Method of processing coal for use as a carbon reductant in the manufacture of silicon

Abstract

A method of processing coal for use as a carbon reductant in the manufacture of silicon. The method comprises the steps of: crushing raw coal in a crusher, conveying the crushed coal beneath an electromagnet to remove ferromagnetic material, passing the coal onto a desliming screen, adding water and vibrating to remove particles less than 3 mm in size. The washed coal is then conveyed to a multi-deck screen to remove material over 14 mm in size. Material is transferred to an inclined frustoconical cyclone, mixed with a magnetite and water suspension, denser material comprising impurities is collected in the lower end underflow and less dense coal product containing less ash is collected in the overflow. The density of the magnetite and water medium is monitored and controlled such that the cyclone overflow output has an ash content of equal to or below 1.5 %. Magnetite is removed from the coal product by electromagnet and dewatered to produce a final product with an ash content less than or equal to 1.5 %.

Description

FIELD OF THE INVENTION This invention relates to a method of processing coal for use as a carbon reductant in the manufacture of silicon and to a carbon reductant made in accordance with such method. BACKGROUND OF THE INVENTION Ferroalloy is a term used to identify various alloys of iron with a high proportion of one or more other elements, for example Silicon, Aluminium, Magnesium, Nickel and many others. The main task of the ferroalloys industry is the primary recovery (reduction) of needed metals from natural minerals. One ferroalloy which has become critically important in recent decades is ferrosilicon and the metal which can be derived from it known as Metallurgical-Grade Silicon (Si). Silicon metal is used in a wide variety of applications in the chemical industry, e.g. in the production of silicones and silanes, and in the production of high-strength aluminium alloys for the automotive industry. It is also a primary input material in the manufacture of photovoltaic modules. Silicon is the dominant material of the digital age, it is a vital component in steel, electronic equipment and when purified further it is the core building block of semiconductor electronics and is essential to the transistors and integrated circuit chips used in most modem technology such as smartphones and other computers. Metallurgical grade silicon is produced by the carbothermic reduction of silica rich materials, e.g. quartz, with a carbon reductant in a submerged arc furnace. Carbon reducing agents are an integral element in the production of Silicon. Carbon reducing agents (otherwise known as carbon reductants) consist mainly of coal, petroleum coke and charcoal. The selection of carbon reductant for the ferrosilicon production process requires consideration of several criteria, such as economic considerations, appropriate chemical composition (low ash content, high reactivity, low volatile substances, high fixed carbon, etc.), high electrical resistance, appropriate granulation, and high mechanical strength. In order to produce high-purity silicon or metallurgical grade silicon, in particular for the semiconductor industry, carbon reductants containing very low levels of impurities are required because the purity of the final silicon product is strongly determined by the amount of impurities present in the carbon reductant. While coal is considered a suitable carbon reductant for the production of metallurgical grade silicon, due to its low cost and availability, the natural high ash content of coal is problematic. Most commercially available grades of coal typical have an ash content of between 5% to 40%. The ash content of the coal is the portion of the overall mass of the coal that contains non-combustible material (i.e. impurities). In the production of metallurgical grade silicon for use in the manufacture of semiconductor electronics a carbon reductant having an ash content of less than 1.5% is desirable, known as Ultra-Clean-Coal (UCC). However, processing coal with known physical washing and grading techniques has not been able to decrease the mineral content of the coal sufficiently to produce UCC. Therefore chemical demineralisation processes, either alone or following physical washing and grading processes, are generally required for the production of UCC. These chemical demineralisation processes are costly and lead to problematic waste streams. An object of the present invention is to produce coal for use as a carbon reductant having an ash content below 1.5%, while still maintaining the additional properties required of a carbon reductant for metallurgical grade silicon production, particularly in terms of particle size and mechanical strength, using physical washing and grading processes and without requiring additional chemical treatments. SUMMARY OF THE INVENTION In a first aspect, the present invention provides a method of processing coal for use as a carbon reductant in the manufacture of silicon comprising the steps of passing raw coal into a crusher adapted to reduce the particle size of the coal and preferentially break up coal containing high levels of impurities; conveying the coal from the crusher upon one or more conveyor belts to pass beneath one or more electromagnets to remove ferromagnetic particles before passing the coal onto a deck of a first desliming screen, adding water and vibrating the first desliming screen in order to wash the coal and remove particles less than 3mm in size, such particles passing though apertures in the deck to be collected in a sump therebeneath, the washed coal passing over the deck of the first desliming screen and onto a further conveyor whereupon it is conveyed to a multi-deck washing and grading screen having an upper deck having apertures selected to remove material over 14mm in size, material less than 14mm in size passing through the apertures of the upper deck to fall onto a lower deck, the lower deck having apertures of a size selected to pass material less than 3mm in size, the greater than 3mm and less than 14mm coal particles being retained on the lower deck and being conveyed across the lower deck to pass onto a further conveyor whereupon it is transferred to