3.1 Principle of utilization and disposal of WOPCBs
The purpose of converter systems in steelmaking is to produce steel with optimal physical, chemical and mechanical properties, by oxidizing carbon, silicon, manganese, and other impurities in pig iron under pure oxygen conditions. Converter systems complete the process using the physical heat generated from molten iron and the chemical heat formed from the reaction of inner components. Hot metal, scrap steel and ferroalloys are the main raw materials in this process. The melting point temperature of pig iron ranges from 1100 to 1250℃, while the temperature of molten steel ranges from 1450 to 1700℃ . Oxygen is lanced to the furnace to oxidize the impurities in the hot metal. After the blowing, smelting temperatures can reach levels of up to 1700℃, the impurities (C, Si, S, etc.) present in steel is utilized in reducing the iron oxide contained in the WOPCBs and melting as Fe or slag; thereafter, metal and slag are collected separately and the iron of WOPCBs provides an effective supplement to scrap steel. The remaining oil in WOPCBs is fully decomposed, the emissions from this process entering the converter off gas purification system through the dust removal system and finally being recovered by gas holder or discharged via a chimney. The possible chemical reactions during disposal process are shown in Figure 3.
3.2. Analysis of influence on steelmaking production
3 pieces of WOPCBs (~540kg per batch) pressed block scrap steel was added, the converter was shaken 4 times at ±50°. The flame and dust concentration at the furnace mouth were observed to be normal. Two minutes later, iron mixing was initiated for a duration of 4 min. The smoke and flame levels at the furnace mouth were increased slightly, although no splashing or shooting was observed during the process. Blowing was performed from the 7 to 16 min, with no splashing or abnormal noises observed in the furnace. At the 31 min, the process was returned to normal functioning and steel was discharged. The chemical components of hot metal were presented in the Table 1.
These results illustrate that it is feasible to use scrap steel from WOPCBs pressed blocks as the test raw material. Compared with the disposal process for ordinary scrap steel, the modified scrap steel disposal process required 4-cycles of shaking, prolonging the production cycle time. According to the presented data, the WOPCBs pressing block scrap steel could be used in ladle furnace desulfurization steel. There were no obvious differences observed in the converter blowing process and both production and safety conditions could be controlled, which did not affect the quality of steel products and allowed control of the sulfur content of molten steel.
3.3. Analysis of influence on the environment
3.3.1 Waste gas emission
For the converter off gas generated in the converter refining process, the dedusting technique of oxygen converter gas recovery(OG) is used. Dust is then scrubbed out of the off-gas from the process through wet venturi scrubber systems before the off-gas is suitable to either collect for gas recovery or to flare to the atmosphere. The flue gas escaping from the converter smelting process, as well as the flue gas produced by slag stripping and desulfurization in ladle argon blowing station, adopts the type of smoke hood in front of the furnace to collect, and then sends it to the bag filter for purification, and then discharges it to the atmosphere through the exhaust pipe. After dosing WOPCBs in the converter, VOCs, PAHs, dioxins and other pollutants may be generated in the incineration process. The environmental indicators during the WOPCBs disposal process were commissioned by a third party company.
Monitoring results showed that the concentrations of benzene, toluene, xylene, NMHC, VOCs and dioxins in emissions were slightly increased from 0.024 mg m-3, 0.207 mg m-3, 0.0235 mg m-3, 0.44 mg m-3, 0.841 mg m-3 and 0.032 (ng-TEQ m-3) prior to testing, to 0.326 mg m-3, 0.378 mg m-3, 0.0455 mg m-3, 2.22 mg m-3, 2.966 mg m-3 and 0.039 ng-TEQ m-3 during testing, as indicated in Table 2. However, the concentrations of all pollutants met the limits of Chinese standards “Integrated Emission Standard of Air Pollutants” in China (GB16297-1996)  and the “Emission Standard of Air Pollutants for Steel Smelt Industry” in China (GB28664-2012) . It should be pointed out that it is unnecessary too much concerned about the increase of dioxin concentration when the WOPCBs disposal process. The boiling points of the dioxins ranged between 421 and 447 ℃ and the decomposition temperature was above 800 ℃ . A key characteristic of steelmaking converter systems was that the instantaneous temperature of added WOPCBs was over 1300 ℃, while the smelting temperature can reach above 1600 ℃. Therefore, the temperature conditions required for the formation of dioxins are not formed during the smelting process. Subsequently, the converter off gas was cooled and atomized efficiently to make the flue gas rapidly cool from above 1000 ℃ to 170~200 ℃. This ensured that dioxins can be completely decomposed and would not be generated again during cooling [19, 20].
