Hydrolysis of cupric chloride in aqueous ammoniacal ammonium chloride solutions

Cupric solubility in the CUCI2-NH4CI-NH3-H2O system for chloride concentrations lower than 4 molal in the temperature range 25-60 °C was studied. The experimental results show that for chloride concentration between 3.0 and 1.0 molal the cupric solubility is determined by the solubility of the cupric hydroxychloride Cu(OH)i 5CI0.5. For a chloride concentration valué of 4.0 molal, there are two cupric compounds, the hydroxychloride Cu(OH)i 5CI0.5 or the diammine chloride Cu(NH3)2Cl2, on which the solubility of Cu(II) depends, according to the temperature and the valué of the ratio [NH3]Total/[Cu]Total.


INTRODUCTION
During the last years, attention has been given to the ammonium chloride based hydrometallurgical processes.Aqueous solutions of high concentration of ammonium chloride are a very interesting leaching médium, being especially appropriate for the treatment of complex raw materials of both oxide and sulphide types (1)(2)(3)(4)(5)(6).The use of this leaching médium is appropriate for the recovery of metáis such as Zn, Cu, Pb, Hg and Ag.During the leaching with ammonium chloride solutions, besides the solubilization of these metáis, ammonia is produced in the proportion of 2 mols of ammonia ^*^ Trabajo recibido el día 20 de septiembre de 1994.^ ^ Centro Nacional de Investigaciones Metalúrgicas (CSIC).Avda.de Gregorio del Amo, 8. 28040-Madrid (Spain).per mol of dissolved metal for the oxides of divalent metáis (3 and 4), and in a somewhat lower proportion for the sulphides.This proportion varíes according to the mineral species of sulphide being leached (7).Therefore, the resultant médium of the leaching step is, in both cases, an aqueous ammoniacal ammonium chloride solution.
Chlorides of metáis such as copper and zinc have a high solubility in concentrated ammonium chloride solutions.This solubility is drastically reduced in the presence of ammonia, due to the precipitation of the diammine chlorides.The solubility of the diammine chloride of these metáis is heavily influenced by both the composition of the solution and the temperature (8 and 9).For process development purposes, it is necessary to have a detailed knowledge of the solubility of these metal chlorides as a function of temperature and solution composition.In a previous study by the present authors (9), the solubility of cupric chloride in concentrated solutions of ammonium chloride in the presence of ammonia was studied.The results there obtained showed that, for the studied ammonium chloride concentration range (total concentration of chloride between 5 and 9 molal), the cupric solubility is restricted by either the solubility of the cupric diammine chloride or the cupric hydroxychloride.
When the concentration of ammonium chloride decreases below 5 molal, Cu(II) solubility diminishes.The knowledge of Cu(II) solubility in the CUCI2-NH4CI-NH3-H2O system for these lower ammonium chloride concentrations is also a very important matter for a suitable hydrometallurgical use of this leaching médium.Especially, these solubility data must be considered to avoid metal losses in pulp washing steps.No literature on Cu(II) solubility in these conditions of low ammonium chloride concentration has been found.The present study investigates this matter.

