Maximizing the exploitation of phosphogypsum wastes using soaking technique with citric acid, recovering rare-earth and residual phosphate contents

Phosphogypsum(PG), the main by-product of phosphoric acid production industries, is considered one of the most important secondary sources of rare earth elements(REEs). The current study focuses on the recovery of REEs content and the residual phosphate content existing in the PG with preserving on the CaSO_(4)skeleton to be used in other various applications. These attainments are carried out using citric acid leaching process via soaking technique. Several dissolution parameters for REEs using citric acid were studied, including soaking time, soaking temperature, citric acid concentration, solid-to-liquid ratio, and recycling of the citrate leaching solutions in the further REEs dissolution experiments. The best-operating conditions were 14 d of soaking time, 7.5% citric acid concentration, and the solid-toliquid ratio of 1/5 at ambient temperature. About 79.57% dissolution efficiency of REEs was achieved using the optimal conditions. Applying four soaking stages by mixing different fresh PG samples with the same citrate solution sequentially, cumulative dissolution efficiency for REEs was found to be 64.7% under optimal soaking conditions. REEs were recovered using Dowex 50X8 resin from citrate solutions with 96% extraction efficiency. Dissolution kinetics proved the pseudo-first-order nature, reversible reactions, and two activation energies for all REEs.

Air renewal times and ventilation rate calculations for underground workings using radioactive measurement

Potential alpha emitters are of prime concern to the ventilation engineer due to their rapid concentration increasing once radon released in the mine atmosphere, causing tissue irradiation and lung cancer.Studying of the time based variations of the natural ventilation in tunnels and their relationship to the external parameters contribute to the air circulation assessment. Due to the continuous and high fluctuation of the meteorological conditions affecting the air circulation and intensity through the underground workings, there is a difficulty in the natural ventilation assessment by only the ordinary meteorological measurements. So, in this paper, the possibility of using the radioactive measurements, allowing for the air aging and ventilation quality to be qualified, is investigated through three different underground structures. Referring to the most confined structure of them, results show that one structure has a better exchange rate by a factor 1.8 and the other has the best rate by a factor 2.1. This parameter can be linked to the operating costs and size of a future ventilation system.

Construction and Application of Quenching Critical Cooling Rate Model

Based on exploring quenching mechanism, three types of quenching critical cooling rate models of mediumcarbon steel and low-carbon steel and low-alloy construction steel were established, as modified Maynier model, Eldis model and isothermal curve model, respectively. During construction process, regression analysis and Newton interpolation method were used for higher calculated precision. Through comparing and analyzing, the superior one was selected to evaluate the full hardening thickness after quenching. According to hardness analyzing, the deviation between predicted hardness distribution and the experimental value is within 6%.