Enhanced Calcium Extraction From Steel Converter Slag Using Wet Extractive Grinding And Comparison With Traditional Mechanical Mixing - Selective extraction of Ca, V, Si and Mg

This thesis belongs to the field of process engineering. The goal of this research is to develop an effective recovery method for calcium from industrial waste streams to produce precipitated calcium carbonate, PCC. This would reduce our dependency on virgin raw material resources, reduce CO2 emissions and offer a pathway to circular economy by closing material loops in industrial processes. We focused on the utilization of steel converter slag as a source of calcium for the pH-swing process named as X2PCC (X refers to Ca-bearing materials). As the first process step, we extract calcium from steelmaking slag using aqueous ammonium chloride solvent and separate solids by filtering. In the second step, the Ca-rich solution is treated with CO2 gas to produce precipitated calcium carbonate (PCC). The main scientific objective of this thesis is to experimentally analyze phenomena in Ca extraction step and develop process solutions for increasing the Ca dissolution rate and yield. As a part of the work, we developed the new wet extractive grinding (EG) method. Traditional mechanical mixing (MM) process is first briefly explained and then compared with the new wet extractive grinding process. The carbonation step was left out of the scope of this thesis. In this thesis, the effects of various process parameters on the performance of these two techniques are systematically and quantitatively investigated. The first part (paper I) focuses on the experimental study on the traditional mechanical mixing with respect to particle size distribution and strength of the solvent. Aqueous ammonium chloride (NH4Cl) was used as an extracting solvent with concentrations of 0-2 mol/L. The slag to solvent ratio was constant 100 g/L for all experiments. The main objective of this part was to determine the optimal concentrations of solvent and particle size for leaching out the maximum amount of calcium from the slag. All the tests were conducted at ambient pressure and temperature. The extraction rate of different size fractions of the slag, 0–50 μm, 50–74 μm and 74–125 μm were studied. If was found that the smaller the particle size, and the higher the solvent molarity, the higher the yield of Ca. We found that Ca extraction is limited by the mass transfer and the availability of Ca within the large particles. We proposed a mechanism that the surface layer of the slag particles could be get blocked by reaction products, mainlySiO2, and this stops the reaction. This would explain the lower yield in the larger particles. The second part (papers II and III) of the work focuses on the new wet extractive grinding method and optimizing it for the maximum extraction efficiency of calcium from steel slag. Extractive grinding was compared with our traditional mechanical mixing to give better understanding of the performance of our new extractive grinding method, and to study the effects of the different process parameters on the calcium extraction and dissolution of other trace elements such as V, Si and Mg. Physical and chemical characterization is performed for the analysis of particle morphology before and after the extraction. Here, we found that with our new method we can achieve up to 73 % Ca-extraction efficiency as compared to below 40 % in MM. EG process, that combines grinding and extraction stages, reduces the overall energy requirement of the process. There are two mechanism that makes this possible: the processing time reduces due to reaction surface grinding effects and the Ca yield also significantly increases. The main scientific contribution of this thesis is in identifying the limiting factors in the Ca extraction stage and presenting and analyzing the new wet extractive grinding method. We were able to show that with the EG method, Ca yield increased from 35 to 73%, processing time decreased from earlier used 30 to 5 min. To obtain Ca yield higher than 70% via mechanical mixing, energy intensive fine grinding is required. With EG method, based on preliminary calculations, energy saving can be up to 56 % compared to fine grinding and mechanical mixing path. We also found that EG method does not significantly affect the particle size distribution, which means that there are fewer filtering issues expected, compared with the fine-grinded slag. We are presenting unique data on the effects of solvent molarity, slag to solvent ratio, particle size distribution, process time. These will be later used in the actual process design and feasibility analysis. Finally, we studied also initially a concept for the integration of CO2-capture Ca-looping integrated with our X2PCC process. Initial results show that CO2 capture potential of the process could be significantly increased by this. This will be further studied in our future work and we will evaluate what would be the optimal use for the PCC produced in terms of circular economy and environment.