EXPERIMENTAL RESULTS
The range of conditions considered in this study corresponds to the ammonium chloride concentrations lower than 5 molal and low or relatively low concentrations of ammonia.The total concentration of chlorides was varied between 1 and 4 molal.Solubility was studied for the temperatures of 25 and 60 °C.
The experimental technique used, as previously described (8 and 9), consisted in determining, at each selected temperature, the compositions of the different saturated solutions in equilibrium with the respective solid phases.Copper was analyzed by atomic absorption spectrometry, ammonia by back titration, adding an excess of standardized hydrochloric acid solution and titrating with standardized NaOH using dimethyl yellow as an indicator, and chloride by the Volhard method.Synthetic solutions were used as a basis, and analytical grade chemical reagents were used in their preparation.Solutions were prepared with concentrations higher than those permitted by the solubility at the chosen temperature.For this purpose, solutions very near saturation were prepared at temperatures above those of each test, achieving over-saturation on cooling to the test temperature.To ensure that crystallization was complete and equilibrium had been achieved in all cases, a period of 2 h was always allowed to pass, with gentle agitation, at the chosen constant temperature.This time was considered sufficient after checking that there were no variations in the composition of the solution for times longer than 1 h.The solid was removed from the liquid phase by filtration, wiped by pressing between filter papers, without washing, and dried at room temperature.Chemical analysis and X-ray diffraction were used to monitor the compositions of the solid phase.
Chemical analysis of crystallized solids showed the existence of two different compounds, with compositions corresponding to the diammine chloride of formula Cu(NH3)2Cl2 and the oxychloride of formula Cu(OH)i 5Clo,5.The analytical valúes obtained for these solid specimens differed something from the theoretical valúes according to these formulae.The differences obtained were very light for the diammine chloride and more important for the oxychloride.They were caused by contamination with NH4CI because of not washing the solid phase and quantitatively agree with the amount of solution imbibed in each solid compound.
Analysis of solid phases by X-ray diffraction showed that in the range studied three different compounds crystallize.For the solids characterized by chemical analysis as diammine cupric chloride of formula Cu(NH3)2Cl2, their diffractograms coincided with that reported in (10) for this compound.The X-ray analysis of the solids characterized by chemical analysis as oxychloride showed that all these solids are mainly constituted by a same compound, whose diffractogram coincides with that reported in (10) for the Cu(II) chloride hydroxide hydrate of formula Cu7Cl4(OH)io(H20).In the major part of these solids, this compound appears as the unique constituent.In the rest, together with the diffractogram lines of this compound, appear other lines that coincided with those reported in (10) for the Cu(II) chloride hydroxide (botallackite) of formula Cu2Cl(OH)i 5. Careful chemical analysis of the solids, whose diffractogram coincides with that reported in (10) for the Cu7Cl4(OH)io(H20), points out that its composition is lightly different to that corresponding to this formula, being however in agreement with the formula CUCIQ 5(0H)i 5.
Table I sets out the experimental solubility results.The composition of the solution and the solid phase in equilibrium is given for each temperature studied.
studied (5 to 9 molal), the Cu(n) diammine chloride is generally the solid phase whose solubility regulates that of the cupric ion; the oxychloride becomes the most insoluble solid compound, and henee the substance which restricts the solubility of the cupric ion, only at the lowest test levéis of chlorides and when, at the same time, the ratio between the concentrations of [NH3]j and [Cu]^ is less than 2. When the solubility of Cu(II) diammine chloride establishes that of the cupric ion, the concentration of chlorides hardly affects this (Continúa) solubility, which in practice is essentially determined by the concentration of total ammonia in solution and, at each temperature, all the solubility data for the different total chloride levéis can be represented in an [NH3]joj3i-[Cu]TQtai by only one mean curve, with almost no error for practical purposes.On the other hand, when the solubility of the cupric ion is controUed by that of its oxychloride, the concentration of chloride has a strong influence.
The results obtained in the present work confirm the previous ones.For the conditions here studied, total chloride concentrations 1 to 4 molal, the Cu(II) solubility is also restricted by either the solubility of its diammine chloride or its oxychloride.In this case, however, the solubility behaviour of these two solid compounds is here quite different.As the experimental results show, the cupric oxychloride is generally the solid phase whose solubility regulates that of the cupric ion.Only, at the highest test levéis of chlorides and when, at the same time, the temperature is low (25 °C) and the ratio between the concentrations of [NH3]jotai and [Culj^jai i^ higher than 2, the cupric diammine chloride becomes the most insoluble compound, and in consequence, the one which regulates the Cu(II) solubility.
As it was exposed before, in the conditions studied here, cupric hydroxychloride crystallizes in two different crystalline forms.However, no relation between experimental conditions and the crystalline form precipitated could be found.
The theoretical treatment of Cu(II) solubility in ammoniacal ammonium chloride solution can be done keeping in mind all the different complexes that are formed in this médium.In the model developed in (9), it was considered that the Cu(II) in solution is distributed, besides the ionic form Cu2+, in form of chloride complexes CuCl^^-^, ammonia complexes Cu(NH3)n^+ and ammonia chloride complexes Cu(NH3)4_nCln2-n^ where n takes valúes from 1 to 4. The total copper concentration, [Cu] jotai' is the addition of the concentrations of all the ionic species in which Cu(II) is distributed.The same balance can be done for the total chloride and ammonia concentrations.The model assumes the existence of eighteen unknown concentrations: [Clljotai-To calcúlate these concentrations, it can be used the twelve equations for the chemical equilibriums of the complex formation and the solubility products, as well as the four corresponding to the material and loading balances.With the use of appropriate complex formation constants valúes from the literature, corresponding in each case to the ionic strength studied, the total concentrations of copper, ammonia and chloride can be estimated.In this way, it is possible to determine the solubility concentrations at equilibrium.
In the above mentioned paper ( 9), the predicted solubility curves obtained with this model fitted well with experimental data obtained for total chloride concentration higher than 5.0 molal, proving the validity of the model for the satisfactory description of the system solubility for these conditions.
For the experimental conditions here studied, total chloride concentration lower than 5.0 molal, it is not posible to make a similar comparative analysis for all the experimental solubility data.This is due to the absence of literature data on complex formation constants for many of the tested ionic strengths.For this reason, this analysis has been only made for the two higher valúes (3,0 and 4,0 molal) of total chloride concentration tested, using the same equilibrium constant data that were selected in ( 9) from the literature (11)(12)(13)(14) for ionic strength 5-6 (Table II).This approach was assumed, considering that the valué of concentration-based formation constant changes slowly (8 and 14) as ionic strength increases above 3 mol/kg.Elsewhere, the hydroxychloride solubility product valué was estimated from the experimental solubility data, using the model such as was made in previous works (8 and 9).The valué of the hydroxychloride Cu(OH)i 5CI05 solubility product so calculated (log Pg = -16,65 at 25 °C) has the same magnitude order although it is slightly lower than the valué reported in (11) (log Pg = -17,3 at 25 °C) for ionic strength zero.
The solubility isotherms of Cu(II) hydroxychloride and chloride diammine predicted with the model for a temperature of 25 °C have been represented in figure 1 for the 4,0 molal chloride level.As it can be seen, the two isotherms cross at one point, which defines the point of the solubility curve where the solid phase in equilibrium with the solution, either the hydroxychloride or the diammine chloride, changes.The Cu(II) solubility curve for this total concentration of chloride and temperature is composed of two different sections; each one belongs to one of the isotherms (continuous lines), and corresponds to that with the least solubility.The experimental points from table I obtained at this level of chloride concentration are also represented in figure 1 along with the predicted curves.As it can be seen, there is a good fit between the experimental points and the predicted curves.This agreement confirms the validity of the theorical model for the conditions here tested, providing a rational prediction model for Cu(II) solubility in the CUCI2-NH4CI-NH3-H2O system, as well as for which is, in each case, the solid compound in equilibrium with the solution.^ ^ Valores calculados para una concentración salina 6 molal con la expresión dada en (13) para un medio NH4NO3 (log K^ = log K^ +0,08C, siendo log B^ = X log K^, from / = 1 to / = n).
^ ) Estimated valúes from experimental solubility data.
In figure 2, the cupric hydroxychloride solubiüty curve predicted for a total chloride concentration 3,0 molal at 25 °C has been represented.Along with this predicted curve, the figure also illustrates the experimental points of solubility (Usted in table I) obtained for this same total chloride concentration and temperature.As can be seen, there is also a very good fit between the predicted solubility curve and the experimental points.In this figure, experimental points obtained for this same total chloride concentration but at a temperature of 60 °C have been also included.As can be seen, the temperature has scarce influence on cupric hydroxychloride solubility.This behaviour, together with the strong influence of the temperature on the cupric diammine chloride solubility, satisfactorily explains the change of compound in the solid phase with the temperature  increases.As experimental results show, this phenomenon is observed for a total chloride concentration valué of 4,0 molal and when the ratio [NH3]Totai/[Cu]Totai is highcr than 2. In these conditions, the solid compound in equilibrium at 25 °C with the solution is the cupric diammine chloride, but changes to the hydroxychloride when the temperature increases to 60 °C.

