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What points should be paid attention to in scientific iron making

2016-07-05

At present, China's steel production capacity has reached 1.1 billion tons, with a surplus of about 300 million tons, and the capacity utilization rate is 76%. There are still a number of ironmaking production equipment to be eliminated. The state has proposed to strictly control the growth of steel production capacity, but also to increase the intensity of overcapacity. Jiangsu tinplate iron and steel enterprises used to have output in the past, and now they are entering the era of low profit. Some enterprises are even in a state of loss. The greater the output, the more losses. Therefore, iron-making enterprises should not focus on the pursuit of output, but should promote the optimization of production indicators, promote energy-saving emission reduction, and maximize benefits.

China's ironmaking industry faces multiple constraints

At present, the development of China's ironmaking industry is severely constrained by resources, energy and the environment. The newly announced national environmental protection standards have been in line with the world level. Enterprises must operate according to the new environmental protection standards. The cost per ton of materials will increase by about 100 yuan. Now the operation of steel enterprises is a small profit.

The steel industry accounts for about 16% of the country's total energy consumption, while the iron-making system accounts for about 70% of the total energy consumption of steel production, with pollutant emissions accounting for about 90% and costs accounting for about 75%. The state has proposed energy total control targets and has also increased energy consumption access standards for blast furnace ironmaking processes. Therefore, the ironmaking system must complete the task of energy conservation and emission reduction, reduce production costs, and achieve environmentally friendly production processes.

In recent years, the quality of iron-smelting raw fuel has declined, the composition has fluctuated greatly, the content of harmful impurities has increased, the production of blast furnaces has been unstable, and the technical and economic indicators have deteriorated. It is imperative to change the management concept, attach importance to scientific development, optimize the structure of the furnace material, strive to reduce production costs, and enhance the competitiveness of the enterprise market. This requires companies to respect the basic principles of metallurgy, eliminate unscientific ironmaking, and optimize ironmaking to obtain the best indicators and performance.

Focus on the scientific concept of operation and operation technology

Change the concept of operation, no longer pursue production, and achieve low cost and maximum profit. At present, a number of small and medium sized blast furnace operations use high air volume and high smelting strength to achieve high yield and increase fuel ratio.

The scientific blast furnace operating policy is to achieve a high utilization factor by reducing the fuel ratio. The optimum smelting strength of the blast furnace is about 1.2t/m3·d. Above this value, the blast furnace output will increase, but the fuel ratio will increase and the cost will increase. Some small and medium sized blast furnaces use high air volume and high smelting strength to achieve high utilization factor, which is an incorrect operation policy. The consumption of tons of iron is high, and the fuel ratio is inevitably high, which also causes high energy consumption in the iron making process. Therefore, the medium smelting strength can obtain the best indicator of iron making.

Optimize the structure of the blast furnace charge and improve the proportion of pellets. Increasing the ratio of pellets and ore can effectively improve the iron content of the blast furnace into the charge, and promote the iron production to increase the coke. The energy consumption of the pellet process is 1/3 lower than that of the sintering process, and the pellet ratio is increased, which can effectively reduce the energy consumption of the ironmaking system. Less production of sinter can reduce pollutant emissions and environmental protection costs. Increasing the proportion of pellets is one of the development directions of ironmaking technology in China.

Imported pellets and production pellets are expensive with concentrate powder, which is the main reason for the reduction of the proportion of iron ore pellets in China. According to statistics, about half of the concentrate powder produced in China is suitable for the production of pellets. Enterprises should, in light of their own circumstances, study how best the pellet ratio is, how to produce neutral pellets, and how the blast furnace operation can adapt to the adjustment of high proportion pellets.

Whether the blast furnace is center-focused depends on the quality of the raw fuel and the level of blast furnace operation. The purpose of centering the blast furnace is to adjust the gas flow distribution (to achieve gas passage in the center of the furnace, there is no dead column), and to promote the blast furnace's smooth and high production. Baosteel does not implement central coke, because Baosteel's original fuel quality is good, the furnace material has good gas permeability, and it can achieve uniform gas flow distribution, high gas utilization rate and low fuel ratio. Therefore, whether or not to perform centering depends on the company's own raw fuel quality and blast furnace operation level. The operation of centering the coke will increase the fuel ratio, mainly due to the decrease in gas utilization.

