Production of Al-Sr master alloy from domestic celestite and investigation of its properties

In this study, aluminum-strontium master alloy, which is widely used in the aluminum casting industry, has been produced using domestic celestite ore. Different methods have been tried to produce Sr master alloy. In these studies, production of strontium carbonate, which has higher commercial value than celestite, has also been carried out. In the first part of SrCO3 production from celestite ore, celestite was reduced with metallurgical coke to obtain SrS, called black ash. With mechanical activation, the reduction temperature was reduced from 965°C to 892°C. In both the planetary type ball mill and the ring mill, the mechanical activation process has resulted in the reduction of the size of the powders as well as the deformation of the crystal structures. As a result, mechanically activated celestite and coke can react at lower temperature and with higher efficiencies. After 45 min of mechanical activation in the ring mill and planetary type ball mill, the average grain size of the powders decreases to 2.22 μm and 1.30 μm. Carbothermic reduction of celestite in a lab-scale rotary kiln was performed with different parameters. After carbothermic reduction in the rotary kiln, the unburned carbon remains in the black ash up to 19.32 %, depending on the furnace temperature, slope and speed of rotation. At the same time, conversion of celestite to strontium sulfide varies between 5 and 75 % depending on the furnace parameters. The highest reduction rate (75 %) is obtained at 2° slope and 6 rpm rotation speed when the optimum kiln temperature is determined as 1100°C. The black ash, obtained by carbothermic reduction, can be dissolved in pure water to remove impurities from the ore. Rising the solution temperature increases the amount of dissolved SrS. At low temperatures, increasing the solid/liquid ratio, reduces the dissolution efficiency despite the possibility of gaining higher Sr concentrations. The black ash is solubilized at 95°C with 1/5 solid liquid ratio, resulting in 184,54 ppm Sr. However, since reducing the solution temperature decreases the solubility during filtration after leaching, most of the strontium is precipitated and separated from the solution. Using both sodium carbonate and ammonium carbonate, SrCO3 was precipitated with yields over 99 % from the SrS solutions. Adding sodium carbonate solution to the SrS solution facilitates SrCO3 precipitation in a very short time. After less than five minutes of precipitation, the Sr concentration in the solution is lower than 15 ppm. The amount of carbonate required for precipitation is sufficient to be the stoichiometric equivalent of strontium in the SrS solution. The increase in carbonate concentration has no effect on the precipitation of SrCO3. Although the solution temperature does not affect the desertification efficiency, rising the temperature increases the crystallinity of the precipitated SrCO3. The addition of ammonia (NH3) and hydrogen peroxide (H2O2) to ammonium carbonate solution does not change the amount of precipitated SrCO3, but the addition of 10 % NH3 and 1.2 % H2O2 made the crystal structure of precipitated SrCO3 more prominent. Celestite (SrSO4), which has a very low solubility in water and concentrated acids, is dissolved with barium chloride in acidic chloride solutions. Since the solubility of barium sulphate is lower than that of strontium sulphate, sulphate ions come from celestite precipitate as barium sulphate. Increasing the solution temperature increases the celestite solubility. NaCl and HCl, which influence the concentration of chlorine in the solution, contribute to the solubility of celestite by increasing the ionic strength. However, the dissolution rate decreases at concentrations of 1 M NaCl and 0.5 M HCl. The highest celestite solubility is obtained at 85°C and 1/300 solid/liquid ratio. The maximum amount of Sr in the solution was determined as 1391 ppm (92.7 % efficiency). SrCO3 was precipitated with sodium carbonate from the acidic chloride strontium solution. Initially, the pH of the solution is increased by adding sodium carbonate. The carbonate ions added to the acidic solution are oxidized by H+ ions and are removed from the solution as CO2. With the formation of carbon dioxide, the concentration of H+ in the solution decreases and the concentration of OH+ increases. Thus, the acidity of the solution decreases. As the solution pH exceeds 5, SrCO3 begins to precipitate. SrCO3 can be precipitated by adding sodium carbonate to the acidic strontium solution as three times that of the Sr concentration. Increasing the solution temperature increases both the precipitation rate and the efficiency. As a result of the study, the SrCO3 precipitation rates at 25, 50, and 75 ° C after 120 min were 83, 98, and 99.5 %, respectively. Mechano-chemical transformation of celestite to SrCO3, takes place very quickly and with high efficiency. The celestite concentrate can be converted to SrCO3 with 99.7 % efficiency in the planetary type ball mill without any pretreatment. Experiments showed that the grinding speed and time were the most important parameters in increasing the reaction efficiency. However, no significant effect of carbonate concentration has been observed. The highest mechano-chemical conversion rate was obtained at a grinding speed of 300 rpm in a 1.5 M Na2CO3 solution for two hours. Calcination of strontium carbonate occurs relatively slowly at temperatures up to 1067°C, although it begins at temperatures above 900°C. 17 % of SrCO3 can be calcined at 900°C in three hours. However, when the calcination temperature is increased to 1100°C, an efficiency over 99 % can be obtained in 90 minutes. At 1200°C, 99 % rate can be reached in 30 minutes. The calcination in the lab-scale rotary kiln furnace was not efficient enough at 1100°C. However, the addition of 15 % metallurgical coke to SrCO3 ensures that calcination efficiency is increased to the completion. SrO, obtained by calcination of SrCO3 can be hydrated under atmospheric conditions because it is a hygroscopic material. Under normal conditions, SrO is converted to Sr(OH)2.H2O by taking 2 moles of water in 24 hours. Strontium carbonate can be reduced in pure Al dust at 1050°C in a vacuum (~ 1 bar) atmosphere. In the reduction process, SrCO3 is first calcinated to SrO and CO2. Subsequently, Al reduces SrO to provide metallic Sr vapor. The Sr vapor is dissolved in excess Al to form Al4Sr intermetallics. Due to the vacuum in the working environment, a part of the Sr vapor is lost. For this reason, the study to reduce the partial pressure of oxygen was done in the Ar atmosphere. Reduction can be achieved by adding SrCO3 and SrO to the liquid metal in an Ar atmosphere. The amount of powders added to the liquid metal exponentially increases the amount of Sr obtained by reduction. Moreover, the reduction efficiency increases as the reduction temperature and the process time increase. Mixing powders in liquid metal is very difficult task as the reaction interface is very low. To overcome this problem, the powders, fed to the liquid metal by mixing system, which provided remarkably good dispersion in melt. The best reduction (1.06 % Sr) was obtained with a reduction of 10 % SrO in the aluminothermic reduction work carried out at 1200°C. The addition of 6 % Mg into liquid aluminum reduces the surface tension of liquid aluminum, thus improving the reduction efficiency. Al-Sr master alloys containing 1.15 % Sr, and 5.15 % Sr are obtained by the reduction of 10 % SrO, and % 10 SrCO3, respectively. The master alloys exhibit different degrees of modification performance in proportion to the amount of Sr contained therein. Master alloys produced with pure Al are generally not sufficient for modification. The master alloy, which was produced with a reduction of 10 % SrO, was able to partially modify the EtiAl 171 alloy, and after modification the amount of Sr in the alloy was determined to be 51 ppm. All of the master alloys obtained as a result of the metallothermic reduction with Al6Mg are able to partially modify the EtiAl 171 alloy. The master alloy produced with the reduction of 10 % SrO exhibits similar modification performance with commercial Al5Sr. After modification, the Sr in the alloy is at the level of 517 ppm.