I TR.ODUCTiOB AND BACKGROUND OF THE INVENTION Ί his invention relates to a process and equipment for the melting of silicon based fines.

Various silicon based products are produced for usa in particularly the metal industry, including elemental silicon, also referred to as metallurgical grade silicon or simply metallurgical silicon, csiclum silicon and ferrosilicon alloy based inocuiants, also referred to as simply ferrosilicon inocuiants.

Elemental or metallurgical silicon, typically above 95% Si, one of the more basic silicon based products, is commonly used in aluminium aiioy production, to make the alloy easier to cast, harder and more resistant, ana is the raw material in the production of many other, down-stream silicon based products.

Calcium silicon (CaSI , is commonly used In steel making, for deoxi-dstio and desu!furlzation of liquid steel, thereby enhancing the continuous casting fluidity, machinahlliiy, ductility and Impact properties of the final product.

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Ferrosilicon (FeSi) Inocuiants are typically added In liquid cast iron, to improve, characteristics such as the matrix structure and to avoid casting defects, inocuiani addition withi the foundry process is made at different locations, such as so-called "In furnace", *tn ladle", n stream" or "in mould" addition.

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The effectiveness and consumption of elemental silicon, calcium silicon and/or ferrosilicon inocuiants In the above metal processes are dependent on the size fractions and distributions of these- silicon based products. These silicon based products are therefore typically procured commercially as powders, grains or lumps at preferred or specific size fractions, with fines being an integral size fraction of the finer products or an unwelcome contaminant in the courser products, wher¾ the average particle size is ell above that of such fines.

However, silicon based fines, Including silicon based ultra-fines, are generated as a •matter of course during the crus ing or milling of silicon bearing bulk material in the production of metallurgical silicon, calcium siiseon and/or ferrosiiicon inoeulants. For purposes of this specification, the term "silicon based fines" shall be interpreted as to mean fines as generated from the processing of silicon bearing material, and to include uitra-fines.

The silicon based fines are therefore not necessarily sellable at their existing size fractions with such products. The accumulation of unsellable yet inherentl valuable silicon based fines during silicon based product production thus often necessitates the re-processing of such fines by means of additional process steps, such as through re- melting and then re-milling or re-crushing to recover the sellable size fractions before recycling the resultant silicon based fines for further reprocessing,

DISADVANTAGES, DIFFICULTY AND SHORT CO INGS OF THE PRIOR ART

The melting of bulk silicon based fines at high temperatures Is however a relatively expensive process, often re uiring an intermediary preparation step, such as pellefisation, before the melting process, rendering the commercial viability of conventional silicon melting processes and equipment unsuitable for silicon based fines reprocessing in many circumstances.

OBJECT OF THE INVENTION

It is therefore an object of this invention to provide a novel, simple and relatively Inexpensive process and corresponding equipment for the melting of silicon based fines, or at least to provide an alternative process to the existing processes and equipment for such melting,

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a process for the melting of silicon based fines, the process comprising the steps of:

feeding silicon based fines into a direct current electric arc furnace;

melting the fines with a direct current arc; and tapping the resultant marten liquid silicon product from the furnace, once the required melting was accomplished.

The melting may be conducted in a nitrogen atmosphere. The process preferably includes the step of introducing nitrogen gas into the molten liquid bath and, more preferably, of introducing the gas by means of at least one lance into the molten silicon..

The fines may be fed into the molten liquid bath. The step of feedi g the fines into the furnace preferably comprise of feeding the fines pneumatically into the molten liquid bath and, more preferably, of feeding the fines pneumatically by means of the nitrogen gas, as transport medium, via the lance into the molten silicon.

The fines may comprise of a minus 15 mm size fraction, preferably minus 3..mm size fraction and, more preferably, betwee 30 and 90 percent mass minus 25 micrometres and less tha 10 percent mass plus IDS micrometres, and, most preferably,, between 45 and 80 percent mass minus 25 micrometres and less than 5 percent mass plus 106 micrometres. The fines may Include a substantial ultra-fine size fraction.

The fines may contain between 0.03 and 0.07 mass percent calcium and, preferably, between 0,04 and 0,055 mass percent calcium. The fines may contai between 0,i§ and 0.35 mass percent aluminium and, preferably, betwee 0.2 and 0.3 mass percent aluminium.

The fines may be melted in a batch flow, direct current electric arc furnace. The fines may be melted at a temperature of above 1450 degrees Celsius. The fines may be melted ai about 1500 kilowatt hour per ton of fines.

The fines may be fed at a controlled mass rate Into the furnace to ensure a predetermined, optimum melting of the fines.

The silicon yield may be between 85 and 98 mass percent, preferably about 94 mass percent.

The resultant molten liquid silicon product may be cooled down Into a silicon cake suitable for subsequent processing by means of crushing, miiiing and/or screening into ^• desired size fractions.

The resultant silicon may contain between 0.008 and 0 01 mass percent calcium and, preferably, between 0.0065 and 0.0095 mass percent calcium. The resultant silicon may contain between 0.05 and 0,3 mass percent aluminium and, preferably, between 0, 1 and 0,25 mass percent aluminium.

The melting of the fines may be conducted substantially simultaneously with refining of the silicon in the furnace.