3.3.2 Waste water emission
The waste water mainly comes from the water for primary converter off gas cooling and dust removal, which is recycled and not discharged. The process flow of circulating water treatment system can be described as: firstly, the backwater of circulating water entered the coarse particle separator through the elevated flow tank, then after separating the coarse particles, the supernatant was pumped into the distribution tank, and then coagulated sedimentation in radial-flow sedimentation tank. At last, the clear water was cooled to room temperature and sent back to the circulating water system for reuse. ICP-MS analysis in circulating water showed that the concentrations of seven heavy metals such as mercury (Hg), cadmium (Cd), nickel (Ni), lead (Pb), chromium (Cr), arsenic (As) and copper (Cu) in circulating water were found to be close to detection limits, and all the concentrations of heavy metals from washing water could meet the corresponding standard requirements of the “Discharge standard of water pollutants for iron and steel industry of China (GB13456-2012)”. The results are shown in Table 3.
3.3.3 Solid waste emission
The main solid waste produced in the disposal process of WOPCBs was steel slag, mud cake produced by circulating water treatment system and dust collected by converter off gas. Ministry of Ecology and Environment of China has developed a regulatory definition and process that identifies specific substances known to be hazardous and provides objective criteria for “Identification standard for hazardous wastes-Identification for extraction procedure toxicity”(GB 5085.3-2007). The sequential leaching test were performed to investigate the leachability of heavy metal ion from the original steel slag, mud cake and dust in aqueous solution according to the Chinese Standard Procedure HJ/T299-2007. The leaching results for these heavy metals were shown in Table 4.
The results in Table 4 indicate that the concentrations of heavy metal leaching solution of all metallurgical solid wastes were far lower than the standard limits of “Indentification standards for hazardous wastes indentification for extraction toxicity of China"(GB5085.3-2007). This means that all metallurgical solid wastes can generally be categorized as a non-hazardous by-product from the steel industry and could potentially be recycled for use elsewhere, which is beneficial to the environmental protection and resource saving. Currently, mud cake and dust were used as raw materials of sintering, and steel slag was used as cement aggregate [10, 21]. WOPCBs contained a small quantity of inorganic substances, mainly Ca and Si compounds and trace amounts of heavy metals, such as barium, lead, antimony, cadmium, chromium, strontium and other heavy metals or other harmful metal elements, which can be reacted with lime under high temperatures and strong liming effects to form a very stable silicate which eventually combined with the steel slag [22, 23]. Meanwhile, The precipitation of calcium carbonate on the surface of the steel slag particles blocked the liquid phase from reaching the surface of the soluble mineral phase, and this action caused a decrease in the dissolution rate of the mineral phases, affecting the leachability of heavy metal ions in the encapsulated crystal phase. Because of the stable properties of silicates, their use did not lead to environmental concern . Meanwhile, Silicates and steel slag can be comprehensively reused as a building material such as cement, avoiding further contributions to overall emissions .
3.4. Analysis of material balance
Most of the coating residue of waste paint barrels is formed of organic substances (about 10wt.%), containing the elements C, H and O, which was reduced in the smelting process to a coating residue proportion of less than 0.004%. The converter smelting temperature was about 1600 ℃, with smelting being an oxidation process in an oxygen environment. All organic substances were decomposed under the high temperature conditions, which did not affect the safety or quality of converter smelting products . Through decomposition and transformation under high temperatures in the converter, the content of N, S and other heavy metals in WOPCBs were lower the limit of emission standard in Chinese iron & steel industry. The inorganic substances component contained the elements Ca and Si in the coating, which reacted with lime at high temperatures to form very stable silicate materials, which entered the steel slag but do not affect the molten steel products. Ferrite also entered the molten iron in the converter, becoming a partial substitute of scrap steel .