a dense medium cyclone comprising an inclined fructoconical cyclone, tapering from an upper end towards a lower end, wherein the coal is mixed with a medium comprising a suspension of magnetite and water, such mixture being introduced tangentially into the cyclone through an inlet at an upper end thereof to generate a swirling motion within the cyclone whereby denser particles, comprising coal containing impurities, migrate towards the outer wall of the cyclone to subsequently pass out of a lower end of the cyclone as an underflow, while the less dense particles, comprising coal with a low ash content, are directed toward the centre to subsequently pass out of an upper end of the cyclone as an overflow, the method comprising the step of monitoring and controlling the density of the medium with the cyclone such that a coal product carried in the overflow of the cyclone has an ash content of less than or equal to 1.5%, removing magnetite from said coal product by means of one or more electromagnets and dewatering said coal product upon a 4 dewatering screen to product a coal product having an ash content less than or equal to 1.5% suitable for use as a carbon reductant in the manufacture of silicon. Preferably the method comprises the further step of conveying said coal product from the dewatering screen to a further grading screen, said further grading screen being adapted to remove material having a particle size greater than 12mm and less than 3mm from said coal product. The particles greater than 12mm may be processed to produce one or more further coal products. In a preferred embodiment the method comprises the step of monitoring the density of the medium passing into the cyclone of the dense medium cyclone and adjusting said density in real time by adding magnetite and / or water to the medium to maintain a target density required to achieve the required ash content in the coal product leaving the overflow of the cyclone. Samples of the medium may be taken at predetermined intervals and the density of said samples is measured to detect any changes in density requiring an adjustment of the composition of the medium over time. In one embodiment samples may be taken and tested every ten to fifteen minutes. Further variables, including temperature and water content of the coal entering the dense medium cyclone, may be monitored, the density of the medium and the operation of the cyclone may be optimised based on such variables. The overall average ash content and water content of the raw coal may be determined before it enters the crusher, such data being used to determine baseline variables of the required density of the medium in the cyclone and other operating parameters of the dense medium cyclone. The rate at which the medium is pumped through the inlet of the cyclone may be controlled to achieve and maintain the target ash content of the coal product. The rate at which the medium is pumped through the inlet of the cyclone may be controlled by varying the current supplied to the pump motor. The step of monitoring and controlling the density of the medium with the cyclone may be carried out such that a coal product carried in the overflow of the cyclone has an ash content of less than 1 %. DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION A method of processing coal for use as a carbon reductant in the manufacture of silicon in accordance with an embodiment of the present invention will now be described, by way of example only. Raw Coal, known as “Run of the Mine Coal” (ROM coal), is first loaded into a crusher adapted to reduce the particle size of the coal and break up coal containing high levels of impurities to facilitate subsequent separation, as described below. The inventors have surprisingly noted that coal particles containing higher levels of impurities are more brittle and have a lower mechanical strength than coal particles containing fewer impurities (and therefore more carbon), such higher purity coal particles having greater mechanical strength and compressibility. The result of this difference in mechanical strength as a function of purity of the coal particles means that coal particles containing higher concentrations of impurities will mechanically break down in the crusher at a higher rate and to a greater extent than coal particles with higher concentrations of carbon. This means that larger particles downstream of the crusher tend to have a greater purity than smaller particles (with greater levels of impurities and hence lower mechanical strength). Therefore crushing and then subsequent grading of the coal to remove these smaller particles (preferably removing particles of a size less than 3mm) has surprisingly been found to lead to a significant reduction in the ash content of the remaining coal. Therefore, crushing the raw coal prior to washing and grading the coal means that more ash content is removed through subsequent washing and grading processes. After crushing, the coal is transported via one or more conveyor belts, during which it is passed under one or more electromagnets to remove ferromagnetic particles from the crushed coal. The one or more electromagnets may comprise a sweeping electromagnet passing at right angles over the respective conveyor belt and / or a fixed electromagnet. The coal is now clean of magnetic elements and is ready for the next stage. The coal proceeds to a first desliming screen, whereupon the coal is washed and particles less than 3mm in size (passing through the deck of the screen and into a sump therebeneath) are removed. In a preferred embodiment the first desliming screen comprises a frame, defined by a pair of substantially parallel side walls interconnected by transversely extending bridging members, upon which is mounted a substantially horizontal deck having small openings or slots for water and / undersize particles to pass through. Spray bars are provided over the deck to add water to the coal. The deck is vibrated at high frequency to shake out excess water and / or undersize material (less than 3mm in size) through the openings and into a sump therebelow and to convey the remaining material across the deck to one end of the screen whereby the oversize material (>3mm) is discharged onto a conveyor. The frame, and thus the deck, is typically vibrated