WO1992007048A1

WO1992007048A1 - Briquettes

Info

Publication number: WO1992007048A1
Application number: PCT/AU1991/000489
Authority: WO
Inventors: Roger Franklin; Michael Pekin
Original assignee: Nufarm Energy Pty Ltd.
Priority date: 1990-10-23
Filing date: 1991-10-23
Publication date: 1992-04-30
Also published as: ZA918453B; IT1251417B; ITMI912811A0; FR2668161A1; CN1061991A; ITMI912811A1

Abstract

A briquette suitable for use in a silicon smelting process, the briquette being formed from a mixture of carbonaceous material and silica fume. The briquette may be made by combining the silica fume and the carbonaceous material with water, forming the briquette in a conventional ring roll press or extrusion process, and hardening the briquette so formed by drying.

Description

"Briquettes"

THIS INVENTION relates to briquettes and a method of forming such briquettes. The invention relates particularly to briquettes that are suitable for use as a raw material in silicon smelting processes that utilise electric arc furnaces.

Briquettes that are used in silicon smelters require unique features due to the extreme furnace temperatures at which they are required to combust. It is thus desirable that such briquettes retain their structural integrity until the high temperature zone of the furnace is reached_/ which is created within the smelter by an electric arc at which reduction of silicon dioxide occurs. It is also desirable that briquettes for use in silicon smelters have high levels of purity with respect to metallic contaminants (other than silicon), such as calcium, magnesium, iron, aluminium, titanium and the like.

Furthermore, silicon smelting processes produce silica fume and carbonaceous materials such as charcoal fines, as by¬ products. While silica fume has some use as a cement or paint additive, and, when pelletised, is useful as a feed stock for return to the silicon smelter, in some countries it is expensive to transport and there is often only a small market for it. Hence, large quantities are often simply dumped. Similarly, the charcoal fines cannot be used directly in an electric arc furnace but must firstly be briquetted to a suitable size. While conventional methods of briquetting, such as the use of a starch binder, make a useful product, the cost of the binder and the poor characteristics of the product render the process unattractive. The aim of the present invention is to provide a briquette and a method for forming such a briquette that is suitable for use in silicon smelting processes that utilise electric arc furnaces.

It is to be understood that throughout this specification the term "silica fume" refers to fine partiσulate silicon in an amorphous form. As described above, this form of silica is a by-product of the processing of silica ores such as the smelting of quartz in a silica smelter which results in the formation of silicon monoxide gas and which on exposure to air oxidises further to produce small particles of amorphous silica. Typically, the silica fume contains up to 10% entrained carbon which imparts a greyish colour to the material while the remainder is almost pure silicon dioxide. Silica fume is also known to result from the reduction of silicon tetrachloride.

It is also to be understood that throughout this specification the term "carbonaceous material" includes charcoal, charcoal fines, coke, coal, coal char, petroleum coke, and any other carbon material derived from the destructive distillation of organic materials or coal.

The present invention provides a briquette suitable for use in a silicon smelting process, said briquette being formed from a mixture of carbonaceous material and silica fume. The briquette may be made by combining the silica fume and the carbonaceous material with water, forming the briquette in a conventional ring roll press or extrusion process, and hardening the briquette so formed by drying.

In a preferred form of the invention, the carbonaceous material consists of charcoal fines, and comprises 10% to 99% by weight of the briquette. These charcoal fines may be those produced as a by-product of a silicon smelting process, although fines from any source may be used. Preferably the fines will have a low ash content. The silica fume is a very finely divided material, having a particle size preferably less than 20 microns. This very fine particle size allows for an intimate mixture of silica and carbon in the briquettes and thus assists in binding the briquettes, whereas briquettes using large particles of amorphous or crystalline silica show little or no tendency to bind together. Thus, there is no essential requirement for the use of additional binding materials that may introduce impurities to the silicon smelting process.

Of course, there may be some instances where it is preferred to use additional binders in the briquettes of the present invention. Without any additional binders the briquettes, while having a high crush resistance tend to be brittle when subjected to shearing action. A less brittle and more robust briquette results from the use of small amounts of certain binders. Furthermore, in high volume production, for example when using a ring roll press, it is difficult to obtain a briquette with sufficient hot strength in the furnace due to the relatively low compression. This can be overcome by the addition of other materials which do not act as binders during manufacture of the briquettes but are, in a sense, activated by the heat of the furnace to bind the briquette in the furnace and thus increase its hot strength.

According to this preferred feature, the additional binding agents may be selected from the group comprising bitumen, pitch, asphalt, wax, wood tar, gilsonite, coal, glues, starches, sugars, lignosulphonates, gum arabic, silicates, poly-vinyl alcohol, casein and like agents.

