Silica Fume Production Process Technica

Silica Fume Production Process Technical Document

1. Process Overview

Silica fume, also referred to as microsilica, is an ultrafine silicon dioxide byproduct with a typical SiO₂ content exceeding 85%. It is generated during the high-temperature smelting of ferrosilicon and industrial silicon (metallic silicon) at ferroalloy plants, when silicon vapor reacts with oxygen in the ambient air. Following dedicated collection and purification, this material becomes a high-value-added industrial commodity.

The production process centers on two core stages: high-temperature smelting fume capture and refined purification treatment. The finished silica fume exhibits exceptional pozzolanic activity, attributable to its ultrafine particle size (average diameter of 0.1–0.3 μm) and large specific surface area (20–28 m²/g). It is widely deployed in high-performance concrete, refractory materials, ceramic products, and other advanced industrial applications.

Beyond delivering superior material performance, silica fume production enables the resource utilization of industrial exhaust fumes and mitigates atmospheric pollution, delivering both profound environmental benefits and tangible economic value.

2. Core Principles

The core logic of silica fume production revolves around "smelting reaction - fume generation - efficient capture - quality purification". The key principles are divided into two steps:

2.1 Silicon Vapor Generation and Oxidation Reaction

In a submerged arc furnace (at a high temperature of 1800-2200℃), silica (SiO₂) and coke are mixed and smelted in proportion to undergo a reduction reaction: SiO₂ + 2C → Si + 2CO↑. The produced ferrosilicon (such as FeSi75) or industrial silicon (purity >98%) melts at the bottom of the furnace, and a large amount of silicon vapor (Si) is generated at the same time. The silicon vapor escapes out of the furnace with the smelting flue gas (containing CO, dust, etc.) and reacts rapidly with oxygen in the air in the flue: Si + O₂ → SiO₂, forming ultra-fine silicon dioxide fume (the original form of silica fume). At this time, the SiO₂ content in the fume is about 70%-85%, and it also contains impurities such as unreacted carbon powder and iron oxides.

2.2 Fume Capture and Purification Principles

The original silica fume has an extremely small particle size (easily diffused with the air flow) and needs to be captured through "negative pressure collection + multi-stage purification":

a. Negative pressure collection: Induced draft fans are used to form negative pressure in the flue system to suck high-temperature fume (300-500℃) into the collection equipment to avoid fume overflow;

b. Classification and purification: Coarse particle impurities (such as unburned carbon blocks and ferrosilicon debris) are removed through cyclone separation, and then ultra-fine silica fume is intercepted by bag filter. At the same time, metal oxides and soluble salts are removed through acid washing/alkali washing (optional) to increase the SiO₂ purity to more than 90% (high-end products can reach 97%).

3. Complete Production Process Flow

Silica fume production needs to connect the entire chain of "smelting flue gas extraction - fume collection - purification treatment - finished product storage" to ensure the fume capture rate >95% and product purity meets the standard. The specific process is as follows:

3.1 Process Framework

Submerged arc furnace (smelting ferrosilicon/industrial silicon) → Flue system (high-temperature flue gas extraction, temperature 300-500°C) → Cooling device (water cooling/air cooling, cooling the flue gas to 150-200°C) → Cyclone separator (primary separation: removing coarse particle impurities >10μm) → Bag filter (secondary collection: intercepting 0.1-10μm ultra-fine silica fume, capture rate >99%) → Crude impurity bin (coarse material from cyclone separation, can be returned to the furnace for re-smelting) → Primary silica fume bin (SiO₂ content 80%-85%, containing a small amount of carbon powder) → Induced draft fan (maintaining system negative pressure, air volume 15000-50000m³/h) → Purification treatment (optional: acid washing to remove metal oxides/alkali washing to remove carbon, increasing purity to over 90%) → Tail gas detection (dust concentration <5mg/m³) → Drying system (if the purified product contains water, dry to moisture content <1%) → Up-to-standard emission (in line with GB28662-2012 "Emission Standard of Air Pollutants for Iron and Steel Industry") → Finished product bin (classified storage: Grade Ⅰ SiO₂ ≥95%, Grade Ⅱ SiO₂ ≥90%)

3.2 Detailed Explanation of the Core Function of Each Link

3.2.1 Smelting and Flue Gas Extraction Stage

c. Submerged arc furnace: The core smelting equipment is usually a closed/semi-closed submerged arc furnace with a capacity of 2500-12500kVA (the closed furnace has a higher fume capture rate of more than 98%; the semi-closed furnace is about 85%). The furnace is heated by electrode arc to reduce silica and coke at high temperature to produce ferrosilicon/industrial silicon, and at the same time generate flue gas containing silicon vapor (the flue gas volume changes with the furnace capacity, for example, a 12500kVA furnace produces 15000-25000 m³ of flue gas per hour).

d. Flue system: Made of high-temperature resistant steel plate (material Q345R) with refractory material lining (such as high alumina brick) to avoid corrosion by high-temperature flue gas. The flue needs to be equipped with a guide plate to ensure the uniform flow of flue gas, and connect with a cooling device to prevent the filter bag of the subsequent bag filter from being damaged by high temperature (the temperature resistance of the filter bag is usually ≤200℃).

