Chromium-free alkaline bricks made of magnesia and ferrous sharp stones have excellent thermomechanical and thermochemical properties, and are suitable for firing zone and upper transition of large pre-decomposition kiln under normal temperature and high thermomechanical stress. Belt and alkali-laden firing belt, the development and application of chrome-free alkaline refractory bricks have become the focus now
Reasons for the development of chrome-free alkaline refractory bricks
In the middle of the last century, the direct bonded magnesia-chromium bricks made of natural magnesia are brittle and easy to crack the bricks. They are only used in the steel industry. In the 1960s, high-quality magnesia was used, and high-quality chromium ore was also used. The direct combination of magnesia-chrome bricks produced through special calcination has excellent elasticity and thermoplastic stress relaxation, kiln skin properties, and thermal shock resistance. The resistance to chemical corrosion is used in the firing zone of various rotary kiln and the transition zone of rotary kiln with low alkali salt erosion, and has gradually developed into the main product of alkaline brick for cement kiln. Taking only a certain brand of magnesia-chrome brick produced by a foreign refractory manufacturing company as an example, since 1967, the total output has exceeded 2 million tons (> 200 million pieces).
Magnesium-chromium brick contains chromium. During the cement kiln clinker calcination process, the alkali (or sulfur) in the kiln and clinker interacts with the stable trivalent chromium in the brick to generate highly oxidizable hexavalent chromium. Salt is easily soluble in water, and the resulting solution is harmful to the human body. From the perspective of environmental protection requirements, it is necessary to develop a chromium-free alkaline refractory brick.
Application of chrome-free alkaline refractory bricks
For many years, refractory manufacturers have been engaged in the reduction of chromium in magnesium-chromium bricks, from chromium content> 8% to less than 4%. On the other hand, research on chromium-free alkaline bricks has been carried out, which is to optimize chromium-free Magnesium-aluminum spinel, dolomite, magnesium dolomite, magnesium-iron spinel, magnesium-zirconium bricks and other chromium-free alkaline bricks have made progress to varying degrees. So far, it is difficult for an alkaline brick to completely replace magnesia-chrome bricks in terms of performance and price. People have been looking for and optimizing chrome-free alkaline refractory bricks to comprehensively replace magnesia-chrome bricks with good performance and relatively low prices.
In recent years, the performance of a ferromagnesium (ferrous) aluminum chromium-free refractory brick made by special calcination using synthetic ferrous spinel as an elastic agent and sintered magnesium oxide containing iron oxide as raw materials has attracted attention. The chemical composition of chrome ore, magnesia-alumina spinel, and ferrous spinel as elastic agents are shown in the table:
- Chemical properties of chrome-free alkaline refractory bricks：
The chemical composition of ferrous spinel made of ferrous spinel as an elastic agent in the MgO-Al2O3-Fe2O3 system differs greatly from that of magnesium-aluminum spinel, but close to the composition of chromium ore. In the brick production process, Al2O3 and MgO or FeO / Fe2O3 are easier to replace Cr2O3, so the refractory bricks made do not contain Cr2O3.
Fused magnesia, iron oxide, and iron spinel react with surrounding materials to form uneven pores and fine structures locally, which reduces the erosion of clinker melt by iron spinel, while ferrous spinel and magnesium aluminum Spinel is similar, but does not react with surrounding materials, so it is resistant to clinker melt erosion.
The chrome ore in magnesia-chrome bricks easily interacts with alkali compounds in the kiln material and kiln gas to form brittle chrome-alkali compounds. According to the comparison of the alkali crucible test, under the condition of 1350 ° C, using potassium carbonate solution as a test, the result is that chromium ore and potassium carbonate form chromate, and a large number of traces are found in the crucible shelter, but ferrous spinel is not associated with carbonic acid. Potassium effect.
Ferrous spinel, magnesia spinel, and C2S, which are made of ferrous spinel and sintered magnesia containing iron oxide, are easy to interact with the kiln material to produce high-viscosity calcium ferrite and Calcium aluminate compound improves the performance of hanging kiln skins, surpassing spinel and magnesia-chrome bricks
- Physical properties of chrome-free alkaline refractory bricks：
Coarse-grained crystals made of ferrous spinel and spinel and produced by a special calcination technique. The ferrous alumina spinel bricks have excellent elasticity and thermal shock resistance, a reduction in creep stress, and magnesium and chromium Compared with brick, it has better thermo-mechanical properties.
Under the appropriate fire resistance, the compressive strength is about 20% higher. The porosity is only 13 ~ 15%, which is about 3 ~ 4% lower than that of magnesia-chrome brick. Without affecting other properties, the melt penetration resistance is increased by about 25%.
- Stress sensitivity index of chrome-free alkaline refractory bricks :
The stress sensitivity index SSI is slightly lower than that of magnesia-chrome bricks. SSI refers to the sum of the thermal expansion stress and ovality stress of the refractory brick minus the cold compression strength of the refractory brick itself, that is:
From the formula chart, the stress sensitivity index is almost related to the physical properties of the refractory bricks, that is, the thermal expansion of the refractory bricks at high temperature, the ovality stress and the strength of the bricks. Practice has confirmed that the smaller the SSI value, the better the physical properties of the refractory brick.
Because of its fine structure, ferrous spinel bricks have lower thermoplastic stress relaxation values than magnesium-chromium bricks. Thermoplastic relaxation means that the smaller the creep value of the refractory brick is, the better the resistance to thermomechanical stress is under normal pressure and normal temperature within a certain period of time (generally 25h).
development and application of chrome-free alkaline refractory bricks have become the focus now