The intimate mixture of silica and carbon which results from the use of the silica fume also provides the briquettes of the invention with unique combustion characteristics and a high silicon yield when used in the harsh temperature conditions of an electric arc furnace. Temperatures in these furnace generally exceed 1200^*C and often go as high as 1700^*C. Furthermore, the method of forming the briquettes of the present invention does not require comparatively expensive high pressure and high temperature hardening processes such as hydrothermal hardening.

The invention will be more fully understood in the light of the following description of one specific embodiment. However, it should be understood that the following description is not to limit the generality of the invention described above.

The embodiment relates to carbonaceous briquettes to be utilised in silicon smelters. As described above, it is a desirable feature of briquettes that are used in silicon smelters that the briquette retain its structural integrity until it reaches the high temperature zone which is created within the smelter by the arc at which the reduction of the silicon dioxide occurs.

It has been found that forming particles from a mixture of carbonaceous material and silica fume provides a composition in which the silica fume acts as a binding agent and also coats the carbonaceous particles to provide a resistance in the carbonaceous particles to combustion.

According to a particular example of the embodiment a carbonaceous briquette having a composition of approximately 70 parts by weight of wood charcoal 25 parts by weight silica fume, and 30 parts by weight of water is formed by conventional techniques utilising a stamp press or roll press. Preferably, the carbonaceous material is mixed with the silica fume and the mixture is passed through a hammer mill to produce a mixture with a particle size less than 1 mm. While it is preferable to have a particle size less than 1 mm to obtain a more uniform mixture, the process will work satisfactorily with a particle size up to about 5 mm. This mixture is preferably then introduced to a paddle mixer and water is sprayed in until the desired moisture content is obtained.

The preferred process may of course be varied in a number of ways. For example, the carbonaceous material may be ground to a fine powder and then mixed with the silica fume. Alternatively, the carbonaceous material can be moistened before mixing with the fume (being either pre- milled or milled when wet) .

After forming the briquettes, they are preferably dried in a conventional drying oven at a temperature from 80'C to 140^*C. Higher or lower temperatures can be used with a corresponding increase or decrease in the rate of drying.

The resultant briquettes have a high green strength and, when dried are resistant to abrasion and firm enough to withstand the normal stresses encountered during handling and transportation. In addition, the dried briquettes exhibit exceptional strength at elevated temperatures (i.e. greater than 1,000^*C) which ensures that when used in a silicon smelter most of the carbonaceous material in the briquette is retained for utilisation in the reduction of the silicon ore in the region of the arc.

The silica fume utilised in the above example is a waste product of the silicon smelting process while the carbonaceous material is derived from charcoal fines which are too small to be utilised as such in the smelting process. Thus the present invention advantageously utilises relatively useless by-products of the smelting process. Further, the briquettes according to the embodiment contain no expensive binders or contaminants, such as the metallic contaminants mentioned earlier, which would interfere with the smelting process by introducing contaminants into the resultant silicon. Further still, the silicon content of the briquettes _t in the form of the silica fume, serves as a feed stock for the smelter and reduces the amount of quartz required in the operation of the furnace.

It should be appreciated that the scope of the present invention need not be limited to the particular scope of the embodiment described above. In particular, the briquettes according to the invention may be utilised in other combustion processes both industrial and domestic with the inclusion of appropriate additives to facilitate combustion, of the carbonaceous material at lower temperatures.

Claims

THE CLAIMS defining the invention are as follows:

1. A briquette suitable for use in a silicon smelting process, said briquette being formed from a mixture of carbonaceous material and silica fume.

2. A briquette according to claim 1 wherein the silica fume is fine particulate silica in an amorphous form having a particle size of less than about 20 microns.

3. A briquette according to claim 1 or claim 2 wherein the carbonaceous material is charcoal fines and comprises from 10% to 99% by weight of the briquette.

4. A briquette according to any on of claims 1 to 3 wherein the briquette has a composition of about 70 parts by weight of char fines, 25 parts by weight of silica fume, and 30 parts by weight of water.

A briquette according to any on of claims 1 to 4 also including an additional binding agent selected from the group comprising:

bitumen, pitch, asphalt, wax, wood tar, gilεonite, coal, glues, starches, sugars, lignosulphonates, gum arabic, silicates, poly-vinyl alcohol, casein and like agents.

6. A method of making a briquette according to any one of claims 1 to 5 said method comprising forming a mixture of carbonaceous material and silica fume, combining the mixture with water, forming the briquette in a conventional ring roll press or extrusion process, and hardening the briquette so formed by drying.

7. A method according to claim 6 wherein the mixture is passed through a hammer mill, before being combined with water, to produce a mixture having a particle size of less than 1 mm.

8. A method according to claim 6 wherein the carbonaceous material is ground to a fine powder prior to mixing with the silica fume.

9. A method according to claim 6 wherein the carbonaceous material is moistened before mixing with the silica fume, being either pre-milled or milled when wet.

10. A method according to any one of claims 6 to 9 wherein the briquettes are dried in a conventional drying oven at a temperature from 80"C to 140^*C.