Investigation of Regeneration Mechanisms of Aged Solar Salt

The scope of our study was to examine the potential of regeneration mechanisms of an aged molten Solar Salt (nitrite, oxide impurity) by utilization of reactive gas species (nitrous gases, oxygen). Initially, aging of Solar Salt (60 wt% NaNO3, 40 wt% KNO3) was mimicked by supplementing the decomposition products, sodium nitrite and sodium peroxide, to the nitrate salt mixture. The impact of different reactive purge gas compositions on the regeneration of Solar Salt was elaborated. Purging the molten salt with a synthetic air (p(O2) = 0.2 atm) gas stream containing NO (200 ppm), the oxide ion concentration was effectively reduced. Increasing the oxygen partial pressure (p(O2) = 0.8 atm, 200 ppm NO) resulted in even lower oxide ion equilibrium concentrations. To our knowledge, this investigation is the first to present evidence of the regeneration of an oxide rich molten Solar Salt, and reveals the huge impact of reactive gases on Solar Salt reaction chemistry.

Laboratory tests of the hydrometallurgical method for processing polymetallic raw material in Central Kazakhstan

Within the scop e of this study the leaching process of Cu, Zn, Co, Mo from the complex polymetallic sulfide concentrate with high Fe, As, which was produced in Central Kazakhstan using two methods is examined. The metal concentrations in the concentrate were 0.27% Cu, 0.026% Zn, 0.464% Co, 0.057% Mo, 15.51% Fe, 7.38% As. Nitrogen oxides formed in the processes of leaching were absorbed with water, the return of absorption product to the operation of leaching allowed reducing the consumption of nitric acid. The single-stage nitric acid leaching experiment was carried out at the leach concentrate productivity of 170±20 g/h, 80±2 °С, liquid/solid mass ratio (L/S) 6/1, leach time of 10.7 h, and nitric acid (57%) consumption of 0.12±0.01 l/h. The degree of capture of nitrous gases reached 97.8%. In these conditions Cu, Zn, Co, Mo, Fe, As were obtained with dissolution efficiencies of 96.98, 64.92, 99.99, 95.39, 80.13 and 99.80% respectively. Concentrations of NO3–, H+ and redox potential in the leachate were 88.7 g/l, 1.82 mol./l and 741 mV, respectively. The two-stage counter-current nitric acid leaching experiment was carried out at the leach concentrate productivity of 170±20 g/h, 80±2 °С, L/S 6/1, leach time of 26.8 h, and nitric acid (57%) consumption of 0.06±0.006 l/h. The degree of capture of nitrous gases reached 97.8%. In these conditions Cu, Zn, Co, Mo, Fe, As were obtained with dissolution efficiencies of 99.11, 85.23, 100.00, 88.60, 77.24 and 85.56%, respectively. Concentrations of NO3–, H+ and redox potential in the leachate were 46.7 g/l, 0.35 mol/l and 658 mV, respectively. Compared to single-stage leaching, two-stage counter-current nitric acid leaching allo ws the significant economy of nitric acid (256 g of 100% HNO3 per kilogram of concentrate), which is reduction of 48%. Moreover, counter-current leaching enables reduction in concentrations of NO3–, H+ and redox potential in the leachate. The further studies should focus on possible reduction in nitric acid consumption by lowering concentration of NO3– in leachate. The authors appreciate participation of I. A. Parygin, VNIPIpromtekhnologii in this study.

