Alumina Rotary Kiln Refractory

Aluminum Smelting Industry Description

The world’s annual aluminum output takes the top position among nonferrous metals production, far larger than other nonferrous metals. Refractory material consumption of aluminum industry is much more than the total consumption of copper, lead and zinc smelting. The production of aluminum including 2 steps, first, produces alumina from bauxite by wet method, second,  uses industrial alumina as raw material, adopts molten salt electrolytic process to produce aluminum. High temperature kilns used for aluminum production are rotary kiln, molten salt electrolytic tank, aluminum smelting furnaces, etc.

Refractory material consumption of industrial aluminum smelting furnaces is large, that’s because, during producing Al2O3, basic material will have very severe corrosion to alumina rotary kiln refractory material. When smelting aluminum, even the temperature is relatively low, aluminum liquid has very strong permeability, once it permeates into firebricks, it will react with SiO2 and reduces Si from the firebricks, which will destroy their structure and cause the looseness, spalling and damage of furnace lining.

So SiO2 containing refractory materials are also not suitable for the masonry of aluminum smelting equipments. High alumina bricks and carbon refractories are usually used for ordinary aluminum industry furnaces.

Alumina Rotary Kiln Description

Rotary kiln is thermal equipment for the sintering of alumina clinker. During manufacturing alumina, alumina bauxite, pure alkali and lime are proportionally loaded into rotary kiln, calcined at 1200~1300℃, the calcined bauxite clinker needs to be further processed to make Al(OH)3. After sintering in 1200℃ rotary kiln, Al2O3 are produced by Al(OH)3.

The sintering process are: high temperature flame and the raw material reversely move in the rotary kiln. Lime and bauxite raw material pulp(contains 40% water) or Al(OH)3(contains 12%~18% water) is loaded form kiln tail. After low temperature drying and dehydration, heating and high temperature sintering, calcined clinker will be unloaded from kiln head, while high temperature gas flows from kiln head to tail. So the kiln can be divided into preheating zone and high temperature sintering zone. To avoid pulp material adhering to kiln lining during sintering and to strengthen heat exchange efficiency, chains are set between refractory material brickwork, when kiln body is in rotary motion, the chains will continuously strike raw materials and lining bricks, which will affluence the service life of kiln lining.

Alumina Rotary Kiln Refractories Properties

The rotary kiln body used for alumina production is steel board welded cylinder, inner lining is constructed by refractories. The working environment of refractory materials are harsh, so they should have these characteristics: strong alkaline corrosion resistance, can continuously work at 1200~1300℃ high temperature and won’t damage, can resist material erosion and impaction, can resist the souring of high temperature air current.

Alumina Rotary Kiln Refractories Category

Reractories used in rotary kiln are mainly high alumina bricks and magnesia bricks. Low temperature drying kiln usually adopts fireclay bricks as inner lining refractory. At sintering zone, there is often an insulating lining between kiln wall and firebricks inner lining, which is often built by diatomite bricks, ceramsite bricks or refractory fiber felts.

Alumina Rotary Kiln Unshaped Refractories

Currently, unshaped refractories are popularly used in aluminum industry. Rotary kiln inlet where constantly suffers materials high temperature abrasion and thermal shock stress, is vulnerable to deformation damage. At alumina clinker sintering kiln transition zone, the temperature is between 400℃~1000℃, alkaline corrosion and mechanical damage are severe, the inner lining often spalls or falls off. So steel fiber castable which has good abrasion resistance is are usually used in rotary kiln.

Alumina Roasting Furnace Refractories

Alumina Roasting Furnace Refractories Description

The roasting of aluminum hydroxide is the last procedure of alumina production. Its main purposes are drying adhered water and crystallization water in aluminum hydroxide and transforming some γ-alumina into α-alumina. Most alumina manufacturer in China have totally or partly adopted fluidized roasting equipments. Fluidized roasting equipments are classified into fluidized flash roasting furnace, circulating fluidized roasting furnace and suspension roasting furnace. Refractories for these 3 types of roasting furnaces are not the same, but large quantity of unshaped refractory( plastic refractory and refractory castable) are used, more specifically, 50%~70% of refractory consumption is unshaped refractory.

Alumina gas suspension roasting furnace is the specialized equipment for aluminum hydroxide roasting. The craft and automation level are both very high, roasting process takes place at high temperature furnace body ( 1200℃) and high speed atmosphere. The processed alumina has high rigidity, good flowability, meanwhile, there are strict quality standard on alumina products, any mixing of impurity can directly affect the product performance. So the refractory materials used in alumina gas suspension roasting furnace should have the following characteristics: high refractoriness, good abrasion resistance, high strength, good thermal stability, good integrity, good tightness.

Alumina Gas Suspension Roasting Furnace Refractory

Aluminum Reduction Cell Refractory

Aluminum Reduction Cell Description

Aluminum reduction cell is the core equipment for the production of electrolytic aluminum. Aluminum reduction cells are usually rectangular steel shells, with carbon bricks inner linings. Carbon anode suspends in aluminum reduction cell, while carbon cathode locates at the bottom. Molten Cryolite, aluminum fluoride, lithium fluoride and so on are usually used as electrolyte. Al2O3 is smelted at 970℃ and electrolyzed into metal aluminum at bottom cathode and oxygen at anode, oxygen can react with carbon anode and produce CO2 or CO. The heat released by electrochemical reaction can keep aluminum and aluminum reduction cell molten state, aluminum liquid is discharged from reduction cell at intervals, and some quantity of alumina and cryolite are added into the cell. Electrolysis  temperature is kept between 900~1000℃.

