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The precipitation of solids in a liquid membrane emulsion: The control of particle size
Powder Technology, 65 (1991) 235-242
235
The precipitation of solids in a liquid membrane emulsion: the control of particle size M. Yang, G. A. Davies* and J. Garside Denartment of Chemical Enaineering, University of Manchester, Institure of Science and Technology, P.O. Box 88, Mkchester, k60 IQD (U.K.)
Abstract The precipitation of a solid from a solution generally involves several steps, viz., the purification of the solution, reaction and precipitation, separation, drying, comminution and classification of the solid. In this work, we consider the precipitation of a salt, copper oxalate, for which we have developed a process whereby separation of a desired cation, Cu 2f from a solution takes place by selective mass transfer through a membrane film. A chemical reaction and precipitation in a confined environment is carried out
so that the particularly precipitation for solution further size
resulting particle size of the precipitate can he controlled within the range 0.1-7 pm and from 0.1-2.0 pm. The process, based on extraction using a liquid surfactant membrane with of the solid in the internal phase of the emulsion membrane, effectively eliminates the need pretreatment to remove competing ions and most importantly will eliminate the need for modification of the precipitate.
Introduction The control of product purity and particle size distribution are two essential factors in the production of fine particles and crystals. When a solid product is produced by precipitation, either by reaction or concentration, a range of particle sizes is produced with often only a fraction of the distribution being in the desired size range. The quest is always to optimise the fraction of the solid precipitate within the desired size range. Since a spectrum of sizes is produced, some being smaller and some larger than the product specification, two subsequent processes are necessary to produce the product and to optimise the yield. These are classification and comminution. When fine particles d,, < 10 pm are to be produced the processing costs associated with these solids handling stages can represent a serious on-cost to the product. If a process could be developed which would eliminate the need for classification and comminution, then this would have significant benefits. Additionally, the solutions from which the solid is produced must be purified to remove co-ions or impurities which would co-precipitate. Therefore conventional processes involve at least five separate stages: (i) solution purification, (ii)
*To whom correspondence
0032-5910/91/$3.50
should be addressed.
precipitation, (iii) solid-liquid separation and washing, (iv) classification and (v) comminution. Furthermore, if the product is to be obtained from dilute solutions, then pre-concentration is usually necessary in conjunction with the solution purification stage(s). A key to improving the first step might be to use a permeability selective membrane to carry out solution purification and, since some membranes are particularly suitable to function with dilute solutions and provide a significant concentration enhancement between the feed and permeate, the use of a membrane system might be extremely effective. Thus, in principle, it might be possible to process dilute feed mixtures and achieve, by selective transport through a membrane, solution purification to a level where the precipitated solid may have acceptable purity. The concentration enhancement achieved would reduce energy costs by eliminating the need for solvent evaporation. This improvement to the front end of the process would not, however, improve or control the particle size distribution of the solid product and expensive downstream solids handling would remain. In this work, we have addressed these problems. A key is to carry out the precipitation process in an environment in which the growth of the particles formed following nucleation is constrained so that the final size is within the required
0 Elsevier Sequoia/Printed
in The Netherlands
236
product range. This constraint is not possible in a conventional stirred tank reactor or crystalliser. One solution we describe here is to return to a membrane system so as to exploit the advantages with regard to solution purification but rather than use porous solid membranes to use a liquid surfactant emulsion membrane (LSM). If it is possible to carry out the precipitation reaction within the dispersed phase of the emulsion then, notwithstanding some technical problems which we will consider later, it should be possible to control the particle size since these particles produced will be constrained within the physical size of the dispersed phase droplets and by the initial concentration of reagents in these droplets (via material balances).
A system to carry out precipitation of a salt from aqueous solution with control of particle size A model system has been used in this work, the precipitation of copper oxalate from simulated commercial acid leach liquors containing copper sulphate (concentrations 0( lo)-0( 103) ppm wt./ unit vol.) with oxalic acid. A copper system was used since a great deal of experience has been obtained in this laboratory on the selective extraction of copper, Cu2+, with liquid surfactant membranes and the most difficult problems, those of the formulation of the emulsion and methods of preparation, have been largely solved for this type of cation transport system. Additionally, contaminants common in acid leach liquid such as nickel will co-precipitate in oxalic acid so that the system will also demonstrate the advantage of selective cation transport achieving purification. The overall reaction is Cu2+ + (COOH), + Cu( COO)21 + 2H +
(1)
Copper ions will be present in the feed leach solution along with a range of other co-ions; principally of interest are Fe2+, Fe3+, Ni’+: since ferrous and nickel oxalates would co-precipitate with copper if present when the reaction, eqn. (l), was carried out. Ferric oxalate, although soluble under the conditions for the reaction, would contaminate the product; Fe3+ is normally present in concentrations many times that of copper in the feed solution. In order to carry out a selective extraction from the feed solution and precipitation of copper oxalate, a liquid membrane containing a copper ligand was used. Following our earlier work on LSM extraction [ 1,2], the ligand used was 2-hydroxy-5nonylacetophenone oxime. This was manufactured by Shell Chemicals under the trade
Aqueous
Feed
Solution
Organ/c
(&I++)
+ 2RH -
CUR,
Membrane
+ 21H+)
Fig. 1. Mass transfer in an idealised membrane.
