There is a certain amount of carbon monoxide (CO >: 20%), so it is called blast furnace gas. Blast furnace gas contains a lot of combustible components and a lot of dust, and the temperature is usually 150 ~ 400℃. The waste gas discharged from the top of the furnace generally passes through a gravity dust collector, and then is washed and deeply dusted. Washing treatment is to realize gas washing and cooling by convection contact between gas and water in washing tower or venturi tube. The water after washing and cooling is blast furnace gas washing wastewater. The water temperature of the wastewater is above 60℃, and the main impurities are solid suspended solids, dust sludge (air sludge), oxides, coke powder, etc. In addition, it also contains some inorganic salts and toxic substances such as phenols, cyanide and heavy metals. Because of the large amount of water and heavy pollution, wastewater must be treated and recycled as much as possible [1]? . ?
1 Status of governance
At present, the precipitation treatment of large and medium-sized blast furnace gas washing wastewater can be divided into natural precipitation and coagulation precipitation.
1. 1 natural precipitation method
Natural sedimentation is the main treatment method of blast furnace gas washing wastewater in Capital Iron and Steel Company, Panzhihua Iron and Steel Company, Xiangtan Iron and Steel Company and Shanghai No.1 Iron and Steel Plant. In the past, the washing wastewater of blast furnace gas in Laigang was treated by two concentration tanks with D= 12m, which could not meet the industrial water consumption and discharge standards. Later, the horizontal sedimentation tank was used for natural sedimentation. The sedimentation efficiency was about 90%, the content of suspended solids in the effluent was less than 100mg/L, the water temperature was about 40℃ after cooling, and the water circulation rate reached 90%, except for some indicators (such as lead and phenol). Foreign blast furnace gas washing wastewater is mostly treated by natural sedimentation method [2], which is characterized by that the wastewater is discharged into sedimentation tank or concentration tank by gravity, and then recycled after being cooled by cooling tower, and the effluent is suspended solids SS.
1.2 coagulation and precipitation method
Coagulation and sedimentation is also a widely used treatment method. For example, the blast furnace gas washing wastewater from Wuhan Iron and Steel Plant, Baosteel General Plant and Shougang Company is mostly treated by coagulation and sedimentation. Treatment index of WISCO blast furnace gas washing wastewater: adding 0.5mg/L polyacrylamide, the suspended matter in the effluent of sedimentation tank is less than 50 mg/L; Inorganic and organic polymer flocculants were added to Benxi Steel, and the precipitation efficiency reached 98%. After coagulation and sedimentation purification in Baosteel General Plant, the suspended solids in water can be reduced from 2000mg/L to below 100mg/L, and the total recovery rate is 97%. The wastewater treatment system operates normally and the treatment effect is good, but the imported water treatment chemicals used are expensive. Polyacrylamide (0.3 mg/L) was used to coagulate and precipitate the washing wastewater of Shougang blast furnace gas, and the precipitation efficiency was over 90%. When the circulation time is long and the circulation speed is high, the combination of polyacrylamide and a small amount of ferric chloride can remove rich fine particles and achieve satisfactory treatment results. Coarse particles were separated from blast furnace gas washing wastewater of Iijima Steel Plant in Japan by a coarse particle separator, then caustic soda was added to improve the pH value, and then polymer coagulant was injected into the coagulation sedimentation tank to precipitate iron and zinc in the form of Fe(OH)2 and Zn(OH)2. In order to remove Zn that pollutes the environment, the pH value should be kept in the range of 7.5 ~ 8.5. The wastewater treated by coagulation and sedimentation is recycled after being cooled by the cooling tower. The content of suspended solids in the treated water is SS.
2. Development of new treatment technology
The removal efficiency of suspended solids in wastewater depends on the solid-liquid separation speed, which in turn depends on the growth particle size and density of suspended solids. The larger the growth particle size, the higher the density and the higher the water treatment efficiency. According to flocculation kinetics, due to the randomness of floc growth process in traditional treatment technology, the effective density of floc decreases exponentially with the increase of particle size. At present, other high-efficiency flocculation technologies studied in China have not fundamentally solved the contradiction between the increase of floc particle size and the sharp decline of effective density, although the particle coagulation speed has increased and the floc growth particle size has increased. However, the speed of solid-liquid separation can be greatly improved by changing the dynamic conditions and physical and chemical conditions of suspended particles to limit the randomness of coagulation process and form high-density agglomerated flocs. This new treatment technology is called agglomeration or agglomeration granulation fluidized bed method. Many achievements have been made in the theoretical research, experiment and application of this process in feed water treatment and sludge concentration [3 ~ 5], and gratifying achievements have also been made in the treatment of high concentration suspended solids wastewater in agglomerated fluidized bed. The treatment results of blast furnace gas washing wastewater in Shaanxi Lueyang Iron and Steel Plant show that the hydraulic load (rising speed of water flow) can be as high as116 cm when the dosage of PAC is 0.5 ~10.5 mg/L and that of PAM is 0.06 ~10.05 mg/L. When the dosage of PAC is 0.75mg/L and PAM is 0.375mg/L, the hydraulic load or surface load can be as high as 1 12cm/min, the total residence time is about 2min, and the effluent turbidity is lower than 2NTU. The surface load of the above two kinds of wastewater can be increased by about 10 times compared with the traditional treatment process by using the agglomeration granulation fluidized bed process. For the treatment of coal washing wastewater, the surface load can be as high as 70cm/5min, the turbidity of effluent is less than 40NTU, and the total residence time is less than 5 minutes. Compared with the traditional treatment process, the surface load can also be increased by more than 6 times.
This new treatment technology has broad application prospects for solving the pollution problem of key pollution sources at present, because this kind of wastewater, such as the above-mentioned coal mine washing wastewater, metallurgical mine dressing wastewater, tailings wastewater, gas washing wastewater from iron and steel enterprises, etc., has the characteristics of large water volume and heavy pollution. Using this technology, not only suspended pollutants and a large number of other pollutants such as heavy metals, phenols and cyanogens in wastewater can be removed to solve the pollution problem, but also wastewater can be recycled to save and make full use of water resources, which has remarkable environmental and social benefits.