3.2.2 Flue Gas Cooling and Coarse Separation Stage

e. Cooling device: Adopt "water-cooled casing" or "air-cooled heat exchanger" to quickly cool the 300-500℃ high-temperature flue gas to 150-200℃ (if the cooling is too slow, silica fume is easy to agglomerate, affecting the subsequent dispersibility). The water-cooled casing absorbs heat through circulating water, and the water temperature is controlled at 40-60℃; air cooling blows air through a fan, suitable for water-scarce areas, and the cooling efficiency is slightly lower than water cooling.

f. Cyclone separator: As a primary pretreatment equipment, it uses centrifugal force to separate coarse particles >10μm in the flue gas (such as unburned coke blocks and small ferrosilicon particles) with a separation efficiency of about 80%-85%. The separated coarse material can be returned to the submerged arc furnace for re-smelting to reduce raw material waste; if the carbon content in the coarse material is high (>15%), it can also be reused as fuel.

3.2.3 Ultra-fine Silica Fume Collection and Purification Stage

g. Bag filter: The core equipment for silica fume collection adopts an "off-line cleaning bag filter" (to avoid secondary pollution of dust during cleaning). The filter bag material is PPS (polyphenylene sulfide) or PTFE (polytetrafluoroethylene), which can withstand a temperature of 150-200℃ with a filtration accuracy of 0.1μm. During operation, the flue gas containing silica fume passes through the filter bag, and the ultra-fine silica fume is intercepted on the surface of the filter bag. The silica fume is regularly shaken off to the primary silica fume bin through pulse jet (compressed air pressure 0.5-0.7MPa) with a capture rate >99%. At this time, the primary silica fume has a SiO₂ content of 80%-85% and a moisture content <3%.

h. Purification treatment (optional): To meet the demand of high-end market (such as high-performance concrete, electronic-grade silicon materials), it is necessary to improve the purity of silica fume:

i. Acid washing for impurity removal: Soak the primary silica fume with 10%-15% hydrochloric acid or sulfuric acid to remove metal oxides such as iron and aluminum (the reaction generates soluble salts, which are separated by filtration), which can increase the SiO₂ purity to 92%-95%.

ii. Alkali washing for carbon removal: If the carbon content in the primary silica fume is >5% (which will affect the concrete strength), react with 5%-8% sodium hydroxide solution by heating (60-80℃) to convert carbon into soluble sodium salt. After filtration, the SiO₂ purity can reach more than 95% with a carbon content <0.5%.

3.2.4 Finished Product Treatment and Tail Gas Emission Stage

i. Drying system: If the moisture content of the purified silica fume is >1% (easy to agglomerate), it needs to be dried by a hot air dryer (the heat source is smelting waste heat or natural gas) to control the moisture content <1% and ensure the fluidity of the finished product. The drying temperature ≤120°C to avoid high-temperature agglomeration of silica fume.

j. Finished product bin: Adopt a silo with stainless steel or carbon steel lined with epoxy resin to prevent silica fume from getting damp. The silo needs to be equipped with an air homogenizer to ensure the uniform particle size of the finished product; at the same time, store it by purity grade (Grade Ⅰ SiO₂ ≥95%, Grade Ⅱ SiO₂ ≥90%), and pack it into bags (usually 25kg/bag) or in bulk (for bulk transportation) through a screw conveyor or pneumatic conveying system.

k. Tail gas treatment: The tail gas purified by the bag filter has a dust concentration <5 mg/m³, which meets the national emission standard. Some enterprises will add an activated carbon adsorption device after the induced draft fan to further remove harmful gases (such as CO) in the tail gas and improve the environmental protection level.