name SME 529 and was dissolved in a kerosene diluent. An idealised system is shown in Fig. 1. The membrane, shown here as a thin planar film, contains the ligand. On one side of the membrane is the feed mixture containing copper as Cu*+ and a number of other co-ions, M”+. At the interface between the feed and membrane film, a surface chemical reaction takes place according to the overall stoichiometric relation Cu;+ + 2RH, = [Cu R210+ 2H,’
(2)
The subscripts a and o refer to the aqueous and organic (membrane) phase respectively. With the ligand (RH) used, the reaction is specific for copper if the pH of the aqueous solution is kept below 2.5. In this work, the pH at the start of the extraction was controlled within the range 1.51.8. Since the copper complex [Cu R2] is formed at the feed/membrane interface, molecular diffusion within the membrane film will cause it to migrate towards the other interface which is again an organic-aqueous interface. In the LSM system, this ‘initial’ aqueous phase is actually the dispersed phase of an emulsion. The object of the process is to selectively transport copper from the feed solution through the membrane into this internal phase where the precipitation reaction (l), can take place. The great advantage of this liquid system is that the reaction (2) is reversible if a source of protons is available at an organic/ aqueous, O/A, interface. Thus, [Cu R210+ 2H,+ + Cu;+ + 2RH,
(3)
In the model system, protons are available, since the internal phase contains oxalic acid (or, in some of our work, mixtures of oxalic acid with other acids). Thus, at the membrane interface, the internal phase reactions (3) and ( 1) take place simultaneously and a solid copper oxalate phase is formed. At this stage, selective transport leading to a pure solid product has been achieved. The
231
aspects of concentration and control of particle size have yet to be considered. In order to understand the concentration enhancement which can be achieved across this membrane, we must examine the mechanisms of transport. This is achieved by selective surface chemical reaction at an aqueous/organic, A/O, interface followed by diffusion of a ligand bound complex and surface chemical reaction at an O/A interface to liberate in this case a cation and regenerate the ligand. This ligand then diffuses in the counter current direction to the A/O interface. There will be diffusion in the mass transfer boundary layers in both aqueous phases but the controlling concentration gradient is not the cation but protons. Unless the proton concentration in the internal phase is greater than that in the feed solution, then no transfer will take place. This then means that the cation concentration of the internal phase is not controlled by that of the feed solution and it will be possible to produce an aqueous internal phase with a cation concentration much higher than that of the feed solution [ 1, 3-61. It is an example of facilitated transport. In the present case, the cation concentration in the internal phase will be reduced by reaction (1) to a concentration related to the solubility product of the oxalate. The control of particle size is achieved by the form of the membrane system used. It is not practically possible to produce a stable planar membrane in the form shown in Fig. 1. It is clear that the membrane must be stable until at least the feed solution is removed, otherwise the oxalic acid would leach into the feed solution, with the resulting competing precipitation of the various salts, and no control of purity or particle size, etc., would be possible. A practical way of achieving a ‘stable’ membrane is that demonstrated by Li [7]. An aqueous in oil emusion is first produced. The internal acid phase containing, in our case, the oxalic acid is emulsified in the organic (membrane) phase. The organic phase contains the ligand, diluentkerosene, and an emulsifying agent. For A/O emulsions, the hydrophilic-hydrophilic balance number of the emulsifying agent should be between 7 and 9. In this study, following our previous work [ 11, Span 20, a commercial grade of the non-ionic surfactant sorbitan laurate, was used. The emulsion phase is ‘quasi-stable’ so that, when this is mixed with the feed solution, the two aqueous phases are separated. The system during mass transfer and selective permeation is shown schematically in Fig. 2. The internal phase is formed as small micro droplets dispersed in the continuous phase of the emulsion. A process flowsheet is shown in Fig. 3. The emulsion is produced in a high-shear ho-
Emulsion
I
Globule ~
0
:
*. *.’
Aqueous
Feed
Solution
Fig. 2. Emulsion membrane system.
mogeniser and then transferred to a stirred tank reactor, where it is dispersed as ‘droplets’ or ‘globules’ in the feed solution. At the outer surface of these ‘droplets’, reaction (2) takes place. The droplets are typically in the size range l-3 mm diameter. The organic membrane is in fact a complex matrix separating the dispersed micro droplets, sketched in Fig. 2. Diffusion of the complexed ligand takes place within the matrix and the liberation of cations by reaction (3) takes place at the surface of the micro droplets. The effective thickness of the membrane film is controlled by the size of the micro droplets in the emulsion and on the acid/oil phase ratio of the emulsion. Both of these parameters are also important in determining the stability of the membrane film. In this work, the emulsions were formulated and manufactured to produce mirco droplets in the size range l-10 pm. The principle object of the work was to produce a solid precipitate in a controlled environment to a particle size range specification, the premise being that, if the reaction takes place inside the micro droplets, then unless either the emulsion is unstable and breaks down or micro droplets coalesce during processing, the particle will be confined within these droplets. The differences in densities between the solid product and liquid reactant solution will mean that the maximum particle diameter d, max will be of the order of 7 pm diameter, since dp max
where d,,, is the maximum diameter of the micro pL and ps the densities of the liquid and solid phases, respectively. In most experiments, d max- 10 pm. drops,
Spent Raffinate
I-Iomogeniser
Membrane Brealcdown
Dilllent f SAA)
Fig. 3. Process Bow sheet.
Results, extraction of copper ions and precipitation of copper oxalate The formulation and methods of manufacture of the emulsion used here was as outlined in our earlier work [l, 21. Extraction experiments were carried out from a range of solutions containing from 100-2 000 ppm wt/unit vol. of copper ions. The initial pH of the solutions was controlled at 1.8. Small samples of the solution were taken during contact with the emulsion and the concentration of copper ions of these samples was determined using atomic absorption spectrophotometry. Examples of the results showing extraction are seen in Fig. 4. These show that extraction can be carried out from dilute solutions and the initial extraction rate is proportional to the copper ion concentration. It is not obvious from these extraction curves that selective transport of Cu2+ has been achieved. However, by analysing samples of both the continuous oil phase and acid internal phase of the emulsion for other ions present in the feed, Fe3+, Fez+, Ni2+, the absence of these ions in the emulsion proved that selective extraction had been achieved. Thus, by using the membrane system, the need for solution purification has been eliminated. Samples of the emulsion were removed from the reactor, Fig. 3, to analyse for the product copper oxalate. The presence of the copper salt was evident from visual inspection since the colollr of the emulsion, which starts as a white opaque solution, was blue and the depth of the blue colour increased as the contact time in the
Fig. 4. Typical liquid membrane extraction curves.