by means of a pair of counter rotating rotors defining eccentric masses, driven by one or more drive motors, to impart circular or reciprocating vibratory motion to the deck. The frame is supported on resilient mounts to permit such vibratory motion while isolating adjacent structure from such vibration. The material passing over the deck of the first desliming screen proceeds via a conveyor belt, preferably moving at 1.5 m / s with a 18° incline, to a multi-deck washing and grading screen, having an upper deck having apertures selected to remove material over 14mm in size, material less than 14mm in size passing through the apertures of the upper deck to fall onto a lower deck, the lower deck having apertures of a size selected to pass material less than 3mm in size, the greater than 3mm and less than 14mm particles being retained on the lower deck and being conveyed across the lower deck to pass onto a further conveyor to be transferred to a dense medium cyclone (DMC). The oversize material (greater than 14mm) from the upper deck may be conveyed to further washing and grading stages to produce several further coal products. All of the conveyor belts outlined above are preferably set to a predetermined incline (preferably 18° to the horizontal) and speed (such as 1.5m / s) as this has been found to act as a further ‘filter’, causing any oversize particles to fall back down the conveyor belt and not proceed to the next stage. The crushed and graded coal (3mm to 14mm particle size) now enters the DMC, where it is immersed in a medium of predetermined and controlled density, said medium consisting of a suspension of magnetite in water. The DMC comprises an inclined fructoconical cyclone, tapering from an upper end towards a lower end. The mixture of coal and dense medium are introduced tangentially into the cyclone through an inlet at an upper end thereof to generate a swirling motion within the cyclone. This swirling motion generates significant centrifugal force, compelling the denser particles to migrate towards the outer wall of the cyclone to subsequently pass out of a lower end of the DMC as an underflow, while the less dense particles are directed toward the centre to subsequently pass out of an upper end of the DMC as an overflow. The Dense Medium Cyclone (DMC) is a density based separation system in which coal with different densities can be accurately separated. The basic working principle of a DMC is that the lighter coal particles (comprising high purity coal) float and the heavier particles (containing higher levels of impurities) sink. The density of the medium is carefully controlled with high accuracy over time to achieve a precise separation of lighter and heavier coal to produce a coal product at the overflow of the DMC of exceptionally low ash content (less than 1 % ash content). The main reason there is a difference in density between coal particles is the presence of impurities. The DMC does not separate ash from coal rather it ‘cleans’ the overall coal product. If you are feeding into the coal washing process tonnes of coal that has an average ash content of 10%, within that will be coal that has 20% ash and coal that has 1.5% ash. The DMCs role is to separate the coal based on its ash content (density). Key to achieving this separation is the density of the medium. The desired density separation is achieved by adding magnetite to water forming the medium used in the DMC. Magnetite is a mineral whose primary component is Iron Oxide, it has a high density, 5.17 g / cm3, and is ferromagnetic. This allows magnetite to be used to increase the density of the medium but also allows the magnetite to be recovered from the medium by means of an electromagnet rotating partly submerged in the medium, after use. The selection and control of the density of the medium used in the Dense Medium Cyclone (DMC) is a key factor in its ability to accurately separate different densities of coal. At any point in time, the accuracy of the density of the medium can allow the operator to be discerning and only select the coal with the composition of ash that they require. However what can be overlooked in prior art systems is the variation of medium density with time. As soon as the density is set to a particular value, with the addition of magnetite, it will begin to change. In fact the density is constantly changing driven by a number of factors, this is the ‘temporal density of the medium’. In the method in accordance with an embodiment of the present invention, the density of the medium is monitored and corrected in real time to maintain the desired density and therefore the desired cut point. The density of the medium is tested continuously with a time base reflecting time release of the magnetite. It is measured by the operator frequently taking samples of the medium and determining its density experimentally, for example via a displacement method. The amount of magnetite is how much magnetite (kg) is introduced to the medium every hour, this is decided by the operator based on measurements of the density of the medium over time (preferably every ten to fifteen minutes). Other variables affecting the operation of the DMC, such as temperature and the water content of the mixture entering the DMC, are also monitored to optimise the operation of the DMC. The environmental temperature is preferably tested on a 12 hour period. Changes in the environmental temperature will cause greater or lesser evaporation of water from the medium, affecting the water content of the medium and therefore the overall density. The overall average ash content of the input coal is known daily testing on the run of mine coal before it enters the crusher. This gives the operator a baseline of the average ash content to be expected during the day from which the operator may need to react. The rate of coal feed is how many tonnes of coal enter the DMC every hour. Other factors in the temporal density equation will impact this rate. If there has been increased precipitation the water content of the coal will be higher, which decreases the amount the DMC can process efficiently. Precipitation, measured daily, has two main impacts on the process, if affects the water content of the ROM coal and it adds water to the medium, so lowering the density. The cyclone