Basic Research On Slag Control Of Spent Automobile Exhaust Catalys

Due to their unique and superior physicochemical properties,such as high melting point,high boiling point,antioxidant and corrosion resistance,long and stable service life,as well as biological and catalytic activity,platinum group metals are widely used in modern industry,and are an important strategic resource irreplaceable in national defense,medicine and health,environmental protection and high-tech industries,and are known as"industrial Vitamin".Spent automotive catalysts（SAC）are the largest application area for platinum group metals and are known as"mobile platinum group metal mines".The recovery of platinum group metals from spent automotive catalysts（SACs）is essential in order to alleviate the increasingly tight supply and demand for platinum group metals.In this paper,basic research and validation experiments on slag regulation are carried out to achieve efficient enrichment of platinum group metals by fire capture of platinum group metal melting process using cordierite type SAC.The details and results of the study are as follows.（1）Characterization,inductively coupled plasma emission spectroscopy（ICP-OES）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and electron microprobe（EPMA）tests of the SAC feedstock showed that the main material phase in the feedstock was cordierite(Mg₂Al₄Si₅O₁₈)and contained Pt（92 g/t）,Pd（1458 g/t）and Rh（293 g/t）,with Pt mainly in the form of oxide（Pt O₂）,while Pd and Rh coexist as monomers and oxides,and the distribution of these three platinum group metals in the feedstock is relatively dispersed.The thermodynamic parameters of the base slag system（Mg O-Al₂O₃-SiO₂）were calculated,and the results showed that the initial/final liquid phase temperatures of the base slag system were 1445.13°C and 1646.03°C,respectively.The actual melting temperature interval of the raw material was determined to be1345°C～1548°C by high temperature confocal microscopy（HT-CSLM）in situ observation.（2）Thermodynamic calculations were carried out for the base slag system（Mg O-Al₂O₃-SiO₂）in the raw material,and the results showed that the initial/final liquid phase temperatures of the base slag system were 1445.13°C and 1646.03°C,respectively.The actual melting temperature range of the raw material was determined to be1345°C～1548°C by high temperature confocal microscopy（HT-CSLM）in-situ observation and thermal analysis.The thermodynamic parameters of the melting slag systems with different additives（Mg O-Al₂O₃-SiO₂-Ca O,Mg O-Al₂O₃-SiO₂-Na₂B₄O₇,Mg O-Al₂O₃-SiO₂-Ca F₂,Mg O-Al₂O₃-SiO₂-Na₂CO₃）were calculated,and the results showed that the addition of both Ca O and Ca F2 could reduce the melting temperature of the slag system up to 200℃～250℃,while the addition of Na₂B₄O₇can gradually dissociate the silicate compound or transform it into a borate compound with low melting point,and its reduction of the melting temperature of the raw material can reach more than 300℃.The above three groups of additives can effectively reduce the melting temperature and slag viscosity,and increase the amount of slag melting at the same temperature.And with the increase of Na₂CO₃content,the initial liquid phase temperature of the slag first decreases and then increases,and the final liquid phase temperature first increases and then decreases.（3）Based on the above thermodynamic calculations,an experimental study on the regulation of the physical and chemical properties of slag was carried out,and the melting temperature,melting volume and solid-liquid distribution of slag were observed in situ by high-temperature confocal microscopy（HT-CSLM）,and the results showed that the addition of Ca O,Ca F₂,Na₂B₄O₇decreased the melting temperature of slag system,while Na₂CO₃decreased and then increased the melting temperature of slag system,which was basically consistent with the theoretical calculations.（4）Based on the slag modulation,Pb S was used as a trapping agent for thermodynamic analysis and experimental investigation.The slag regulation is effective in promoting the separation of matte phase from slag phase to achieve slag-gold separation and increasing the content of platinum group metals in the lower matte phase.Through slag optimization,the melting temperature was lowered from 1548°C of the raw material to below 1300°C under the optimized slag type,and the slag phase was changed from a high polymerization cordierite phase to an amorphous amorphous glass phase with low polymerization,which realized the separation of slag and gold.The optimal slag type for Pb S trapping of platinum group metals in failed auto exhaust catalysts was determined by considering the melting temperature and additive input:Ca O addition of 8.58 wt.%,borax addition of 25.29 wt.%,Na₂CO₃addition of 2.22 wt.%,and SAC input of 23.91wt.%.