239
reaction increased. (Compared with the initial feed solution the colour of the emulsion after mode of the solid distribution being 0.6 pm and that of the droplets 4.5 pm. This suggests that the nuclei formed in the micro droplets do not coagulate to form a single particle, since l/3 h
0
0 0.1
10
1 Particle
Fig. 5. Size distribution alate particles produced.
size(pm)
of typical
emulsions
loo
4 and copper
ox-
reaction was much more intense.) The emulsion was separated from feed solution, washed with water and the wash water decanted. It was then mixed with warm toluene to break the emulsion. The internal aqueous phase was then allowed to settle, together with particles of copper oxalate. After decanting off the solvent, the copper oxalate was filtered and examined by a scanning electron microscope. A sample of the aqueous slurry after solvent decanting was also analysed chemically to confirm the presence of copper oxalate in the form of the hemihydrate, Cu( COO), fH,O. The samples were also analysed using a Malvern Mastersizer to determine the particle size distribution of the solid. Results from the size analysis are shown in Figs. 5 and 6. In both figures, the size distribution of the emulsion micro droplets are shown prior to extraction (these were also measured on the Mastersizer diluting the emulsion in toluene solvent). In Fig. 5, the important features are that the copper oxalate particles are less than the maximum emulsion drop size, the 20
100
f
k
\
a conclusion confirmed by SEM examination. The drop size distribution in the emulsion can be controlled by varying the manufacturing conditions [8-lo]. For example, changes in the system to achieve an increase in the drop size produced the results in Fig. 6. The distributions become skewed as seen in the diagram. However, the important aspect of this is to see if a change in the particle size distribution can be achieved by modifying the emulsion and by adapting the concentrations. Comparing Figs. 5 and 6, it can be seen that there is a change in the size distribution of the copper oxalate with the proportion of bigger particles increasing. Again, the particles were all confined within the maximum of the drop distribution. The method of breaking the emulsion could influence the size distribution, since an increase in temperature on adding warm toluene could influence the growth rate if any unreacted copper ions were present in the internal phase of the emulsion. This was assessed by breaking the emulsion in the cell of the Mastersizer by initially part filling the cell with ethanol and adding the emulsion directly into ethanol. The results are shown in Fig. 7, the initial emulsion corresponding to that used in Fig. 5. The particles obtained under these conditions are now smaller than those in Fig. 5, suggesting that further particle growth by reaction or agglomeration may take place during emulsion breaking. A number of additional analytical techniques were employed to supplement the chemical analy20
loll
1 10
% 50
% 50
0
0 0.1
1
10 Particle
Fig. 6. Size distribution alate particles produced.
sue
him)
of typical
100 31
emulsions
and copper
ox-
Fig. 7. Size distribution alate particles produced.
of typical
emulsions
and copper
ox-
240
Fig. 8. A photomicrograph sion.
from Cryo-SEM,
of a virgin emul-
ses and light-scattering measurement to confirm the form of the precipitate. Scanning electronmicroscopy using a cryogenic stage and microtome section facility was used to examine the emulsion and precipitate. An example of emulsion before extraction and reaction showing a section of micro drops is shown in Fig. 8. The sectioned micro drops are shown as cavities in the photograph. Droplets lying in a plane below the section are also shown; the ‘caps’ of these appear as highlights. Two sections of emulsions after producing a copper precipitate are shown in Figs. 9 and 10. In these examples, the conditions under which the emulsion in Fig. 9 was processed produced less precipitate than that in Fig. 10 (lower internal concentration of copper in the feed and shorter contact time). In these figures, small spherical particles are seen ‘attached’ to the internal interface of the droplets. The form and size of these coincided with the copper oxalate particles extracted from the emulsion after solvent breakage and settling. A definitive confirmation that these particles seen in Figs. 9 and 10, where indeed the
Fig. 10. A photomicrograph containing copper oxalate
from Cryo-SEM particles produced.
of an emulsion
product was attempted using X-ray diffraction and scanning TEM is apparent. A confirmation from X-ray diffraction that copper was present was obtained, but the resolution of the machine was such that one could not differentiate the result from the area of the ‘particles’ and the surrounding background emulsion. The emulsion may contain copper complexed with the ligand. Further careful work stripping the emulsion with an acid which will not react with copper oxalate is necessary before final conclusions can be drawn from the results of X-ray diffraction. Similarly, much more effort is required to extract the potential out of scanning TEM for this problem. 20
104
0
Fig. Il. Size distribution of copper oxalate tated by mixing two aqueous solutions.
Fig. 9. A photomicrograph containing copper oxalate
from Cryo-SEM particles produced.
of an emulsron
0
particles
precipi-
An interesting feature arising from the work on direct precipitation, Fig. 11, concerns the morphology of the precipitate and the influence on this of surfactants. During the experiments, some of the particles were seen to adsorb at the aqueous-air interface. These particles were examined using an optical microscope. The particles first aggregated
241
Fig. 12. A photomicrograph from SEM of a copper oxalate cluster formed at the air-water interface.