Trapping nitrous gases during nitric acid leaching of sulfide concentrates

Gas-air mixtures that form in nitric acid leaching of sulfide raw materials possess the following peculiarities making a negative impact on trapping of nitrogen oxides: elevated temperature, different oxidation level of nitrogen oxides, slow oxidation of NO in region of low concentrations, and instability of the resulting gas-air mixture flow. Therefore, well-known methods of trapping nitrous gases shall be adapted to specific sulfide raw material. We propose a process flow diagram for trapping nitrous gases formed during nitric acid leaching of sulfide concentrates at atmospheric pressure on the example of Zhezkazgan concentrate. The paper addresses theoretical aspects of the use of water-ore pulp, concentrated sulfuric acid, process water and alkaline agents for trapping nitrous gases, and typical reactions of interaction of the proposed absorbents with nitrogen oxides. The choice of water-ore pulp as an absorber was made because of similarity between the mechanism of absorption of nitrogen oxides for neutral and alkali ore suspensions and the one for alkali solutions: nitrogen dioxide and nitrous anhydride are absorbed with formation of a solution of nitrates and nitrites. Due to availability in a liquid phase of ferrous iron along with NO2 and N2O3, acidic suspensions are also capable to absorb nitric oxide, to some extent, with formation of Fe(NO)SО4 complex. Process water absorbs only nitrogen dioxide, with formation of nitric and nitrous acids. Nitrous acid is an unstable compound in acidic environments and decomposes with formation of water and nitrogen oxide. At the stages of trapping nitrogen oxides with water-ore pulp and process water (circulating solution), it is recommended conditioning of gas-air mixtures by choosing the volume of additionally introduced air, in an amount to provide the highest rate of nitrogen oxide oxidation. At the stages of sulfuric acid and alkaline trapping of nitrogen oxides, it is recommended conditioning of gas-air mixtures by selecting the volume of additionally introduced air and the oxidation time of nitrogen oxide that provide an equimolecular mixture of NO and NO2. A distinctive feature of the use of water-ore pulp, concentrated sulfuric acid, process water and alkaline agents for trapping nitrous gases is possibility to use the products of absorption at the stage of sulfide concentrate leaching. The extended tests of trapping nitrous gases have been conducted. The plant capacity by the gas-air mixture ranged 17–21 m3/h, and by leached concentrate — 12–15 kg/h. In this case, the degree of capturing nitrous gases reached 96.8%. Return of the products of absorption of nitrous gases in the form of condensate, water-ore pulp, nitrosyl sulfuric acid, nitric acid solution, nitritenitrate lye allows to reduce the nitric acid consumption by 7–10 times relative the values obtained without using the trapping system. In this case, the degree of copper extraction into the leaching solution was 97.7%. The extraction degree of silver, rhenium, zinc was respectively 98.0%, 99.0%; 98.5%.

Two-Stage Catalytic Abatement of N2O Emission in Nitric Acid Plants

Different variants for abatement of N2O emission from nitric acid plants with the use of catalysts developed at Łukasiewicz-INS were analyzed. Activity tests on a pilot scale confirmed the high activity of the studied catalysts. A two-stage catalytic abatement of N2O emission in nitric acid plants was proposed: by high-temperature decomposition in the nitrous gases stream (HT-deN2O) and low-temperature decomposition in the tail gas stream (LT-deN2O). The selection of the optimal variant for abatement of N2O emission depends on the individual characteristics of the nitric acid plant: ammonia oxidation parameters, construction of ammonia oxidation reactor and temperature of the tail gas upstream of the expansion turbine. It was shown that the combination of both deN2O technologies, taking into account their technological constraints (dimensions of the catalyst bed), allows for a greater abatement of N2O emission, than the use of only one technology. This solution may be economically advantageous regarding the high prices of CO2 emission allowances.

Optimization of Ammonia Oxidation Using Response Surface Methodology

In this paper, the design of experiments and response surface methodology were proposed to study ammonia oxidation process. The following independent variables were selected: the reactor’s load, the temperature of reaction and the number of catalytic gauzes, whereas ammonia oxidation efficiency and N2O concentration in nitrous gases were assumed as dependent variables (response). Based on the achieved results, statistically significant mathematical models were developed which describe the effect of independent variables on the analysed responses. In case of ammonia oxidation efficiency, its achieved value depends on the reactor’s load and the number of catalytic gauzes, whereas the temperature in the studied range (870–910 °C) has no effect on this dependent variable. The concentration of nitrous oxide in nitrous gases depends on all three parameters. The developed models were used for the multi-criteria optimization with the application of desirability function. Sets of parameters were achieved for which optimization assumptions were met: maximization of ammonia oxidation efficiency and minimization of the N2O amount being formed in the reaction.

The Use of Response Surface Methodology in Ammonia Oxidation Reaction Study

The design of experiments (DoEs) with response surface methodology (RSM) were used to investigate the effect of operating parameters on the ammonia oxidation process. In this paper, the influence of reactor’s load and temperature of reaction as independent variables was investigated. The efficiency of NH3 oxidation to NO and N2O concentration in nitrous gases gas was identified as response variables. As a result of these studies, statistically significant models for two responses variables were developed.

Avoidable and Unavoidable Exergetic Destruction Analysis of a Nitric Acid Production Plant

Exergy analysis for Nitric acid production plants are very few and many are outdated. This study aims to support existing scientific studies and incite new investigations of exergy analysis in modern times. An advanced exergy analysis was applied to a production plant with a capacity to process 350 tons/day of nitric acid at a concentration of 55%. The catalytic oxidation of ammonia, condensation and absorption of nitrous gases are considered as the principal process in the nitric acid production. The total destroyed exergy was 46772,55 KW. The component with the greatest impact was the catalytic converter, which presented 75.1% of the total avoidable exergy destruction rate of the plant. These findings are relevant as they can potentially reduce costs of nitric acid production.

Tetrazine functionalized zirconium MOF as an optical sensor for oxidizing gases

Dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate was introduced into the chemically stable UiO-66 structure by a postsynthetic linker exchange reaction to create an optical sensor material for the detection of oxidative agents such as nitrous gases.