Aluminum Reduction Cell Cathode Refractory

Refractory materials used in aluminum reduction cell cathode should have good electricity conductivity and good cryolite, NaF and aluminum liquid corrosion resistance. The bottom working lining of aluminum reduction cell is formerly lined by carbon block, but it can react with sodium and form new compound, the working lining’s structure becomes loose, mechanical strength deceases, and crack also appears in the carbon blocks. Electrolyte and aluminum liquid can permeate into carbon block through the cracks,  and react with carbon to form Al4C3, which causes the further expansion of carbon block cracks, and finally leads to the severe corrosion of inner lining and the deformation of aluminum reduction cell shell. Therefore, amorphous carbon bricks are gradually replaced by semi graphitized carbon bricks or graphitized carbon bricks.

Aluminum Reduction Cell Side Wall Refractory

The damage reasons of aluminum reduction cell side wall inner lining are air  caused material oxidization, high temperature molten cryolite, NaF and aluminum corrosion, moving molten liquid scouring and erosion, temperature fluctuation and thermal expansion caused thermal stress.

Amorphous carbon blocks and graphite carbon blocks are used to mason the side wall of aluminum reduction cell, the lethal defect of carbon based refractories is bad oxidization resistance and low mechanical strength. To prevent oxidization, and keep relatively large electric resistance, SiC refractory materials are gradually developing, among them, silicon nitride bonded silicon carbide bricks have the best application effect. The advantages of silicon nitride bonded silicon carbide bricks are: excellent high temperature mechanical performance, good thermal conductivity, easy to form condensed slay at inner side, large electric resistance, can reduce side way electricity loss, good oxidization resistance, won’t react with aluminum liquid, cryolite and other molten body, can large scale reduce lining bricks thickness and increase reduction cell volume. For example, the carbon bricks lined side way thickness is about 200~400mm, while the thickness of silicon nitride bonded silicon carbide bricks lined wall is only 75mm.  

Silicon Nitride Bonded Silicon Carbide Bricks Physiochemical Index

Aluminum Reduction Cell Bottom Blockage Layer

During electrolytic aluminum production, Na and NaF vapor, liquid can permeate into insulating lining from bottom cathode materials. So the thermal conductivity of insulating lining will increase and correspondingly decrease aluminum reduction cell thermal efficiency, the working atmosphere will degenerate till the damage of reduction cell. A new type of dry impervious material can be paved between cathode refractory and insulating lining, it can not only prevent the permeation of electrolyte but also has good heat preservation performance.

Dry Impervious Material Physiochemical Index

Aluminum Smelting Furnace and Holding Furnace Refractory

Aluminum Smelting Furnace and Holding Furnace Description

Gas-combustion or oil-combustion fixed or tilting reverberators are usually used for aluminum ingots and scrap aluminum melting and alloying, sometimes, resistance reverberator or induction crucible furnace are also adopted.

Although the temperature of aluminum liquid and aluminum alloy is only 700~800℃, magnesium, silicon, manganese and other element in aluminum and aluminum alloys are very active, easy to react with some components in refractory material and cause refractory damage. The main damage mechanisms of aluminum smelting furnaces are:

1. Aluminum liquid has very low viscosity, is easy to permeate into refractory material.
2. Some alloy elements in aluminum and its alloys have very strong reduction ability, their reduction-oxidization reaction is strong heat release reaction. Some alloy elements such as Mg have very high vapor pressure, their vapor is more liable to penetrate into refractory material and then be oxidized, finally cause the metamorphism , structure looseness and damage of refractory material.
3. During large scale aluminum smelting furnace working, continuously added aluminum ingots and alloy will have great impaction and abrasion to furnace inlet and furnace bottom.
4. The addition of aluminum ingots, the discharge of aluminum liquid and temperature fluctuation in furnace can do thermal shock damage to refractory lining.

Aluminum Smelting Reverbertator Refractories

Aluminum smelting reverbertator refractories should resist aluminum liquid and magnesium vapor infiltration, have excellent abrasion resistance and thermal stress impaction, etc. Furnace lining where contact with aluminum liquid usually adopts high alumina bricks containing 80%~85% of Al2O3. For high purity aluminum smelting, corundum bricks or mullite bricks are usually adopted. At furnace bed slope and scrap aluminum feeding parts, where corrosion and abrasion is frequently happened, silicon nitride bonded silicon carbide bricks are often used. At aluminum runner and aluminum tapping hole, aluminum scouring is severe, so self bonded or silicon carbide bonded silicon carbide bricks are adopted, zircon bricks are also used for the masonry of inner lining. For furnace lining, where don’t contact with aluminum liquid, clay bricks, fireclay castable or plastic refractory are usually used. The inner lining of aluminum running trough usually adopts silicon carbide bricks or fused foam silica precast blocks.

With the enlargement aluminum smelting furnace and the strengthening of smelting, high strength anti-aluminum permeation castable which has excellent permeation resistance, abrasion resistance and thermal impaction resistance are widely used in aluminum smelting furnace.

High Strength Anti-Aluminum Permeation Castable