Fig. 13. A photomicrograph from SEM of a copper oxalate cluster formed at the air-water interface.
into small clusters, doublets and triplets. These slowly developed into large two-dimensional clusters at the interface. The large clusters were carefully removed on filter paper, dried and examined using a SEM. Two examples of the clusters formed are shown in Figs. 12 and 13. The concentration of the surfactant used, cetyl trimethyl ammonium bromide in this case, was the same in both experiments, 3.33 g/l. The aggregation of small particles to form large structures has been observed in many different conditions. A question arises as to how these
are formed in the present work, Vold [ 1l] and Sutherland [ 12, 131 observed cluster aggregates in colloidal systems and proposed a model based on particle-cluster agglomeration. A random walk process was used to simulate the motion of single particles. The basis of the model was that a single particle attaches to the largest cluster in the system. The simulations derived from these assumptions produce complex irregular cluster shapes with some resemblance to experimental observations. The problem to compare theory with experiment is to find a quantitative method to assess the form and characteristics of these assemblies. Two important milestones in achieving this were the development of Fractal Mathematics, presented first as an important and powerful branch of analysis by Mandlebrot [ 14, 151and the discovery of the diffusion-limited aggregation, D-L-A, models by Witten and Sander [ 161. These have some similarity to the basic ideas of the aggregation models proposed by Eden [ 171, Void [ 1I] and Sutherland [ 12, 131, but Witten and Sander demonstrated that simple processes could lead to the generation of complex fractal patterns which closely resembled some natural processes. Furthermore, the resemblance could be tested quantitatively by comparing the fractal dimension of the model results with experiment. A further advance has been established by Meakin [l&20], who demonstrated that the fractal dimension of disordered structures could be related via statistical simulation models to the mechanism of formation. Thus, the fractal dimension for cluster structures in 2-D space, as at a planar aqueous-air interface, is unique to the method of formation. The possibilities that arise in the present work are first that the process could be limited by the reaction taking place near the interface or by diffusion of species or single particles at the interface, so-called reaction limited or diffusion limited aggregation. Secondly, the cluster may form by particle aggregation, particle-cluster and polydispersed cluster-cluster mechanisms. In clustercluster aggregation, the clusters may initially be held together by weak van der Waal forces but then reorganise by particle cluster rotation to form strong bonds, for example metallic or co-valent bonds between adjacent particles, to form rigidly bound structures [21,22]. The patterns change as this reorganisation takes place. To start to explore the mechanisms of formation of the structures we observed, the fractal dimension was computed from measurements of up to 40 different clusters analysed. The maximum lengths of the clusters were determined from photomicrographs, such as Figs. 12 and 13. This was plotted against the number of particles in the cluster, i. The slope from a logarithmic graph is
242
The surface active agent used to stabilise the membrane emulsion has been shown to influence the morphology of the precipitate produced.
Acknowledgements The authors wish to thank SERC for the provision of a research contract under the Membrane and Separation Processes Initiative. Mr. M. Yang wishes to thank the British Council for the award of a Research Scholarship.
Fig. 14. Maximum distance-number relationship clusters to determine the fractal dimension.
of particle
the reciprocal fractal dimension, Dr. The results are shown in Fig. 14. From the graph, fitting a linear regression through the points, the value of D, obtained was 1.81. This corresponds to a particle-cluster aggregation process and is intermedifor a ate between the theoretical results diffusion-limited aggregation process in 2-D space of 1.71 and a reaction limited process in 2-D space of 2.00. The role of the surfactant is crucial in this process; stearic forces between the surfactant chains and the particles will govern the particle-particle organisation in the surface layer. More work is being carried out to confirm this. An improved method of computing D, by measuring the perimeter and area of the clusters on a digitised image analyser is being carried out.
Conclusions This work has demonstrated that it is possible to carry out a chemical reaction and precipitation within the dispersed phase of a liquid surfactant membrane emulsion. The particle size of the solid produced is controlled by the drop size distribution of the emulsion, and in this work, using water in oil emulsions, particles of copper oxalate within the range 0.1-7 pm were produced. By carrying out the reaction within a liquid membrane emulsion structure, separation of the reagents can be accomplished and under conditions of facilitated transport a concentration can be simultaneously obtained. Thus, the method can be used to produce solids of controlled size from dilute liquid mixtures without pre-separation and concentration.
List of symbols micro drop diameter mean micro drop diameter particle diameter mean particle diameter fractal dimension length of cluster density of copper oxalate solid phase density of aqueous phase of the emulsion
References 1 T. P. Martin and G. A. Davies, Hydrometallurgy, 2 (1976) 315. 2 T. P. Martin and G. A. Davies, Proc. Inr. Solv. Extr. Con& I (1980) 230. 3 N. N. Li, Ind. Eng. Chem. Proc. Des., 10 (1971) 215. 4 A. G. Kopp, R. J. Marr and F. E. Moser, I. Chem. E. Symp. Ser., 54 (1978) 321. and E. L. Cussler, Sep. Purif. Methods, 5 A. M. Hochhauser 3 (1974) 399. G. A. Davies, Hydrometallurgy, I (1981) Dl. N. N. Li. U.S. Patenf 3.410.794 (1968). G. A. Davies, U.S. Patent 4,047,;193 (i977). V. B. Menon and D. T. Wasan, Sep. Sci. and Tech., 19 (1984) 555. 10 V. B. Menon and D. T. Wasan, Coil. & Surfaces, 19 (1986) 89, 107. 11 M. J. Vold, J. CON. Sci., 18 (1963) 684. 12 D. N. Sutherland, J. Coil. Inferf: Sci., 22 (1966) 300. J. Coil. Interf Sci., 25 (1967) 373. 13 D. N. Sutherland, 14 B. Mandelbrot, Form, Chance and Dimension, Freeman, New York, 1977. 15 B. Mandlebrot, The Fractal Geometry of Nature, Freeman, New York, 1982. 16 T. A. Witten and L. M. Sander, Phys. Rev. Letters, 47 (1981) 1400. 17 M. Eden, in F. Neymal (ed.), Proc. 4th Berkeley Symp. Math, Statistics and Probability, Vol. 4, Univ. of California Press, Berkeley, 1960. p. 133. 18 P. Meakin, Phys. Rev., B29 (1984) 3722. 19 P. Meakin, Ann. Rev. Phys. Chem;, 39 (1988) 420. in H. E. Stanlev and N. Ostronskv feds.). 20 P. Meakin, Random Fluctuations and Pa;tern Growth, Kluwdn,‘ Len: don, 1988, 170. 21 A. T. Skjeltorp, Phys. Rev. Letters., 5g (1987) 1444. 22 P. Meakin and R. Jullien, J. Chem. Phys., 89 (1988) 246.