CONCLUSIONS
In the range of ammonium chloride studied (1 to 4 molal), the solubility of Cu(II) in ammoniacal aqueous solutions of ammonium chloride is restricted by either the solubility of the Cu(II) diammine chloride or the Cu(II) hydroxychloride.The hydroxychloride is generally the solid phase whose solubility regulates that of the cupric ion.The diammine chloride becomes the most insoluble compound, and henee the substance which restricts the solubility of cupric ion, only at the highest test level of chloride and when, at the same time, the ratio between the concentrations of [NH3]jofai and [Cu] jotai is higher than 2.  When the solubility of the cupric ion is controlled by that of its hydroxychloride, the concentration of chloride has a strong influence while the temperature scarcely influences.This behavior represents the opposite to that found for the diammine chloride solubility (9).
There is a very good fit between experimental data and solubility isotherms predicted by the theoretical model developed in (9).This agreement confirms the validity of this model, which also describes satisfactorily the solubility of the cupric ion under the conditions here tested.

FiG. 2 .
FiG.2.-Cu(II) solubility as a function of total ammonia concentration in the system CUCI2-NH4CI-NH3-H2O for a total chloride concentration 3,0 molal.Continuous line: Solubility predicted curve of Cu(OH)i 5Clo,5 at 25 °C obtained using the model for the total chloride concentration 3,0 molal.Symbols: Experimental results • at 25 and A at 60 °C.