1 Introduction major domestic steel companies desulfurization of hot metal pretreatment station Most mixed mixed blowing method (referred to as mixed blowing) or a composite blowing (Blowing abbreviation) is desulfurized hot metal pretreatment. Substantially desulfurizing agent to passivate the metal magnesium and calcareous material as a raw material, in accordance with known kinetic conditions under thermodynamic data blowing metallic magnesium and molten iron pretreatment state, metallic magnesium granules is desirable desulfurization sorbent. However, since the magnesium metal has a low melting point (649 ° C) and a low boiling point (1088 ° C), it is extremely flammable and explosive. If the magnesium metal particles are not passivated during spraying, the function of magnesium desulfurization cannot be performed normally, and it still exists. A security risk. In order to give full play to the demagnetization effect of magnesium, improve the utilization rate of magnesium, the yield, reduce the consumption of magnesium, reduce the unfavorable factors such as splashing, blocking guns and sticky slag, and ensure that the production of the desulfurization process is smooth, so passivation is emphasized. The passivation quality of metal particles has become one of the important topics of metallurgical workers. The main research is on the principle of passivation, the selection of passivating agent and the implementation of passivation method to meet the requirements of different process parameters of hot metal pretreatment and desulfurization. 2 Metal magnesium particles passivated metal magnesium particles can be manufactured in two types. One is to melt the magnesium ingot, and the spray granulation method is used to form regular circular or elliptical particles with different particle sizes; the second is to mechanically process the ingot into pieces. After the chips are scraped, the appropriate particle size is used to form the metal magnesium particles, and the powder that does not meet the requirements is recycled. At present, the common metal magnesium particle size is 1.6-0.5mm, and the passivation technology is to passivate the surface of the metal magnesium particles suitable for blowing, for use in blowing.
2.1 Chemical properties of magnesium The chemical nature of magnesium is very active. Its solid is relatively stable in normal temperature and dry air, and is not easy to burn. However, it is extremely flammable in the molten state. 2Mg+O2=2MgO (1) At 300 °C, magnesium and air N2 start to act 3Mg+N2=Mg3N2 (2) The surface of magnesium is brownish yellow. When the temperature reaches 600 °C, the reaction is rapid. If there is water, Mg3N2+6H2O=3Mg(OH)2+2NH3 occurs. (3) During the steam heating process, Mg+H2O(g)= MgO+H2 (4) Therefore, magnesium is not easily contacted with water. Magnesium can dissolve in the acid, such as: HCl, H2SO4, HNO3, H3PO but can withstand HF, chromic acid (Cr2O3) corrosion. Halogen salts (or salt concentrates) MgCl2, MgCl2.6H2O have an etch effect on magnesium. Sulfide, nitride, sodium bicarbonate solution also has an etch effect on magnesium. Magnesium is stable in NaOH and Na2O3 solutions, but organic acids can destroy the stability of magnesium. Magnesium also reduces some oxides (TiO2, VO2, LiO2) and chlorides (TiCl4, ErCl4). Magnesium does not form an alloy with iron, and the solubility of iron in magnesium increases with increasing temperature. It can be seen from the chemical properties of magnesium that the chemical properties of magnesium are quite active and it is difficult to form a self-passivating oxide film. Generally, the refined magnesium ingots need to be treated with oxidation resistance - passivation. Passivation is a compound in which chromate passivation is used to soak the magnesium ingot with chromic acid, chromate or potassium dichromate as the main component to form trivalent chromium and hexavalent chromium and magnesium metal itself. Film, this film has a passivation protection against magnesium metal corrosion. Magnesium metal is one of the metals with the least corrosion resistance. Because of its high chemical activity, it is a thermodynamically unstable base metal. However, it is worth noting that magnesium ingots containing extremely low levels of metal impurities do not require protection on the surface of the magnesium ingot. 2.2 Passivation method of metallic magnesium particles 2.2.1 Chemical passivation At present, the magnesium passivation method of metal particles mostly utilizes the principle of removing non-metallic impurities in crude magnesium, and can form stable or unstable partial oxides CaO or MgO with halogen salts. The surface of the metal magnesium particles is protected by the principle of the complex. MgO+MgCl2 MgO.MgCl2 (5) 5MgO+MgCl2 5MgO.MgCl2 (6) CaO+MgCl2 CaO.MgCl2 (7) Therefore, MgCl2.6H2O (halogen or MCl2 430g/L) is used as a passivating agent to remove magnesium metal particles. After the surface is coated with CaO or MgO powder, the passivation solution is prepared in advance to form CaO.MgCl2 (or MgO.MgCl2). However, this method ignores an important fact that is the metal magnesium after passivation. The granules are sprayed with a spray gun by molten iron (in the transient process of molten iron at around 250 °C, H2O in the brine will react with Mg to generate the formula (4), causing flare until the molten iron and slag are splashed. There have been ways to change the structure of the spray gun, and the