235
The precipitation of solids in a liquid membrane emulsion: the control of particle size M. Yang, G. A. Davies* and J. Garside Denartment of Chemical Enaineering, University of Manchester, Institure of Science and Technology, P.O. Box 88, Mkchester, k60 IQD (U.K.)
Abstract The precipitation of a solid from a solution generally involves several steps, viz., the purification of the solution, reaction and precipitation, separation, drying, comminution and classification of the solid. In this work, we consider the precipitation of a salt, copper oxalate, for which we have developed a process whereby separation of a desired cation, Cu 2f from a solution takes place by selective mass transfer through a membrane film. A chemical reaction and precipitation in a confined environment is carried out
so that the particularly precipitation for solution further size
resulting particle size of the precipitate can he controlled within the range 0.1-7 pm and from 0.1-2.0 pm. The process, based on extraction using a liquid surfactant membrane with of the solid in the internal phase of the emulsion membrane, effectively eliminates the need pretreatment to remove competing ions and most importantly will eliminate the need for modification of the precipitate.
Introduction The control of product purity and particle size distribution are two essential factors in the production of fine particles and crystals. When a solid product is produced by precipitation, either by reaction or concentration, a range of particle sizes is produced with often only a fraction of the distribution being in the desired size range. The quest is always to optimise the fraction of the solid precipitate within the desired size range. Since a spectrum of sizes is produced, some being smaller and some larger than the product specification, two subsequent processes are necessary to produce the product and to optimise the yield. These are classification and comminution. When fine particles d,, < 10 pm are to be produced the processing costs associated with these solids handling stages can represent a serious on-cost to the product. If a process could be developed which would eliminate the need for classification and comminution, then this would have significant benefits. Additionally, the solutions from which the solid is produced must be purified to remove co-ions or impurities which would co-precipitate. Therefore conventional processes involve at least five separate stages: (i) solution purification, (ii)
*To whom correspondence
0032-5910/91/$3.50
should be addressed.
precipitation, (iii) solid-liquid separation and washing, (iv) classification and (v) comminution. Furthermore, if the product is to be obtained from dilute solutions, then pre-concentration is usually necessary in conjunction with the solution purification stage(s). A key to improving the first step might be to use a permeability selective membrane to carry out solution purification and, since some membranes are particularly suitable to function with dilute solutions and provide a significant concentration enhancement between the feed and permeate, the use of a membrane system might be extremely effective. Thus, in principle, it might be possible to process dilute feed mixtures and achieve, by selective transport through a membrane, solution purification to a level where the precipitated solid may have acceptable purity. The concentration enhancement achieved would reduce energy costs by eliminating the need for solvent evaporation. This improvement to the front end of the process would not, however, improve or control the particle size distribution of the solid product and expensive downstream solids handling would remain. In this work, we have addressed these problems. A key is to carry out the precipitation process in an environment in which the growth of the particles formed following nucleation is constrained so that the final size is within the required
0 Elsevier Sequoia/Printed
in The Netherlands
236
product range. This constraint is not possible in a conventional stirred tank reactor or crystalliser. One solution we describe here is to return to a membrane system so as to exploit the advantages with regard to solution purification but rather than use porous solid membranes to use a liquid surfactant emulsion membrane (LSM). If it is possible to carry out the precipitation reaction within the dispersed phase of the emulsion then, notwithstanding some technical problems which we will consider later, it should be possible to control the particle size since these particles produced will be constrained within the physical size of the dispersed phase droplets and by the initial concentration of reagents in these droplets (via material balances).
A system to carry out precipitation of a salt from aqueous solution with control of particle size A model system has been used in this work, the precipitation of copper oxalate from simulated commercial acid leach liquors containing copper sulphate (concentrations 0( lo)-0( 103) ppm wt./ unit vol.) with oxalic acid. A copper system was used since a great deal of experience has been obtained in this laboratory on the selective extraction of copper, Cu2+, with liquid surfactant membranes and the most difficult problems, those of the formulation of the emulsion and methods of preparation, have been largely solved for this type of cation transport system. Additionally, contaminants common in acid leach liquid such as nickel will co-precipitate in oxalic acid so that the system will also demonstrate the advantage of selective cation transport achieving purification. The overall reaction is Cu2+ + (COOH), + Cu( COO)21 + 2H +
(1)
Copper ions will be present in the feed leach solution along with a range of other co-ions; principally of interest are Fe2+, Fe3+, Ni’+: since ferrous and nickel oxalates would co-precipitate with copper if present when the reaction, eqn. (l), was carried out. Ferric oxalate, although soluble under the conditions for the reaction, would contaminate the product; Fe3+ is normally present in concentrations many times that of copper in the feed solution. In order to carry out a selective extraction from the feed solution and precipitation of copper oxalate, a liquid membrane containing a copper ligand was used. Following our earlier work on LSM extraction [ 1,2], the ligand used was 2-hydroxy-5nonylacetophenone oxime. This was manufactured by Shell Chemicals under the trade
Aqueous
Feed
Solution
Organ/c
(&I++)
+ 2RH -
CUR,
Membrane
+ 21H+)
Fig. 1. Mass transfer in an idealised membrane.
