Iraqi quartz silica is the purest silica sand among silica deposits around the world. The purity of Iraqi silica more than 98% SiO2 (1). According to Geological Survey of Iraq, more than 850 m.t. of proved reserves are available of almost pure quartz-sand (1). Presently mined from one site in Anbar Governorate and used in glass, ceramics and refractories production. It is the first time to direct the exploitation of this huge natural resource to be used in the production of silicon metal in an exact property for the state company for mining industries (SCMI) as it new industrial efforts to build metallurgical plants to exploit the natural resources in Iraq followed by establishing an integrated metallurgical plant to produce ferrosilicon alloy, the key alloy in Iron and steel production.
One of the important property of this Iraqi high quality silica sand is the micron size of its particles (1). It is for the first time, it is a big difficulty facing the industrial application but, the company’s decision is to establish the plant and overcome all the difficulties face the project, so agglomeration of the micron size particles of Iraqi quartz sands by briquetting is the way to the company to be able to continue its plans in the mining identity of company share.
The present research is related to providing briquetting process for the particle size of the high quality micron size Iraqi silica sands which the state company for mining industries make a decision to exploit in the silicon metal production project. Briquetting of micron size silica sands into lumps must have a total properties suitable to be a feed charge to production furnace, arc furnace type.
State company for mining industries is a company specialist in the establishing downstream products mining plants to exploit the natural resources abundant in Iraqi soil or by recycling scrap of metals such as aluminum metal scrap like profiles, aluminum cans, etc.
In 2013 the company made a decision to establish silicon metal production plant to exploiting high silica Iraqi sands about 98% SiO2. The published articles belong the industry of production of silicon metal for its ores demonstrates that the preferred ore is quartzite rocks, therefore 90% of silicon production plants around the world depending quartzite rocks as raw material to feed the production furnace. The purity of the quartzite rocks between 80-99% SiO2 (2).
Industrial production of metallurgical-grade silicon.
Silicon is produced industrially by carbothermic reduction of silicon dioxide in submerged-arc electric furnaces. Feed materials include a silicon source such as quartz, and a typical reductant as charcoal.
A typical industrial silicon furnace has a shell. Three pre-baked carbon electrodes, submerged into the charge materials, supply a three-phase current that heats the charge materials up to about 2000 deg. Celsius. At this temperature, silicon dioxide is reduced to molten silicon. The furnaces are operated such that there is a layer of solid material (outer reaction zone) on top of the molten bath (inner reaction zone) to maximize the silicon yield. The silicon is tapped from the furnace through a tap-hole at the bottom, and refined by gas purging. During refining, inclusions are removed, and the composition is adjusted to the specified value. Subsequent to refining, the molten alloy is allowed to cool in a mold, and then crushed to a specific size. Metallurgical-grade silicon has a typical specification of 98.5–99.5% for Si.
A by-product of the silicon smelting process is condensed silica fume. A silicon furnace produces about 0.2 to 0.4 tons of condensed silica fume per ton of silicon metal. The fume reports to the filter plant where it is collected. Condensed silica fume has found a wide array of applications in industry.
The electrical energy consumption for the production of metallurgical-grade silicon is about 11–13 MWh/ton of silicon metal. The off-gas from the silicon furnace has an energy content of the same order of magnitude as the electrical energy input to the furnace.
The metallurgical-grade silicon is produced in excess of 1 million metric tons/year at a cost of few US dollars/kg depending on quality, purity, and particle size. Metallurgical-grade silicon is produced in countries with cheap electricity, reductants, and good quartz deposits. Demand for metallurgical grade silicon is primarily from the aluminum and chemical industries, and a small fraction is refined into semiconductor-grade silicon1. However, this picture is expected to change in the future, as the fastest growing market for silicon metal is the photovoltaic market, with projected consumption exceeding current silicon for all other applications combined by 2020 (3).
Physio-chemical conditions of the process
The production of silicon can be guaranteed when the kinetics of the chemical reaction for the reduction of silicon dioxide to silicon metal match the theoretical estimation of the furnace charge with some quantity of byproduct of valuable material called silica fumed. The quantity of silica fume produced at working in stable condition of silicon production estimated practically. It is found to be approximately 1:6 to 1:5 by weight of the total furnace production capacity. The deviation from the stable conditions of silicon production causes one or more of these cases:-
- Disappearing of the long yellow flame from furnace top.
- Eroding the carbon electrode (quartzitization) by excess of liquid silica.