4. Key Equipment Parameters and Selection Points

The selection of core equipment for silica fume production directly affects the fume capture rate and product quality. The key parameters and selection suggestions are as follows:

5. Process Advantages and Application Scenarios

5.1 Process Advantages

• High resource utilization rate

As an industrial by-product, silica fume will be directly discharged to pollute the atmosphere if not collected. Through this process, the fume capture rate of more than 95% can be achieved. A 12500kVA submerged arc furnace with an annual output can recover 800-1200 tons of silica fume per year. At the same time, coarse particle impurities can be returned to the furnace for reuse, increasing the comprehensive utilization rate of raw materials by 10%-15%.

• High product added value

The market price of purified silica fume (Grade Ⅰ) is about 8000-15000 CNY/ton, which is 40-50 times that of the original submerged arc furnace by-products (such as slag, price 200-300 CNY/ton), significantly improving the economic benefits of enterprises; in addition, silica fume can replace part of cement (dosage 10%-15%), reducing the production cost of concrete and forming industrial chain value-added.

• Significant environmental benefits

The closed submerged arc furnace + bag filter system can make the dust emission concentration <5 mg/m³, far lower than the national limit (30 mg/m³); at the same time, it reduces the emission of harmful gases such as CO and SO₂ (the effect is better if equipped with desulfurization and denitrification devices), helping enterprises achieve "ultra-low emission".

5.2 Application Scenarios

• Supporting production in ferroalloy plants

Ferrosilicon and industrial silicon production enterprises can directly support the silica fume production line, use the existing smelting flue gas resources without additional raw material investment, and realize double profits of "main product + by-product" (e.g., an enterprise with an annual output of 50,000 tons of ferrosilicon can support an annual output of 1000 tons of Grade Ⅰ silica fume, with an annual additional income of 8-15 million CNY).

• High-performance concrete field

Produce high-purity silica fume (SiO₂ ≥95%, carbon content <0.5%) for high-performance concrete such as high-speed railway track slabs, long-span bridges and high-rise buildings, which can improve the compressive strength of concrete (28-day strength increased by 20%-30%) and impermeability (permeability coefficient reduced by more than 80%).

• Refractory material and ceramic industry

Silica fume can be used as a binder for refractory materials (replacing clay) to improve the high-temperature stability of refractory bricks (service temperature increased by 100-200℃); it can also be used in ceramic blanks to reduce the sintering temperature (reduced by 50-100℃) and save energy consumption.

6. Key Points of Process Control

• Control of smelting temperature and flue gas volume

The smelting temperature of the submerged arc furnace should be stabilized at 1900-2100℃ (if the temperature is too low, the production of silicon vapor will decrease and the output of silica fume will drop; if the temperature is too high, too many coarse particle impurities will be generated); at the same time, the flue gas volume is stabilized by adjusting the electrode current (e.g., the current of a 12500kVA furnace is controlled at 1800-2200A) to avoid load fluctuation of the bag filter.

• Control of flue gas cooling rate

The high-temperature flue gas should be cooled from 300-500℃ to 150-200℃ within 10-15 seconds (if the cooling is too slow, silica fume particles are easy to agglomerate and the particle size becomes larger, affecting product activity; if the cooling is too fast, condensed water is easy to generate, causing silica fume to get damp and agglomerate). The cooling rate can be controlled by adjusting the circulating water volume of the water-cooled casing (e.g., 25000 m³/h flue gas matches 50-80m³/h circulating water) or the rotation speed of the air-cooled fan.

• Control of bag filter operation parameters

l. Filtration speed: Control at 0.8-1.2m/min (if the wind speed is too high, the filter bag will wear out quickly and silica fume is easy to penetrate; if the wind speed is too low, the treatment efficiency is low);

m. Ash cleaning cycle: Pulse jet once every 3-5 minutes (adjust according to the pressure difference of the filter bag, timely ash cleaning is required when the pressure difference >1500Pa), the jet pressure is 0.5-0.7MPa to avoid damage to the filter bag due to excessive pressure;

n. Filter bag temperature: Ensure the flue gas temperature ≥120°C (below 120℃, condensed water is easy to generate, causing silica fume to adhere to the filter bag and affect the ash cleaning effect).

• Control of product purity and moisture content

The SiO₂ content of primary silica fume should be ≥80% (if it is lower than 80%, it is necessary to check the raw material ratio of the submerged arc furnace and adjust the ratio of silica to coke to 1:0.4-0.5); the moisture content of purified silica fume should be <1% (the drying temperature is controlled at 100-120℃, and the drying time is 2-3 hours) to avoid agglomeration during storage.

Through the above process design and precise control, silica fume production can achieve the goal of "environmental protection compliance, high-efficiency resource utilization, and high product added value". It provides a mature solution for the resource utilization of by-products in the ferroalloy industry and helps the upgrading of the downstream high-performance material industry.