name SME 529 and was dissolved in a kerosene diluent. An idealised system is shown in Fig. 1. The membrane, shown here as a thin planar film, contains the ligand. On one side of the membrane is the feed mixture containing copper as Cu*+ and a number of other co-ions, M”+. At the interface between the feed and membrane film, a surface chemical reaction takes place according to the overall stoichiometric relation Cu;+ + 2RH, = [Cu R210+ 2H,’
(2)
The subscripts a and o refer to the aqueous and organic (membrane) phase respectively. With the ligand (RH) used, the reaction is specific for copper if the pH of the aqueous solution is kept below 2.5. In this work, the pH at the start of the extraction was controlled within the range 1.51.8. Since the copper complex [Cu R2] is formed at the feed/membrane interface, molecular diffusion within the membrane film will cause it to migrate towards the other interface which is again an organic-aqueous interface. In the LSM system, this ‘initial’ aqueous phase is actually the dispersed phase of an emulsion. The object of the process is to selectively transport copper from the feed solution through the membrane into this internal phase where the precipitation reaction (l), can take place. The great advantage of this liquid system is that the reaction (2) is reversible if a source of protons is available at an organic/ aqueous, O/A, interface. Thus, [Cu R210+ 2H,+ + Cu;+ + 2RH,
(3)
In the model system, protons are available, since the internal phase contains oxalic acid (or, in some of our work, mixtures of oxalic acid with other acids). Thus, at the membrane interface, the internal phase reactions (3) and ( 1) take place simultaneously and a solid copper oxalate phase is formed. At this stage, selective transport leading to a pure solid product has been achieved. The
231
aspects of concentration and control of particle size have yet to be considered. In order to understand the concentration enhancement which can be achieved across this membrane, we must examine the mechanisms of transport. This is achieved by selective surface chemical reaction at an aqueous/organic, A/O, interface followed by diffusion of a ligand bound complex and surface chemical reaction at an O/A interface to liberate in this case a cation and regenerate the ligand. This ligand then diffuses in the counter current direction to the A/O interface. There will be diffusion in the mass transfer boundary layers in both aqueous phases but the controlling concentration gradient is not the cation but protons. Unless the proton concentration in the internal phase is greater than that in the feed solution, then no transfer will take place. This then means that the cation concentration of the internal phase is not controlled by that of the feed solution and it will be possible to produce an aqueous internal phase with a cation concentration much higher than that of the feed solution [ 1, 3-61. It is an example of facilitated transport. In the present case, the cation concentration in the internal phase will be reduced by reaction (1) to a concentration related to the solubility product of the oxalate. The control of particle size is achieved by the form of the membrane system used. It is not practically possible to produce a stable planar membrane in the form shown in Fig. 1. It is clear that the membrane must be stable until at least the feed solution is removed, otherwise the oxalic acid would leach into the feed solution, with the resulting competing precipitation of the various salts, and no control of purity or particle size, etc., would be possible. A practical way of achieving a ‘stable’ membrane is that demonstrated by Li [7]. An aqueous in oil emusion is first produced. The internal acid phase containing, in our case, the oxalic acid is emulsified in the organic (membrane) phase. The organic phase contains the ligand, diluentkerosene, and an emulsifying agent. For A/O emulsions, the hydrophilic-hydrophilic balance number of the emulsifying agent should be between 7 and 9. In this study, following our previous work [ 11, Span 20, a commercial grade of the non-ionic surfactant sorbitan laurate, was used. The emulsion phase is ‘quasi-stable’ so that, when this is mixed with the feed solution, the two aqueous phases are separated. The system during mass transfer and selective permeation is shown schematically in Fig. 2. The internal phase is formed as small micro droplets dispersed in the continuous phase of the emulsion. A process flowsheet is shown in Fig. 3. The emulsion is produced in a high-shear ho-
Emulsion
I
Globule ~
0
:
*. *.’
Aqueous
Feed
Solution
Fig. 2. Emulsion membrane system.
mogeniser and then transferred to a stirred tank reactor, where it is dispersed as ‘droplets’ or ‘globules’ in the feed solution. At the outer surface of these ‘droplets’, reaction (2) takes place. The droplets are typically in the size range l-3 mm diameter. The organic membrane is in fact a complex matrix separating the dispersed micro droplets, sketched in Fig. 2. Diffusion of the complexed ligand takes place within the matrix and the liberation of cations by reaction (3) takes place at the surface of the micro droplets. The effective thickness of the membrane film is controlled by the size of the micro droplets in the emulsion and on the acid/oil phase ratio of the emulsion. Both of these parameters are also important in determining the stability of the membrane film. In this work, the emulsions were formulated and manufactured to produce mirco droplets in the size range l-10 pm. The principle object of the work was to produce a solid precipitate in a controlled environment to a particle size range specification, the premise being that, if the reaction takes place inside the micro droplets, then unless either the emulsion is unstable and breaks down or micro droplets coalesce during processing, the particle will be confined within these droplets. The differences in densities between the solid product and liquid reactant solution will mean that the maximum particle diameter d, max will be of the order of 7 pm diameter, since dp max
where d,,, is the maximum diameter of the micro pL and ps the densities of the liquid and solid phases, respectively. In most experiments, d max- 10 pm. drops,
Spent Raffinate
I-Iomogeniser
Membrane Brealcdown
Dilllent f SAA)
Fig. 3. Process Bow sheet.
Results, extraction of copper ions and precipitation of copper oxalate The formulation and methods of manufacture of the emulsion used here was as outlined in our earlier work [l, 21. Extraction experiments were carried out from a range of solutions containing from 100-2 000 ppm wt/unit vol. of copper ions. The initial pH of the solutions was controlled at 1.8. Small samples of the solution were taken during contact with the emulsion and the concentration of copper ions of these samples was determined using atomic absorption spectrophotometry. Examples of the results showing extraction are seen in Fig. 4. These show that extraction can be carried out from dilute solutions and the initial extraction rate is proportional to the copper ion concentration. It is not obvious from these extraction curves that selective transport of Cu2+ has been achieved. However, by analysing samples of both the continuous oil phase and acid internal phase of the emulsion for other ions present in the feed, Fe3+, Fez+, Ni2+, the absence of these ions in the emulsion proved that selective extraction had been achieved. Thus, by using the membrane system, the need for solution purification has been eliminated. Samples of the emulsion were removed from the reactor, Fig. 3, to analyse for the product copper oxalate. The presence of the copper salt was evident from visual inspection since the colollr of the emulsion, which starts as a white opaque solution, was blue and the depth of the blue colour increased as the contact time in the
Fig. 4. Typical liquid membrane extraction curves.