- Evolution of white powder come from the furnace top and darkened spots appear.
- Movement of the electrode upward because high consumption of silica or silicon carbide.
All of these cases of condition deviation can be overcome by the proper preparation of the furnace feed charge, the key property of the normal run. In each state there is a decrease in the silicon taken from tap hole in accordance with the charge supplied and the power consumed.
The reduction of quartz by carbon can be summarized by reaction 1:
SiO2 + 2C => Si + 2CO – (1)
this reaction does not occur directly but proceeds via a series of intermediate reactions which form solid silicon carbide (SiC) and gaseous silicon monoxide (SiO). At the bottom of the furnace is the main reaction zone (metal production zone), at temperatures exceeding 1820°C.
The first reaction to take place is the reduction of molten silica (SiO2) with carbon from the reductants to produce two gaseous products.
SiO2 + C => SiO + CO – (2)
the gaseous silicon monoxide (SiO) further reacts with carbon to form solid silicon carbide (SiC) in the furnace.
SiO + 2C => SiC + CO – (3)
the silicon carbide can then react with molten silica to form both silicon and silicon monoxide.
SiC + SiO2 = Si + SiO +CO – (4)
This silicon is drained from the furnace via a tap hole into ladles which refine the silicon and transfer the silicon to the casting area.
Silicon monoxide which fails to react within the furnace oxidizes in the atmosphere to form SiO2 (a dust-like material called amorphous silica fume). The silica fume is vented away for collection in a large filtration facility (baghouse) as a by-product of the silicon production (4).
2SiO + O2 => 2SiO2 – (5)
OBJECTS OF THE RESEARCH
The object of the research is to provide a method for forming briquettes made from the micron size Iraqi high silica sands between 600 and 850 micron suitable for use in silicon smelting processes that utilize electric arc furnaces.
SUMMARY OF THE RESEARCH
The high silica Iraqi sand is a small particle size of sands between 600 and 850 micron, so this micron size particles is up-siding down the industrial point of view to the preparing unit for the silicon bearing material to make feed suitable to the arc furnace. As the aim in using quartzite rocks is to minimizing the lump size to the required size, the aim in using micron size Iraqi high silica sands is to enlarge the lump size to the required size.
In the case of Iraqi plant for silicon production the process using micron size high quality silica sands form Ardhuma quarry more than 98% SiO2. The industrial production methodology designed by the most experienced Chinese company demonstrates that the requirement of major raw materials, silica and carbonaceous reducing agent for metal silicon smelting.
Although this micron size particles of the Iraqi silica sand, the designed requirements for the industrial process demonstrates that the particle size for quartz sand must be 8-80 mm, therefore the size of particles must be enlarged by a suitable industrial mean as part of preparation unit for the raw materials to prepare the feed charge for the production furnace.
About 90% of metallurgical silicon production plants in the world uses quartzite rocks as a source material of silicon. The rocks mined from the mine area then crushed in jaw crusher to the sizes depending on the volume of arc furnace, electric power, and the production capacity, the high production capacity the high sizes of quartzite rock.
The major problem of the research is concentrate in the micron size of the Iraqi high grade silica sands, which be used by the silicon plant as a silicon bearing material.
In Iraq, the quartzite rock has a very low occurrence causes unable to use it as a raw material for silicon production for long time, because industrial plants has a basic design and a methodology in the design of preparation unit of raw materials feed to the production furnace, this means the industrial philosophy cannot adopt Iraqi quartzite to be exploited in the silicon metal production plant.
MAJOR PROBLEMS OF THE RESEARCH
The major problem of the Iraqi high grade silica sands is the micron size of its particles, it is between -850 and +600 micron, which is the big difficulty to use this natural resource in the production of silicon metal.
MEANS FOR SOLVING THE PROBLEMS
Agglomeration process is the mean to solve the problem by designing and building a preparation unit for the raw materials which is silica\charcoal by using suitable binder. The process is based on the phenomena of briquetting to produce briquettes to feed the arc furnace.
ADVANTAGE OF THE RESEARCH
The advantages of the research is to make a breakthrough in the industrial exploiting of the quartz sands that have the ultimate purity among the deposited quartz sands around the world to be used in silicon metal production for new and renewable energy, first stage, then the production of silicon alloys like ferrosilicon alloy.