Pay attention to the side effects of harmful impurities in the furnace on the blast furnace. At present, some companies are not scientific in order to reduce the procurement cost and purchase low-grade inferior ore. In particular, some low-cost ore mines have high levels of harmful impurities, which greatly exceed the standards required by the “Blast Furnace Ironmaking Process Design Code”, which have large side effects on the stability and longevity of blast furnace production, and cause losses in production.

Each iron-smelting enterprise shall establish standards for the content of harmful impurities in the charge, and calculate the quality standards for the procurement of iron ore by the purchaser. The iron ore should be scientifically evalsuated, the ore blending should be optimized, and detailed technical and economic calculations should be carried out to properly purchase low-grade inferior ore (generally controlled at 20% to 30%). The use of iron ore ironmaking below 50% grade, the process energy consumption does not meet the national energy consumption limit standards, and pollutant emissions are also high. The promotion of the use of low-cost limonite has little effect on the blast furnace index and the effect of reducing costs.

Try to increase the hot air temperature. At present, the biggest gap between China's ironmaking indicators and the international advanced level is the hot air temperature. The low hot air temperature causes the fuel ratio to be high (deducting the factor of entering the iron grade). The effect of high air temperature on energy saving and cost reduction should be highly valued. The air temperature can be increased by 100 °C, and the fuel ratio can be reduced by about 15 kg/t. High air temperature is an inexpensive energy source (obtained with 45% blast furnace gas) and is a good way to improve energy efficiency.

At present, China has completely mastered the design, material selection, equipment manufacturing, installation, operation and maintenance of high-temperature warm stoves (wind temperature >1200 °C). The existing hot blast stoves should be technically modified in a timely manner so that the air supply system can withstand high wind temperatures. In order to reduce construction investment and purchase low-priced inferior refractory materials, the long-term high wind temperature is lost, and the overall economy is not suitable.

Carry out scientific fabrics to improve gas utilization. At present, most of China's blast furnace gas utilization rate is not high, so that the fuel ratio is increased, and it has certain energy saving potential. The CO2 content is increased by 1%, and the iron-to-fuel ratio is decreased by 4kg/t~5kg/t. The way to increase the CO2 content is mainly to carry out scientific cloth to make the gas flow evenly distributed. To achieve multi-ring fabric (greater than 8 rings) using a bell-free device, the angular difference between the ore and coke is about 5 degrees. The good cloth effect is: the gas curve is double-swallow; the top of the furnace is a large platform with a small funnel in the center; the gas utilization rate is high.

Discussion of key issues in longevity technology. After the use of copper stave, the problem of longevity of the blast furnace has been transferred from the reflow zone to the hearth. The phenomenon of the burning of the hearth and the increase of the sidewall temperature in some blast furnaces in China highlights the seriousness of the longevity problem of the blast furnace.

The longevity of blast furnace is a systematic project, and it is necessary to conduct in-depth research from design, material selection, construction, operation, testing and maintenance. At present, China's blast furnaces are generally produced with high smelting strength and high utilization coefficient. The scouring of the hearth by molten iron is more serious, and the quality of refractory materials used in each blast furnace hearth is uneven. All enterprises should formulate scientific countermeasures for realizing the longevity of blast furnace according to their own specific conditions.

Focus on the applicability of equipment and technology

China should control the construction of blast furnaces above 4000m3. The quality of the raw fuel required for the blast furnace above 4000m3 is high, especially the thermal performance index of coke is required to be good, so that the production cost is increased. For example, Shougang Jingtang 5500m3 blast furnace has excellent coke quality, which is about 100 yuan/ton more expensive than other enterprises. China's main coking coal resources are tight, the price is high, and the cost of producing high-quality coke is high.

The energy consumption and cost of the smelting reduction unit compete with the blast furnace process. There is no hot blast furnace in the smelting reduction device (19% of the blast furnace ironmaking is provided by the hot blast stove), and the ore is directly reduced (the blast furnace is about 50% indirect reduction, the ore reduction is exothermic), and high quality is needed. Fuel (high price), equipment smelted with oxygen has a short life and low operating rate. The author suggests that China is not suitable for the construction of smelting reduction devices and should avoid unnecessary economic losses.

