How to recover platinum, rhodium and palladium from nitric acid furnace ash, acid sludge and rust scale

Recovery of platinum, rhodium and palladium from ammonia oxidation tower ash, acid sludge and rust scale in nitric acid industry Details
The Pt-Pd-Rh alloy catalytic network in the ammonia oxidation tower, at high temperature (800 ~ 950 ℃) and high pressure (304 ~ 912kPa), catalytic oxidation of NH3 to NO2 to produce HNO3, part of the Pt, Rh will also be oxidized to PO2, Rh2O3 volatile losses. Each production of 1t HNO3, the average loss of Pt0.1 ~ 0.15g. There are three loss paths: about 50% of them are taken away by the high-temperature airflow, and in the lower part of the tower colder parts decomposed into Pt, Rh metal particles, deposited in the bottom of the tower ash; 25% of the airflow to the absorption tower, deposited in the acid sludge at the bottom of the nitric acid storage tank; the rest deposited in the equipment rust scale. The furnace ash at the bottom of the tower and the acid sludge containing precious metals at the bottom of the nitric acid storage tank are easy to collect and recover. Clean up the equipment rust scale is an important measure to recover precious metals: certain removable parts are soaked in a plastic box with 10% concentration of phosphoric acid solution for more than 36h to make the rust scale fall off for recycling; non-removable parts or pipes are loosened with a rotating nylon brush or wire brush driven by a motor, and cleaned with water at the same time. Fire, wet metallurgy enrichment - iron trapping and smelting: rust scale containing very high iron oxide, using iron as trapping agent for smelting and enrichment is an effective method. That is, reduction smelting trapping → acid leaching to remove iron → extraction of platinum and palladium concentrate → separation and refining. According to its composition, according to the requirements of the generation of silicate slag with SiO2, CaO, Al2O3, Ca2F and other melting agents, and with the right amount of coke powder (C) as reducing agent, in the electric arc furnace 1500 ~ 1600 ℃ reduction melting. 3 kinds of slag type can make the melting process go smoothly, so that the reduction of iron oxide for iron metal trapping platinum group metals:
2Fe2O3+2CO=4FeO+2CO2.
6FeO+6CO=6Fe+6CO2
2FeO + C = 2Fe + CO2 trap Pt, Pd, Rh iron alloy, poor settlement, some of the particles are mixed in the slag, slag containing platinum group metals up to 90 g / t, need to crush magnetic separation recovery, slag content can be reduced to 5g / t, the recovery rate of ferroalloys > 99.3%. After separating the slag and then adding FeO decarburization to the molten iron alloy, the very chemically inert platinum group metal carbide is destroyed, so that the iron alloy contains C < 0.5%, and the iron is leached out with hydrochloric acid or sulfuric acid to obtain a crude concentrate containing 10-15% platinum palladium rhodium, with a recovery rate of about 95%. The solubility of the crude concentrate depends on the decarburization, i.e. whether the destruction of the platinum group metal carbides is complete or not, so the decarburization in the smelting stage is strictly operated. Crude concentrates with good solubility can be dissolved with aqua regia, HCl + NaClO3, etc., and separated and refined into pure metal products of platinum, palladium and rhodium from the solution by selective precipitation or solvent extraction techniques.