DESCRIPTION OF PRODUCTION PLANT
The production plant is a 1000KVA industrial furnace designed to adopt the local quartz sand to make lumps by briquetting process, and adds reducing agent charcoal to smelt. The industrial submerged arc furnace is designed for the smelting of industrial silicon.
The function is: putting the smelt raw material (quartz briquettes and charcoal) of industrial silicon after prepared in the preparation unit consist of mixer for the quartz and\or charcoal with bonding agent(s) then the briquetting machine to yield green briquettes which entered drying furnace to yield solid briquettes into the specifically-made crucible,
Next the three-phase electrode that put into the raw material generates arc heat, and then heats the furnace charged material, the qualified industrial silicon is generated after reduction.
As crucible part of the furnace, furnace body is the container for raw material to react in. The raw material produces reduction reaction inside the furnace body through electrode-heating according to a fixed proportion. The reaction zone can be divided into preliminary heating zone, conversion zone and finished product zone according to the depth of material surface to the furnace bottom. The uppermost is the preliminary heating zone, and the next is the reaction zone, the bottom of the furnace is the finished product zone. After the reaction carries out to a certain degree, the finished product discharges from the side tap hole of the furnace at regular time.
Short net system: The electric conduction system of furnace includes a serious of devices that from the furnace transformer to the short net then to the copper tile and the electrode. The function of short net is to send electric energy to the furnace inside from the transformer. The electrode generates electric arc, the temperature of electric arc achieve the aim that smelting the alloy. The function of the furnace transformer is to change the low current of high voltage to large current of low voltage.
Electrodes & Holding system of electrode: The electrode adopts graphite electrode mounted by lifting system. The function of electrode lifting and down device is to lift or descend the electrode that can change the electric resistance between the electrode and the furnace material so that the aim of regulating current can be achieved.
Water cooling system is an important part of the plant the furnace working in high temperature, so many components have to adopt water cooling structure so that the furnace can operate normally and the service life can be ensured. The furnace runs in high temperature, great attention has to be paid because the cooling effect has a direct relationship to the operation and service life of the furnace.
Smoke exhaust system: As the main equipment of the furnace, smoke exhaust system aims to collect smoke, it collects and exhausts the smoke and gas that generated during the smelting process of the furnace. The whole smoke exhaust system mainly includes three parts: smoke hood, smoke pipe and vice flue.
Furnace tapping operation: The furnace tapping of industrial silicon means that the fuse silicon generated by the furnace inside reaction is discharged from the furnace tap-hole (5).
THEORETICAL CONCEPTS OF BRIQUETTING
The process of enlarging the particle size of fine particles is called Agglomeration. There are many techniques lay under this methodology. Palletization and briquetting is the most suitable for the process but the former has been made without pressing stage which is the important property of the produced lump of sand, so the briquetting is the required process for agglomerating of the micron sized Iraqi high silica quartz sand.
Pelletization is the process of rolling fine material into little balls using a drum or disc. A binder is usually mixed into the raw material to make the pellets strong and water is added to increase the capillary forces between the particles and bind them together. Briquetting is a cold bonding method which involves pressing mixtures of ﬁne particles with a binder into lumps of composite particles using rolls, extruders or similar devices(6).
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, aluminum, titanium and the like.
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 utilization in the reduction of the silicon ore in the region of the arc.
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 utilize electric arc furnaces in the specific following points:-
- Specifying the probable type of binders can make use to bind quartz sand.
- Specifying the required input particle size of raw materials to yield briquettes.
- Specifying the required moisture content in the briquettes.
- Suggesting the proper mixing process.
- Choosing the proper briquetting machine
- Finding the proper dryer for drying green briquettes.
- The production rate of the preparation unit.
- Testing the compacting stability during manipulating the briquettes
- Testing the thermal stability of the briquettes in about 1000 deg. Celsius.
APPLICATION OF PRACTICAL CONCEPTS
Raw material preparation unit (equipment & conditions)
Binders: – The expected suitable binders to meet the above requirements are bitumen primer, starch, sugar, date syrup, and finally sodium silicate.
Input particle size: – It is found that particle size range between 0.5-1 mm and preferably 600-900 micron is the best range of particle size for briquetting process.
Moisture content: – It is found that moisture content between 6-16% and preferably about 8-12% is suitable moisture content in the green briquettes.
Mixing process: – it is found that using pug mill is suitable machine to reduce the particle size of charcoal to the required size then mix it with sands wetted by binder.
Briquetting machine: – it is found that using the strong force pressure briquettes making machine is proper machine to get ball\oval shape proper briquettes.
