A research program was conducted to improve processing technology for extracting alumina from nonbauxitic. The success of this program had provided the technology necessary for producing an adequate supply of high-grade alumina from domestic sources, thus meeting national economic and strategic needs relative to aluminum production. The hydrochloric acid process for recovering alumina from low-grade ores is one of the processes being investigated in the research program.
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One phase of this process uses the effect of chloride-ion concentration on the solubility of aluminum ion in solution. At 25° C, the solubility of AlCl3 in pure water is approximately 31 pct by weight, and decreases with increasing concentration of hydrochloric acid (HCl), becoming 0.1 pct when the HCl concentration is increased to 37 pct. This behavior has been the subject of numerous investigations, processes, and patents. Thus it is possible to leach a nonbauxitic ore with HCl, decant and filter the liquor, and add gaseous HCl to precipitate out AlCl3·6H2O. The remaining acid solution can then be used for leaching additional ore and the precipitated AlCl3·6H2O can be calcined to obtain Al2O3 . An extensive discussion and evaluation of this process is presented by Peters, Johnson, and Kirby. Thermodynamic data, such as the ionic activities and the solubilities, required for complete analysis, interpretation, and improvement of this process are scarce, and the existing solubility data are limited to a narrow range of temperature. Therefore, the aims of this investigation were (1) to determine the solubility of AlCl3, in a temperature range of 25° to 85° C with HCl concentration from 0.0 to 37.2 pct; (2) to calculate the activities of AlCl3 and of H2O in solutions containing various electrolytes , and the equilibrium constant for the dissolution of AlCl3·6H2O; and (3) to suggest areas of research for generating useful data.
The apparatus used for measurements of solubilities is shown in figure 1. A 100-ml flask, A, contains a solution of electrolytes and some excess salt. A fritted glass filter, B, fused on a 6-mm-OD Pyrex tube is joined to an outer tube attached to two Teflon stopcocks, D and E. The top assembly is attached to A at C by a threaded nylon compression fitting with a neoprene 0-ring. A 6-mm-OD polyethylene tube, F-L, delivers a sample of solution filtered at B into the second flask, K, when A is pressurized through stopcock D, and E is opened. The fitting at J is similar to that at C. The flasks are clamped on a bar, G, which is attached to a rocking mechanism, with H as the axis of rocking for shaking the flasks, A and K, alternately up and down. A covered thermostat, 0, is controlled to within ±0.2° C. A closer temperature control is unnecessary since the solubility is a very weak function of temperature.
Measurements of solubilities at 25°, 45°, 65°, and 85° C were made under conditions of phase equilibrium with analytical reagent-grade materials. Appropriate amounts of AlCl3·6H2O, 37-pct HCl solution, and distilled water were placed in flask A to form approximately 90 ml of solution with a small amount of excess salt crystals (hereafter “salt” refers to aluminum chloride hexahydrate). The amounts of materials were estimated from the existing solubility data on the ternary system AlCl3·6H2O-HCl-H2O. After the solution was formed in flask A and the apparatus was assembled, it was immersed in the water to level N, and the thermostat was set to the desired temperature. A continuous mechanical agitation of the flasks was used during each run lasting 2 to 3 days. Periodically, vigorous manual agitation was also used to assure
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The measurements of activities are useful in interpreting thermodynamics and kinetics of leaching, precipitating, and purifying desired components of various ores. There are two reasons that the activities can be determined accurately from the measurements of vapor pressure of water: (1) The vapor pressure can be measured with a high degree of precision by differential pressure transducers to obtain aw by equation 7 and then to calculate the values of the parameters in equation 11 so that all the related properties can be computed as discussed previously in detail (2) The solubility product and the activities can both be determined accurately for multivalent salts that contain large numbers of water molecules in their hydrated crystals. The second reason arises from the fact that the exponent of the activity of water in equation 4 is equal to the moles of water in the hydrated crystal.
Aluminum Chloride in Aqueous Hydrochloric Acid Solutions
Another interesting type of experiment for the ternary system discussed here is the measurement of the partial pressure of HCl(g) over various solutions containing sufficiently large amounts of dissolved HCl. The experimental procedure for this purpose is described elsewhere. Group IIA, IIIA, and transition metal salts are excellent candidates for these types of investigations since electrolytes with higher valences than 1:1 usually form hydrated salts.
The solubility and the activity of aluminum chloride hexahydrate, AlCl3·6H2O, in aqueous solutions were determined as a part of the Bureau of Mines, U.S. Department of the Interior, research to improve mineral processing technology for recovering alumina from low-grade domestic resources. The solubility of AlCl3 in pure water was found to be 31.09 and 31.77 wt-pct at 25° and 85° C, respectively, and decreased drastically to 10 and 0.1 wt-pct when the respective amounts of 20.3 and 37 wt-pct hydrochloric acid (HCl) were added in solution. The equilibrium constant for the solubility of hexahydrate, activities, and activity coefficients of AlCl3 and H2O were calculated and compared with the results from the available methods of estimation. The results were shown to be useful in leaching nonbauxitic ores for extraction of alumina by changing the solubilities of component salts upon addition of HCl.
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Simultaneous solubilities of AlCl3 , FeCl3, KCl, and NaCl in solutions of HCl-H2O were also determined and their effects on the purity of AlCl3·6H2O crystallized from mixed salt solutions are briefly discussed.