicon?Mr Sciarone:

Iscor uses 15 per cent ferrosilicon for upgrading theiriron ore. Ore with a high silicon content is float material,and the sink material is a high-grade iron ore. Amcor areupgrading their chromium ore at their Potgietersrus minewith 15 per cent ferrosilicon.

Nearly all diamond mines use heavy-medium separa­tion to remove most of the gangue material, and at GrootEylandt heavy-medium separation is used to split high­and low-silica manganese ores; the high-silica manganeseore is then used for the smelting of silicomanganese.Professor G.T. van Rooyen t:

Please elaborate on the influence of chromium addi­tions to 14/16 ferrosilicon on the corrosion resistance,magnetic properties, and specific gravity.Mr Sciarone:

The chromium additions were made in the furnace toobtain complete mixing. A chromium addition of 2 percent was the maximum level to which additions could bemade without influencing the grindability. An improvedresistance to corrosion could not be established becausethe results were too scattered.

No tests have been done on magnetic properties.Chromium additions did not show a noticeable effect onspecific gravity. There was only a tendency to a slightlyhigher carbon content in the fi nal alloy.Dr R.E. Robinson+:

How does the shape of the 14/16 ferrosilicon particleaffect its properties?Mr Sciarone:

The particle shape is very important. The particle mustbe smooth and solid. The smooth surface and near­spherical shape result in lower viscosity of the pulp and ina marked increase in corrosion resistance.

These properties make it possible to work at muchhigher bulk densities in the heavy-medium separationthan with milled 15 per cent ferrosilicon.

*Middelburg Steel and Alloys (Pty) Ltd, South Africa.t University of Pretoria, South Africa.:j: National Institute for Metallurgy, South Africa.