- Time:Mar 29, 2023
1. What harm does the alkali component have on the quality of cement?
As we all know, no matter what form alkali exists, it will bring the following disadvantages to cement production and quality.
(1) Make cement fast setting and early strength. When the alkali content is high, the early strength can be increased, but the 28-day strength will be reduced, and it is prone to rapid setting and unqualified stability.
(2) Cement with high K2O and Na2O content is prone to alkali-aggregate reaction with alkali-active aggregates, resulting in destructive expansion of concrete, which is commonly known as "concrete cancer".
(3) Excessive alkali (K, Na) may easily cause ring formation in the kiln, blockage of the preheater and the crust at the end of the kiln during production, especially affecting the crust of the smoke chamber behind the kiln.
2. What is the adverse effect on production and quality if the sulfur content in raw meal and coal is excessive?
Although based on the burning base of raw meal or clinker, SO3 only accounts for 0.1%-2%, and the total amount of alkali is less than 1%, but their influence on the normal operation of the kiln and the quality of cement and concrete cannot be ignored of.
(1) Affect the normal operation of the calcination process: Excessive sulfur can easily cause ring formation in the kiln. In order to meet the low-alkali regulations, especially for the production of low-alkali cement, excessive SO3 will prevent the formation of C3S in the clinker during calcination.
(2) Unreasonable sulfur-alkali ratio will affect the quality and burnability of clinker.
(3) The higher the SO3 in the clinker, the worse the grindability in the cement mill, and limit the amount of gypsum added, which in turn affects the setting time of cement.
(4) Affect the strength of cement and concrete: when SO3 is high, it is beneficial to increase the strength of 28 days; the optimal amount of SO3 can maximize the strength of mortar for one day, and it is also beneficial to the later strength or the maximum strength at elevated temperature. For cement with coarser fineness, higher C3A and alkali content, more SO3 is needed, which is beneficial to strength; concrete with water reducing agent also needs higher SO3.
(5) The formed sulfides will corrode the equipment, especially cause serious corrosion to the flue gas pipeline.
3. What is the mechanism of the cycle of sulfur and the effect of sulfur-alkali ratio on production?
(1) The main form of sulfide from the oxidation of sulfur in the fuel and the decomposition of sulfate is SO2. They volatilize at high temperature in the kiln, and the process of coagulation after cooling in the preheater is the cycle of sulfur in the kiln. Theoretically, SO2 can be oxidized to SO3 at low temperature, but in fact 99% of the gaseous sulfides will be SO2, which are almost all combined with K2O, Na2O and CaO in the preheater, and volatilized again in the firing zone, and then to The kiln tail and the fourth and fifth preheaters form a sulfur cycle crust, and the non-volatile alkaline sulfate exists in the clinker. This cycle will eventually cause the preheater skin or blockage.
(2) The occurrence state of alkali in clinker depends on the molar ratio of sulfur to alkali:
SG=SO3/(1.292Na2O+0.85K2O)
When SG<1, in addition to forming K2SO4 and Na2SO4, K2O and C2S also form irregular round KC23S12 particles and more residual f-CaO; Na2O and C3A form dark long strips of NC8A3.
When SG>1, in addition to forming K2SO4 and Na2SO4, the remaining SO3 forms CaSO4.
(3) The ratio of sulfur to alkali content has a great influence on the production of precalciner kiln, but keeping the ratio of sulfur to alkali constant is dynamic. Because the material stays in the kiln for a very short time, the alkali (K, Na) is easily taken away, and the sulfide is still volatilized and circulated in the kiln. At this time, due to the lack of alkali, a crust will form at the back of the kiln. The vicious influence of the circle. If the calcination temperature of the kiln is too high, or the flame is swept directly on the material, this cycle will be intensified until skinning or blockage of the preheater occurs. And once in the low temperature area of the kiln, these sulfides will be released, so that the SO3 in the generated clinker will become higher. When the chemical equivalent of SO3 exceeds the base, its volatility is greater
In addition, when designing the specific resistance of electric dust collection, it should be considered that SO3, Cl8 and H2O will all reduce the specific resistance of alkaline dust.
4. How to reduce the sulfur-alkali content in production?
(1) In the ingredients, sufficient attention must be paid to the content and proportion of sulfur-alkali and maintained, and hard ingredients should be avoided as much as possible. For example, when producing low-alkali clinker, it is necessary to strictly limit the sulfur content in raw meal and fuel; for raw materials of high-alkali limestone, phosphogypsum can be added appropriately, which not only reduces the harm of alkali to calcination, but also improves the ease of raw meal. burning.
(2) When the content of sulfur and alkali poses a serious threat to the kiln tail crust, only the bypass discharge of gas (or material) can be set to alleviate the above phenomenon.
(3) Foreign countries adopt the scheme of "cleaning" the exhaust gas with activated carbon filter to reduce the impact of sulfide on production. When it is unavoidable to introduce sulfide in the raw meal, especially when pyrite is used as the iron raw material, when the kiln exhaust gas passes through the drying system and the raw meal mill system, it can absorb CO2 at low temperature. At this time, the drying system is extremely important. The atmosphere, temperature, partial pressure of various gases (such as O3, SO2, CO2 and water vapor, etc.) Influence.
5. Can high-alkali clinker produce alkali-free cement?
The alkali-aggregate reaction in concrete can cause damage to buildings. The raw materials in northern my country generally contain high alkali. Therefore, it is very meaningful to use these raw materials to produce alkali-free cement.
Alkali in concrete mainly comes from cement clinker, admixtures and admixtures. The alkali in the clinker is divided into two forms: soluble alkali and insoluble alkali. Most of the soluble alkali is mainly sulfate alkali, and the insoluble alkali exists in C2S, C3A, and C4AF in the form of solid solution. Among them, soluble alkali has the greatest influence on strength.
In this regard, ZHENJIN REFRACTORIES has carried out research and experiment work. The macroscopic manifestation of the alkali-aggregate reaction is expansion cracking, and the damage effect of this reaction on concrete can be tested by measuring the expansion value. The determination of the amount of soluble alkali is carried out according to the method of ASTM C114. The conclusion is that when the high-alkali clinker is mixed with 20% fly ash and ground together, its specific surface area can be appropriately increased to 390m²/kg. Although it has a large amount of soluble alkali, it can well inhibit the alkali-aggregate reaction.
ZHENJIN REFRACTORIES has a production and office area of 7,000 square meters, 3 ultra-high temperature tunnel kilns with intelligent automatic control system, 20 sets of 400-2,500 tons of various types of CNC automatic brick presses, with an annual output of 100,000 tons of refractory materials. As one of the well-known Chinese refractory brands, Zhenjin Refractory has been established for nearly 40 years. It has rich project experience and has completed a number of large-scale high-temperature kiln masonry projects, providing cost-effective refractory solutions for many users at home and abroad. . Zhenjin Refractory goes deep into the project site to understand the actual needs, provides each user with a complete set of suitable program design, establishes a good brand image of Chinese refractory materials, and is favored by many overseas customers.
