Apparatus

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.

Experimental Procedure

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 fo...

Analysis

The solutions were analyzed after appropriate dilutions for total chloride-ion and aluminum-ion concentrations. The total chloride-ion concentration was determined by potentiometric titration with Ag+. The Al+++ concentration was determined by first co...

Results

The results are summarized in table 1 and shown in figure 2. The maximum HCl concentration for the data given in table 1 for 85° C is approximately 30 wt-pct. Measurements were stopped at this concentration because of the excessive equilibrium pressures in the system at higher concentrations. It should also be noted that fewer determinations were carried out at 65° and 85° C than at 25° and 45° C. Replicates were run at the two lower temperatures to give an indication of satisfactory repeatability of results. Figure 2 shows that the solubility of aluminum salt decreases drastically with increasing HCl concentration. Therefore, leaching nonbauxitic ores with HCl solution and precipitating out the pure salt for extraction of aluminum by adding HCl is a feasible process; such a process has been the subject of several patents. The results show that the solubility does not greatly change with temperature, and for a temperature increase of 60° C (25° to 85° C), the solubility of pure salt alone in water increases only 2.2 pct relative to solubility at 25° C.

Comparison With Other Data

The solubility of pure salt in water at 25° C was found to agree exactly with the data of Palitzch and with Ehret and Frere. Results given by Malquori are about 10 pct higher. The solubility obtained by Tanaka is about 3.3 pct higher than in the present investigation at 45° and at 65° C.The effect of HCl in water on the solubility of salt has been investigated by Seidel and Fisher at 0° C and by Malquori at 25° C. The results of the former investigators at 0° C agree very closely with the data in table 1, extrapolated to 0° C, whereas those of the latter investigator at 25° C scatter somewhat and disagree roughly by 3 pet at low HCl concentrations; however, the agreement in the range of 5 to 30 wt-pct HCl is within 1 pct.

Thermodynamic Treatment

Solubility Product and Activities in AlCl3·6H2O-H2O System

The ionic activity coefficient Y12 of AlCl3 in water has been measured up to 1.8 m by Mason, and recalculated values are given by Robinson and Stokes. Since the activity coefficient from 1.8 m to the saturation value of 3.3835 m is expected to increase mo...

Estimation of Activities in AlCl3·6H2O-H2O System

Two existing reliable methods of estimation are used to obtain the activities in AlCl3·6H2O-H2O system beyond the existing direct experimental data at m = 1.8. The first method, as described by Kusik and Meissner, gives an estimated value of γ12 = 9.94 at 3.3835 m. This estimated value is in close agreement with 10.251, the value obtained in the present investigation. The corresponding estimated value of the activity of water, aw, is 0.47, again in close agreement with the value of 0.509 found in the present investigation. The equilibrium constant Kp calculated from these values is not far from 6.8 x 10 5, considering the fact that the fourth power of a12 and the sixth power of aw are multiplied to obtain Kp.The second method of estimation due to Bromley yields 10.42 for γ12, a value closer than that obtained by the preceding method. The activity coefficient of water has been obtained by the present authors, by integrating the Gibbs-Duhem equation in which the values of a12 estimated by the Bromley method were used. The result is aw = 0.540. The average of both methods is 0.505, which is in very good agreement with the Bureau’s value of 0.509. The Bromley method, however, does not contain a convenient method for determination of aw included in the Kusik-Meissner method.The agreement of the estimated values with the values from the solubility data is not always as good as shown here for every electrolyte at every composition. For example, even for AlCl3 at 1 m, the two estimates of γ12 differ by only 13 percent from each other.

Solubility product and activities in AlCl3·6H2O-HCl-H2O system

The effect of increasing concentration of HCl on the solubility of the salt is largely due to the increasing concentration of the chloride ions. The equilibrium constant in this case with the activities a1 and a2 of individual ions iswhere γ12 = γ1γ2 by...

Estimation of Activities in AlCl3·6H2O-HCl-H2O System

The values of activities, activity coefficient, and the solubility product estimated in the present report according to the method devised by Kusik and Meissner, are listed in table 3. The activity coefficient γ12 for AlCl3 is roughly twice as high as the estimated value for the selected solutions containing HCl. The activity of water, aw, however, agrees well with the estimated values except for m = 2.620. It is evident that the calculations would have been considerably more reliable if the vapor pressure of water, aw, were measured because aw enters in Kp as aw, so that small errors in aw cause large errors in γ12 and Kp . For this purpose, a limited number of vapor pressure measurements are recommended to test the accuracy of the present calculations and the methods of estimation.The temperature dependence of the thermodynamic properties listed in table 3 can be estimated as described by Kusik and Meissner. However, the estimate is likely to yield a higher degree of temperature dependence than that indicated by the weak temperature dependence of the solubility. No attempt could be made to calculate the effect of temperature by the Bureau’s method because (1) a12 at low concentrations of AlCl3 is not known at various temperatures, (2) the solubility of AlCl3 could not be determined with an accuracy approaching 0.01 pct in Al+++, and (3) the range of concentration of HCl at 65° and 85° C is too small for selecting a sufficient number of widely differing solubility data for AlCl3 to determine accurately the parameters in equations 10 and 11.

AlCl3-FeCl3-KCl-NaCl-HCl-H2O System

Preliminary data on solubilities of AlCl3, FeCl3 , KCl, and NaCl in aqueous solutions containing 24 to 31 wt-pct HCl at 25° C were obtained to assess contamination of AlCl3·6H2O extractable from nonbauxitic ores. For this purpose, 2 liters of a solution co...

Areas Recommended for Research

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.