According to a second aspect of the invention there is provided a process for t e melting of silicon based fines, the process comprising the steps of:

feeding silicon -based bulk fines of 100 percent mass minus 3 millimetre into a direct current electric arc furnace;

melting the fines with an electric arc; and

tapping the resultant molten liquid silicon product from the furnace, once the required melting of the fines was accomplished,

The melting may be conducted in a nitrogen atmosphere. The process preferably includes the step of introducing nitrogen gas info the molten liquid bath and, more preferably, of introducing the gas by means of at least one lance into the molten silicon.

The floes may be fed into the molten- liquid bath. The step of feeding the fines Into h furnace preferably comprise of feeding the fines pneumatically into the molten liquid bath and, more preferably, of feeding the fines pneumatically by means Of the nitrogen gas, as transport medium, via the lance into t e molten silicon.

Tha process may Include the step of refining the silicon In the furnace, the refining step preferably being conducted substantially simultaneously with the melting of the fines in the furnace.

According to a third aspect of the invention there is provided equipment for use in a process for the melting of silicon based fines, the process being substantially as hereinbefore defined, the equipment comprising a batch flow, direct current electric arc furnace, having a furnace cavity suitable fo receiving a controlled flow of silicon based fines therein, the cavity being heated with a direct current upper cathode and a direct current bottom anode, the furnace being provided with a fines feed opening and a tap hole discharge opening, the furnace being pivoiabie by means of a tilting mechanism for discharging the resultant molten silicon product in a batch wise manner.

The furnace may be provided with means for injecting gas Into 3 rnoiten liquid bath and, more preferably, with means for introducing the gas into the molten- silicon product, the moans preferably comprising a lance.

The furnace may be provided with means for feeding the fines into a rnoiten liquid bath and, more preferably, with means for feeding the fines into the molten silicon product. The means may comprise Of a lance, suitably dimensioned and .configured to feed the fines pneumatically into the molten liquid bath and, more preferably, to feed the fines pneumatically by means or nitrogen gas, as transport medium, into the molten silicon;

The furnace preferabl is able to melt th silicon at a temperature of above 1450 degrees Celsius, The furnace preferably is able to melt the silicon at about 1500 kilowatt hour per ton of fines.

The furnace is preferably able to ^' refine ^' the. silicon substantially simultaneously with the melting of the fines In the furnace.

DETAILED DESCRIPTION OF THE INVENTION

A. preferred embodiment of the invention will now be described by means of a non- limiting example wherein silicon based fines are melted In a batch flow furnace (not shown).

The furnace ^' is a baich flow, direct current {"DC") electric arc furnace, provided with a furnace bowl suitable for receiving- a controlled flow of the fines therein and to produce a molten liquid bath of substantially refined silicon, and a separate slag layer of gangue material. The fines are heated with. a direct current upper cathode and a direct current bottom anode. The furnace has a fines feed opening and a tap hols discharge opening and is pivotsbie by means of a tilting mechanism for discharging the resultant molten liquid silicon product in a batch wise manner.

The furnace is provided with a lance for injecting nitrogen gas into the molten silicon to create a substantial nitrogen atmosphere In the furnace.

The lance- Is suitably dimensioned, configured and arranged to feed the fines pneumatically by means of the nitrogen gas, as transport medium, into the molten silicon.

Two bulk samples of more than 10 tonnes in total and of minus 3 millimetre silicon based fines were taken from a silicon based fines stockpile collected from conventional silicon bag houses and fines. The samples were taken to be reprocessed In accordance with the Invention. The two samples were first screened and assayed representatively, the fines having the following characteristics:

Characteristics Sample A Sample S

Minus 25 micrometres 60% 75%

Plus 1.06 micrometres 2% 0%

Calcium (¾ mass) 0.08 0.043

Aluminium {% mass) 0.23 Q..26

Silicon ( mass) > 08 > 96

Th two bulk samples wer then fed separately, in two separate tests, in a controlled manner into the DC electric arc furnace and melted at a temperature of above 1450 degrees Celsius and at about 1500 kilowatt hour per ton of fines.

Once properly melted, the. resultant .molten liquid silicon product was tapped off info a bulk receptacle and allowed to cool off. The resultant silicon cakes were then broken Into lumps, crushed and milted for representative samples.

The representative samples from the milled silicon cakes from Samples A and 8 were then sampled and assessed, showing the following results: Characteristics Sample A Sample B

Calcium (% mass) 0.00? 0.009

Aluminium (% mass} 0.14 0.19

Silicon recovery (% mass} ~ 94 ~ 94

The results indicated successful silicon recovery of about 84 percent mass from the bulk silicon fines feed at a smelting specific energ of about 1500 kilowatt hour per tonne of fines treated, it will be appreciated further that many embodiments of th invention are possible without departing from the scope or spirit of the invention as defined in the cisims hereinafter., such as firstly, that the silicon based fines could comprise primarily of silicon based nitra-flnes, secondly that, such fines could be generated during t e processing of sicon based products other than metallurgical silicon, calcium silicon and/or ferrosilicon based inoculants, and thirdly, that the equipment and the method of introducing the nitrogen gas and/or the fines could be modified to enhance the effect of the gas and/or fines introduction to the furnace or the bath.