pump rate is how much medium and coal the DMC processes over time. This is controlled by the operator in the control booth by varying the current supplied to the pump motor. The rate of magnetite addition determines how the magnetite is added to the medium slowly over a 30 minute period. Magnetite is released by a pump feeding the magnetite into the medium in a time released manner. The rate of release can be varied by the operator based on the medium density measurement results. Recycled water is added to the medium on a continual basis, this can be varied by the operator reacting to the other variables, but primarily precipitation and the rate of coal feed. Magnetite recovery from the coal downstream of the DMC may be by means of a rotating electromagnet which is partially submerged in the medium, causing magnetite to adhere to it. The rate of recovery may be controlled by the operator to allow the magnetite level to be controlled in a reliable manner, allowing for a more precise control of the density of coal output. Balancing the variables of the ‘temporal density of the medium’ allows accurate control the density of the medium over time and therefore accurately controls the level of ash content contained in the final coal product. By controlling these variables, the inventors have been able to produce a lower peak to trough of variation in the density of the medium. The magnetite may be added every sixty minutes and may be released over time to produce a slowly varying medium density. This is in contrast to what happens in the prior art if the time base is not controlled, where large spikes and dips in the medium density occur. The coal product leaving the Dense Medium Cyclone in the overflow from the cyclone is passed onto a dewatering screen to remove water and is then transferred via a conveyor belt to a final grading screen, whereupon any remaining material below 3mm in size is removed and also oversize over 12mm is separated before the final product, having a particle size distribution of 3mm to 12mm and an ash content of less than 1%, is conveyed onto a stockpile. Water used in the Dense Medium Cyclone and the desliming / dewatering screens may be recovered and processed, initially through two settlement tanks. The settlement tanks allow impurities in the water, mostly coal particles that have remained in the water, to settle to the bottom to be removed and recovered. The water, still containing finer coal particles, is transferred to a water filter where the water is held and a rotating wheel, ~ 8 m in diameter, covered in fine gauze. The wheel rotates into the water capturing the coal particles as the water is forced through it. The wheel then rotates out of the water. This rotation happens purposefully slowly allowing the coal particles to dry on the gauze and be removed as it dries. The coal particles are transported by conveyor belt to be reused. The water is now mixed with flux and goes into a further settlement tank where the last of the contaminants are removed from the water as they settle out. The water is now clean and is reused into the process both in the screens and the DMC. The above-described process achieves a final coal product having the properties listed in Table 1 below. Parameter Typical Max Min Total Moisture (%) 12 Ash Content (%) 0.8-1.0 1.2 Volatile Matter 35-36 Fixed Carbon (%) 49-52 48 Total Sulphur (%) 0.4-0.5 0.6 Gross Calorific Value (kcal / kg) 6800-7000 Net Calorific Value (kcal / kg) 6500-6700 HGI* 49 49.5 Table 1. *HGI, the Hardgrove Grindability Index is a measure of the grindability of coal. The smaller the HGI the harder and less grindable the coal is. The invention is not limited to the embodiment described herein but can be amended or modified without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A method of processing coal for use as a carbon reductant in the manufacture of silicon comprising the steps of passing raw coal into a crusher adapted to reduce the particle size of the coal and break up coal containing high levels of impurities; conveying the coal from the crusher upon one or more conveyor belts to pass beneath one or more electromagnets to remove ferromagnetic particles before passing the coal onto a deck of a first desliming screen, adding water and vibrating the first desliming screen in order to wash the coal and remove particles less than 3mm in size, such particles passing though apertures in the deck to be collected in a sump therebeneath, the washed coal passing over the deck of the first desliming screen and onto a further conveyor whereupon it is conveyed to a multi-deck washing and grading screen having an upper deck having apertures selected to remove material over 14mm in size, material less than 14mm in size passing through the apertures of the upper deck to fall onto a lower deck, the lower deck having apertures of a size selected to pass material less than 3mm in size, the greater than 3mm and less than 14mm coal particles being retained on the lower deck and being conveyed across the lower deck to pass onto a further conveyor whereupon it is transferred to a dense medium cyclone comprising an inclined fructoconical cyclone, tapering from an upper end towards a lower end, wherein the coal is mixed with a medium comprising a suspension of magnetite and water, such mixture being introduced tangentially into the cyclone through an inlet at an upper end thereof to generate a swirling motion within the cyclone whereby denser particles, comprising coal containing impurities, migrate towards the outer wall of the cyclone to subsequently pass out of a lower end of the cyclone as an underflow, while the less dense particles, comprising coal with a low ash content, are directed toward the centre to subsequently pass out of an upper end of the cyclone as an overflow, the method comprising the step of monitoring and controlling the density of the medium with the cyclone such that a coal product carried in the overflow of the cyclone has an ash content of less than or equal to 1.5%, removing magnetite from said coal product by means of one or more electromagnets and dewatering said coal product upon a dewatering screen to product a coal product having an ash content less than or equal to 1.5% suitable for use as a carbon reductant in the manufacture of silicon.