239
reaction increased. (Compared with the initial feed solution the colour of the emulsion after mode of the solid distribution being 0.6 pm and that of the droplets 4.5 pm. This suggests that the nuclei formed in the micro droplets do not coagulate to form a single particle, since l/3 h
0
0 0.1
10
1 Particle
Fig. 5. Size distribution alate particles produced.
size(pm)
of typical
emulsions
loo
4 and copper
ox-
reaction was much more intense.) The emulsion was separated from feed solution, washed with water and the wash water decanted. It was then mixed with warm toluene to break the emulsion. The internal aqueous phase was then allowed to settle, together with particles of copper oxalate. After decanting off the solvent, the copper oxalate was filtered and examined by a scanning electron microscope. A sample of the aqueous slurry after solvent decanting was also analysed chemically to confirm the presence of copper oxalate in the form of the hemihydrate, Cu( COO), fH,O. The samples were also analysed using a Malvern Mastersizer to determine the particle size distribution of the solid. Results from the size analysis are shown in Figs. 5 and 6. In both figures, the size distribution of the emulsion micro droplets are shown prior to extraction (these were also measured on the Mastersizer diluting the emulsion in toluene solvent). In Fig. 5, the important features are that the copper oxalate particles are less than the maximum emulsion drop size, the 20
100
f
k
\
a conclusion confirmed by SEM examination. The drop size distribution in the emulsion can be controlled by varying the manufacturing conditions [8-lo]. For example, changes in the system to achieve an increase in the drop size produced the results in Fig. 6. The distributions become skewed as seen in the diagram. However, the important aspect of this is to see if a change in the particle size distribution can be achieved by modifying the emulsion and by adapting the concentrations. Comparing Figs. 5 and 6, it can be seen that there is a change in the size distribution of the copper oxalate with the proportion of bigger particles increasing. Again, the particles were all confined within the maximum of the drop distribution. The method of breaking the emulsion could influence the size distribution, since an increase in temperature on adding warm toluene could influence the growth rate if any unreacted copper ions were present in the internal phase of the emulsion. This was assessed by breaking the emulsion in the cell of the Mastersizer by initially part filling the cell with ethanol and adding the emulsion directly into ethanol. The results are shown in Fig. 7, the initial emulsion corresponding to that used in Fig. 5. The particles obtained under these conditions are now smaller than those in Fig. 5, suggesting that further particle growth by reaction or agglomeration may take place during emulsion breaking. A number of additional analytical techniques were employed to supplement the chemical analy20
loll
1 10
% 50
% 50
0
0 0.1
1
10 Particle
Fig. 6. Size distribution alate particles produced.
sue
him)
of typical
100 31
emulsions
and copper
ox-
Fig. 7. Size distribution alate particles produced.
of typical
emulsions
and copper
ox-
240
Fig. 8. A photomicrograph sion.
from Cryo-SEM,
of a virgin emul-
ses and light-scattering measurement to confirm the form of the precipitate. Scanning electronmicroscopy using a cryogenic stage and microtome section facility was used to examine the emulsion and precipitate. An example of emulsion before extraction and reaction showing a section of micro drops is shown in Fig. 8. The sectioned micro drops are shown as cavities in the photograph. Droplets lying in a plane below the section are also shown; the ‘caps’ of these appear as highlights. Two sections of emulsions after producing a copper precipitate are shown in Figs. 9 and 10. In these examples, the conditions under which the emulsion in Fig. 9 was processed produced less precipitate than that in Fig. 10 (lower internal concentration of copper in the feed and shorter contact time). In these figures, small spherical particles are seen ‘attached’ to the internal interface of the droplets. The form and size of these coincided with the copper oxalate particles extracted from the emulsion after solvent breakage and settling. A definitive confirmation that these particles seen in Figs. 9 and 10, where indeed the
Fig. 10. A photomicrograph containing copper oxalate
from Cryo-SEM particles produced.
of an emulsion
product was attempted using X-ray diffraction and scanning TEM is apparent. A confirmation from X-ray diffraction that copper was present was obtained, but the resolution of the machine was such that one could not differentiate the result from the area of the ‘particles’ and the surrounding background emulsion. The emulsion may contain copper complexed with the ligand. Further careful work stripping the emulsion with an acid which will not react with copper oxalate is necessary before final conclusions can be drawn from the results of X-ray diffraction. Similarly, much more effort is required to extract the potential out of scanning TEM for this problem. 20
104
0
Fig. Il. Size distribution of copper oxalate tated by mixing two aqueous solutions.
Fig. 9. A photomicrograph containing copper oxalate
from Cryo-SEM particles produced.
of an emulsron
0
particles
precipi-
An interesting feature arising from the work on direct precipitation, Fig. 11, concerns the morphology of the precipitate and the influence on this of surfactants. During the experiments, some of the particles were seen to adsorb at the aqueous-air interface. These particles were examined using an optical microscope. The particles first aggregated
241
Fig. 12. A photomicrograph from SEM of a copper oxalate cluster formed at the air-water interface.