method of using a gasification chamber at the outlet of the spray gun to reduce the occurrence of splashing, although it has certain effects, it is easy to cause cracks in the muzzle of the wall and the slagging tumor of the gasification chamber. On the way, it is necessary to carry out heavy cleaning work on the gun gasification chamber at regular intervals, otherwise it will affect normal production. In this passivation method, MgCl2+H2O=MgOOHCl+HCl (8) MgOHCl=MgO+HCl (9) may occur. Reaction. Therefore, there is a strong HCl gas smell during the process of passivating the magnesium particles and during the spraying process. It is harmful to the operator's body and environment. In addition, due to the addition of CaO or MgO powder during chemical passivation, the reaction formula CaO+H2O=Ca(OH)2 also occurs in the case of aqueous solution. And MgO+H2O=Mg(OH)2 (11) This method is relatively inexpensive, so most of the passivated magnesium metal particles supplied in the domestic market use this method. 2.2.2 Physical passivation physical passivation is the magnesium metal The granule is placed in the CaO powder for high-speed spin coating and the additive is added with heating to coat the surface layer with a protective film. This method does not chemically react, only coating, then it is an additive. The choice of this kind of additive must have: a certain viscosity, easy to adhere CaO powder and metal magnesium particles; its density should be about 18/cm3; volatile matter ≤0.5% and does not chemically react with magnesium metal particles. In the process of coating, a solidifying agent should also be added to ensure the flowability of the sprayed magnesium. This substance has a dilution function for the slag thickening of the sprayed magnesium particles. At present, only a few manufacturers in China use this method to passivate.
3 Passivated magnesium metal physical index Mmg H2O S Particle size ≥92% ≤0.5% ≤0.002% 0.5-1.6mm Natural pile angle specific gravity flame retardant time ≤300 ≥900Kg/cn3 ≥10S Coating thickness ≤8% Burning point ≥580 °C
4 Factors affecting the desulfurization effect of hot metal pretreatment The influencing factors of the metallurgical effect of hot metal injection of magnesium particles are different. The specific data differs depending on the process parameters of each plant, the blowing method, the blowing device and the structure of the spray gun. However, the aspects of the influencing factors should be common. The following is only a brief analysis of the publicity for the iron pretreatment desulfurization operator. 4.1 The quality of passivated magnesium metal quality desulfurization powder directly affects the desulfurization effect. The quality requirement of passivated magnesium metal is mainly the content of magnesium metal. Generally, Mg≥92% is to ensure the desulfurization of injection. It should be proposed that the higher the magnesium content of the passivated metal magnesium desulfurizer, the better the desulfurization effect, because There is a problem of utilization of magnesium, and the magnesium content is high. Since the selection of the blowing parameters is unreasonable, it is also possible that the magnesium consumption is increased, and the target value cannot be achieved. However, the particle size range of the passivated metal magnesium should be selected according to the structure of the spray gun to ensure its proper fluidity; the thickness of the passivation layer and the flame retardant time are also different depending on the parameters of the injection process, and the flame retardant time is generally short. Magnesium particles are burnt out and are easily splashed; too long is unfavorable for magnesium to react in the molten iron. In short, the choice of passivated metal magnesium particles and the length of flame retardant need to adapt to the structure of the spray gun, can not be generalized. 4.2 Hot metal temperature The temperature of molten iron should not be too high, nor should it be too low, generally in the range of 1250-1350 °C. 4.3 Passive metal magnesium injection production practice found that when the sulfur content in molten iron is low, the dissolution loss of magnesium in molten iron increases sharply. When the molten iron contains 0.05% sulfur, the magnesium consumption in molten iron only accounts for the desulfurized magnesium consumption. When 5% is sulfur and 0.005% is sulfur, the melt loss magnesium is 5-10 times that of desulfurization. Therefore, the amount of magnesium metal sprayed should be adjusted in time according to the original sulfur content in the molten iron. Domestic data shows that the magnesium consumption is about 0.5kg/t iron. 4.4 The blowing time, the blowing speed and the blowing pressure are long, which generally increase the magnesium consumption, and it is easy to block the gun. If the sticking slag is too short, the amount of magnesium is insufficient to reach the desulfurization target. Usually 5-7min. The blowing speed is generally ≤25kg/min; the blowing pressure is generally ≤500kPa, but it should be adjusted with the different desulfurization station equipment and spray gun structure. To achieve the final desulfurization target value. In addition, the structure of the spray gun and the depth of the spray gun inserted into the molten iron are also one of the important factors affecting the desulfurization effect. (Tohoku University Yuki Mori)
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