Drying furnace: – it is found that using multi stage vertical dryer furnace is proper furnace to dry green briquettes at 220 deg. Celsius with maximum output moisture content 3% max.
Production rate: – it is found if the production rate of the preparation unit be around 5-6 ton per hour is fair enough to meet the production rate and the future expansion.
PROCESS TO MAKE QUARTZ SAND BRIQUETTS
Brief introduction to the preparation unit
Quartz sand is conveyed to blender by conveyor. Quartz, charcoal and adhesion agent are evenly mixed in the blender. The mixture is conveyed to the bottom of blender and sent into strong force pressure pellet making machine. Green briquettes have to be dried in briquettes dryer. The dried briquettes are sent out by conveyor to reserve for use of production.
- Blender: evenly blend and crush of raw material charging take over by belt conveyor Equipment parameters.
- Strong force pressure pellet making machine: pressing and shaping powder materials, enabling them to be in a certain particle size to meet the smelting requirements. Requirement of material as follows
- Particle size of material shall be between 0.5-1 mm.
- Moisture: 8~12%. Material shall be with the same moisture, neither too dry nor too wet.
- Iron blocks and hard bulk sundries in material is not allowed.
- Vertical dryer: after shaped, wet pellets are sent to furnace top by belt conveyor. They are uniformly distributed on furnace top surface. Pellets shift down with their own weight. At the meantime, hot air from hot air pipeline is blowing on the pellets. Moisture in pellets is steamed and blows away by the air. Speed of pellets is adjusted by duck-billed discharging port. Pellets are steamed and dried sufficiently in dryer. Parameters of vertical dryer:
- Moisture before dried: 8~12%
- Moisture after dried: 1.5~3%
- Drying temperature: 150~250℃
- Power needed: 22KW
- Production rate: 5-6 T/H
APPLICATION OF RESEARCH TO PRODUCE SILICON
Table (1) Briquette composition determination
|Quartz bed||Silicon weight|
|Quartz||Charcoal||Percent||mm||P.S. micron||Quartz||Particle size|
|650 kg||234 kg||36%||30-35||-850 +600||335 kg||-850 +600||193 kg|
|700 kg||280 kg||40%||30-35||-850 +600||335 kg||-850 +600||179 kg|
|600 kg||276 kg||46%||30-35||-850 +600||335 kg||-850 +600||215 kg|
Table (2) Briquette size determination
|Quartz bed||Silicon weight|
|Quartz||Charcoal||Percent||mm||P.S. micron||Quartz||Particle size|
|600 kg||276 kg||46%||30-35||-850 +600||335 kg||-850 +600||199 kg|
|600 kg||276 kg||46%||40-45||-850 +600||335 kg||-850 +600||211 kg|
|600 kg||276 kg||46%||50-55||-850 +600||335 kg||-850 +600||223 kg|
Table (3) Quartz bed weight determination
|Quartz bed||Silicon weight|
|Quartz||Charcoal||Percent||mm||P.S. micron||Quartz||Particle size|
|600 kg||276 kg||46%||50-55||-850 +600||325 kg||-850 +600||204 kg|
|600 kg||276 kg||46%||50-55||-850 +600||335 kg||-850 +600||213 kg|
|600 kg||276 kg||46%||50-55||-850 +600||342 kg||-850 +600||230 kg|
- Alaa M.Kh. Mustafa, Ibtihal Kh. Fleah and Tanya V. Khachiek, “Preliminary upgrading of Silica Sand for Silicon and Silicones Industries from Ardhuma, Iraqi Western Desert” Iraqi Bulletin of Geology and Mining, Vol. 9, No. 1, 2013, p75-84.
- Erhard Klar, “Metals Handbook“, 9th Edition, Powder Metallurgy, Vol. 7
- S.Xakalashe and M. Tangstand, “Silicon Processing: From Quartz to Crystalline Silicon Solar Cells“, Paper Presented to South African Institute and Metallurgy Conference, Johannesburg, 6-9 March 2011.
- T. Edneral, “Electro-Metallurgy of Steel and Ferro-Alloys, Manufacture of Ferro-Silicon Alloy“, MIR Publisher, Moscow Part 2, p107-120.
- Marius Sunde, ” Organic Binder as Substitute for Bentonite in Ilmenite Pelletization“, Norwegian University of Science and Technology, Department of Materials Science and Engineering, June 2012