Fig. 13. A photomicrograph from SEM of a copper oxalate cluster formed at the air-water interface.
into small clusters, doublets and triplets. These slowly developed into large two-dimensional clusters at the interface. The large clusters were carefully removed on filter paper, dried and examined using a SEM. Two examples of the clusters formed are shown in Figs. 12 and 13. The concentration of the surfactant used, cetyl trimethyl ammonium bromide in this case, was the same in both experiments, 3.33 g/l. The aggregation of small particles to form large structures has been observed in many different conditions. A question arises as to how these
are formed in the present work, Vold [ 1l] and Sutherland [ 12, 131 observed cluster aggregates in colloidal systems and proposed a model based on particle-cluster agglomeration. A random walk process was used to simulate the motion of single particles. The basis of the model was that a single particle attaches to the largest cluster in the system. The simulations derived from these assumptions produce complex irregular cluster shapes with some resemblance to experimental observations. The problem to compare theory with experiment is to find a quantitative method to assess the form and characteristics of these assemblies. Two important milestones in achieving this were the development of Fractal Mathematics, presented first as an important and powerful branch of analysis by Mandlebrot [ 14, 151and the discovery of the diffusion-limited aggregation, D-L-A, models by Witten and Sander [ 161. These have some similarity to the basic ideas of the aggregation models proposed by Eden [ 171, Void [ 1I] and Sutherland [ 12, 131, but Witten and Sander demonstrated that simple processes could lead to the generation of complex fractal patterns which closely resembled some natural processes. Furthermore, the resemblance could be tested quantitatively by comparing the fractal dimension of the model results with experiment. A further advance has been established by Meakin [l&20], who demonstrated that the fractal dimension of disordered structures could be related via statistical simulation models to the mechanism of formation. Thus, the fractal dimension for cluster structures in 2-D space, as at a planar aqueous-air interface, is unique to the method of formation. The possibilities that arise in the present work are first that the process could be limited by the reaction taking place near the interface or by diffusion of species or single particles at the interface, so-called reaction limited or diffusion limited aggregation. Secondly, the cluster may form by particle aggregation, particle-cluster and polydispersed cluster-cluster mechanisms. In clustercluster aggregation, the clusters may initially be held together by weak van der Waal forces but then reorganise by particle cluster rotation to form strong bonds, for example metallic or co-valent bonds between adjacent particles, to form rigidly bound structures [21,22]. The patterns change as this reorganisation takes place. To start to explore the mechanisms of formation of the structures we observed, the fractal dimension was computed from measurements of up to 40 different clusters analysed. The maximum lengths of the clusters were determined from photomicrographs, such as Figs. 12 and 13. This was plotted against the number of particles in the cluster, i. The slope from a logarithmic graph is
242
The surface active agent used to stabilise the membrane emulsion has been shown to influence the morphology of the precipitate produced.
Acknowledgements The authors wish to thank SERC for the provision of a research contract under the Membrane and Separation Processes Initiative. Mr. M. Yang wishes to thank the British Council for the award of a Research Scholarship.
Fig. 14. Maximum distance-number relationship clusters to determine the fractal dimension.
of particle
the reciprocal fractal dimension, Dr. The results are shown in Fig. 14. From the graph, fitting a linear regression through the points, the value of D, obtained was 1.81. This corresponds to a particle-cluster aggregation process and is intermedifor a ate between the theoretical results diffusion-limited aggregation process in 2-D space of 1.71 and a reaction limited process in 2-D space of 2.00. The role of the surfactant is crucial in this process; stearic forces between the surfactant chains and the particles will govern the particle-particle organisation in the surface layer. More work is being carried out to confirm this. An improved method of computing D, by measuring the perimeter and area of the clusters on a digitised image analyser is being carried out.
Conclusions This work has demonstrated that it is possible to carry out a chemical reaction and precipitation within the dispersed phase of a liquid surfactant membrane emulsion. The particle size of the solid produced is controlled by the drop size distribution of the emulsion, and in this work, using water in oil emulsions, particles of copper oxalate within the range 0.1-7 pm were produced. By carrying out the reaction within a liquid membrane emulsion structure, separation of the reagents can be accomplished and under conditions of facilitated transport a concentration can be simultaneously obtained. Thus, the method can be used to produce solids of controlled size from dilute liquid mixtures without pre-separation and concentration.
List of symbols micro drop diameter mean micro drop diameter particle diameter mean particle diameter fractal dimension length of cluster density of copper oxalate solid phase density of aqueous phase of the emulsion
References 1 T. P. Martin and G. A. Davies, Hydrometallurgy, 2 (1976) 315. 2 T. P. Martin and G. A. Davies, Proc. Inr. Solv. Extr. Con& I (1980) 230. 3 N. N. Li, Ind. Eng. Chem. Proc. Des., 10 (1971) 215. 4 A. G. Kopp, R. J. Marr and F. E. Moser, I. Chem. E. Symp. Ser., 54 (1978) 321. and E. L. Cussler, Sep. Purif. Methods, 5 A. M. Hochhauser 3 (1974) 399. G. A. Davies, Hydrometallurgy, I (1981) Dl. N. N. Li. U.S. Patenf 3.410.794 (1968). G. A. Davies, U.S. Patent 4,047,;193 (i977). V. B. Menon and D. T. Wasan, Sep. Sci. and Tech., 19 (1984) 555. 10 V. B. Menon and D. T. Wasan, Coil. & Surfaces, 19 (1986) 89, 107. 11 M. J. Vold, J. CON. Sci., 18 (1963) 684. 12 D. N. Sutherland, J. Coil. Inferf: Sci., 22 (1966) 300. J. Coil. Interf Sci., 25 (1967) 373. 13 D. N. Sutherland, 14 B. Mandelbrot, Form, Chance and Dimension, Freeman, New York, 1977. 15 B. Mandlebrot, The Fractal Geometry of Nature, Freeman, New York, 1982. 16 T. A. Witten and L. M. Sander, Phys. Rev. Letters, 47 (1981) 1400. 17 M. Eden, in F. Neymal (ed.), Proc. 4th Berkeley Symp. Math, Statistics and Probability, Vol. 4, Univ. of California Press, Berkeley, 1960. p. 133. 18 P. Meakin, Phys. Rev., B29 (1984) 3722. 19 P. Meakin, Ann. Rev. Phys. Chem;, 39 (1988) 420. in H. E. Stanlev and N. Ostronskv feds.). 20 P. Meakin, Random Fluctuations and Pa;tern Growth, Kluwdn,‘ Len: don, 1988, 170. 21 A. T. Skjeltorp, Phys. Rev. Letters., 5g (1987) 1444. 22 P. Meakin and R. Jullien, J. Chem. Phys., 89 (1988) 246.