The design of the thin fireplace lining cannot be equal to the operating furnace type of the blast furnace. The thin fireplace lining technology is characterized by the combination of wall tiles, enhanced cooling, saving refractory materials, saving investment and reducing construction time. In terms of furnace design, the furnace type with thin fireplace lining technology should be close to the operating furnace type of the blast furnace, but it cannot be equal to the operating furnace type of the blast furnace. It must have a certain amount of sufficiency, allowing conditions of different furnace materials and different operating levels. Under the production process, a reasonable furnace shape is gradually formed to achieve the best blast furnace index.

Focus on the economics of raw material use

Scientific evalsuation of coke quality. The coke thermal performance test is carried out under specific conditions, which is completely different from the conditions in the blast furnace (temperature, reducing atmosphere, residence time, etc.). The thermal performance index of coke can only be a reference value. The quality of coke depends mainly on the lithofacies index of coal, such as J value, Y value, gel layer, reflectivity, etc., rather than tamping coke, CD quenching, etc. Therefore, scientific coal blending is the key to coke quality. It is scientific to take coke from the blast furnace tuyere and test the degree of deterioration of coke (grain size composition, strength change, etc.) to judge the quality of coke and to guide coking coal blending. M10 is the most important in the coke index. It is generally expected that the M10 index is less than 7% and the giant blast furnace is less than 6%.

Blast furnace ironmaking should also be based on concentrate. Scientific evalsuation of the economics of iron ore can not only look at the cost of procurement, but also the effect of smelting smelting and the impact on steelmaking and rolling processes. The gangue of the ore contains SiO2 and Al2O3, which will reduce its economical efficiency; the ore contains harmful impurities increased by 1%, and the smelting costs 30 yuan/ton~50 yuan/ton. The proportion of low-quality inferior ore used in ironmaking should be controlled at 20% to 30%. Otherwise, the blast furnace index will deteriorate and the fuel ratio will increase, which is not good for reducing costs. The ore grade of the blast furnace above 3000m3 should be above 58% in order to maximize the efficiency.

Scientific use of MgO. The iron ore contains Al2O3 and is required to be added with MgO to improve the fluidity and desulfurization efficiency of the slag. Most companies add MgO to the sinter, and the MgO content in the sinter is more than 2.0%. This will degrade the quality of the sinter and increase the pulverization rate. In order to solve this problem, the sinter should not be added with MgO, and MgO is added to the pellet or directly into the blast furnace.

Scientific evalsuation of sputum coke. The author believes that the coke index measured by coke-solidification is better than the coke of top-loaded coke (which should be multiplied by 0.85% factor compared with top-loaded coke). The combustibility is not as good as that of the top-loaded coke, which causes the CO content in the blast furnace gas to decrease, the indirect reduction of the ore to decrease, and the phenomenon that the fuel ratio is increased and the output is decreased. Tamping coke is not resistant to alkali metal erosion and is easy to powder. Therefore, it is recommended to use tamping coke with a blast furnace of 3000 m3 or more.

Promote economic coal injection. The amount of coal injected in the economy means that the pulverized coal injected into the blast furnace can be digested by the blast furnace and will not escape with the blast furnace gas. There are two main ways to digest pulverized coal: one is to burn before the tuyere (high air temperature, oxygen enrichment, dehumidification), and the other is that unburned coal powder is digested in the direct reduction zone during the ascent process (gas flow) The distribution is reasonable and the gas permeability of the furnace is good). Therefore, in order to achieve economical coal injection, in addition to increasing the pulverized coal combustion rate before the tuyere, it is also necessary to increase the digestion capacity of unburned coal powder, which is closely related to the shape of the blast furnace gas flow distribution.

The suitable coal injection ratio of the blast furnace is: after increasing the coal injection ratio, the fuel ratio is not increased, and the coal powder has a high substitution ratio of coke. Increasing the coal injection ratio is the technical development direction of blast furnace ironmaking and one of the effective measures to reduce the cost of ironmaking.


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