2. A method as claimed in claim 1, comprising the further step of conveying said coal product from the dewatering screen to a further grading screen, said further grading screen being adapted to remove material having a particle size greater than 12mm and less than 3mm from said coal product.

3. A method as claimed in any preceding claim, comprising the step of monitoring the density of the medium passing into the cyclone of the dense medium cyclone and adjusting said density in real time by adding magnetite and / or water to the medium to maintain a target density required to achieve the required ash content in the coal product leaving the overflow of the cyclone.

4. A method as claimed in claim 3, wherein samples of the medium are taken at predetermined intervals and the density of said samples is measured to detect any changes in density requiring an adjustment of the composition of the medium over time.

5. A method as claimed in claim 4, wherein samples are taken and tested every ten to fifteen minutes.

6. A method as claimed in any of claims 3 to 5, wherein further variables, including temperature and water content of the coal entering the dense medium cyclone, are monitored, the density of the medium and the operation of the cyclone being optimised based on such variables.

7. A method as claimed in any of claims 3 to 6, wherein the overall average ash content and water content of the raw coal is determined before it enters the crusher, such data being used to determine baseline variables of the required density of the medium in the cyclone and other operating parameters of the dense medium cyclone.

8. A method as claimed in any of claims 3 to 7, wherein the rate at which the medium is pumped through the inlet of the cyclone is controlled to achieve and maintain the target ash content of the coal product.

9. A method as claimed in claim 8, wherein the rate at which the medium is pumped through the inlet of the cyclone is controlled by varying the current supplied to the pump motor.

510. A method as claimed in any preceding claim, wherein the step of monitoring and controlling the density of the medium with the cyclone is carried out such that a coal product carried in the overflow of the cyclone has an ash content of less than 1%.