NEWS

6Membrane Technology May 2010

FEATURE

The prospective application of membrane distillation in the metallurgical industry

Membrane distillation is a membrane separa-tion process during which water evaporates from the feed stream (hot side) and then passes through the pores of a hydrophobic membrane to the strip stream (cold side or receiver stream) where it re-condenses. The only driving force for the mass-transfer of water vapour in the pores of the hydrophobic

membrane is the water-vapour pressure gradient.As a new member in the ‘membrane fami-

ly group’, research work into membrane dis-tillation started in the 1960s and developed from 1980s onwards. With the improvement of hydrophobic membrane materials, mem-brane distillation has been widely applied in many metallurgical industries.[1]

The common features of all kinds of mem-brane distillation are described as follows:

be achieved by a small temperature difference, under atmospheric pressure and at a relatively low temperature, using waste heat, geothermal or solar power as the energy resource;

stream to the receiver stream results in an improvement in the feed solution, and pure water is obtained in the receiver stream; and

Therefore, with a short supply of energy, lack of water resources and strict environmental pro-tection, membrane distillation is becoming an important and promising technology that can be used in the development of novel metallurgi-cal processes which are highly efficient, con-serve energy and generate little or no pollution.

Process of membrane distillation

Classification of the membrane distillation process

The process of membrane distillation is generally classified into four types. These are direct contact membrane distillation (DCMD), air spacing or air gap membrane distillation (AGMD), air blow or sweep gas membrane distillation (SGMD) and vacuum membrane distillation (VMD), based on the different methods of water condensation in the receiver stream. VMD has been widely applied because this process is easy to operate.

Figure 1 illustrates the underlying principles of the four kinds of membrane distillation.

Technics index and parameters of membrane distillation – rejection

For a non-volatile solute, in theory the rejec-tion should be 100%. However, this cannot be achieved in practice.

One reason for this is that the membrane may contain defects, such as cracks in the

Li Zeng, Metallurgical Science and Engineering School, Central South University, Changsha 410083, China, and Congjie Gao, The State Oceanic Administration, Hangzhou, China

Membrane distillation technology has been studied thoroughly in China and since the beginning of the 1980s it has developed quickly. This article provides an introduction to the basic concept of membrane distillation and includes details of its application in the metallurgical industry. Membrane processes related to membrane distillation are also briefly discussed, as is the future development of this technology.

Figure 1. Four types of membrane distillation.

FEATURE

May 2010 Membrane Technology7

material, partial big pores or pinholes, and a wide pore distribution.

The other reason relates to the moisture con-tent of the membrane during operation – which has an effect on the solution in the pores, through a loss, locally, of hydrophobic characteristics.

Technics index and parameters of membrane distillation – water flux

Water flux can be influenced by: water flux generally

decreases with increasing concentration of the feed stream;

water flux increases with temperature difference;

water flux increases with an improvement in the flow state on both sides of membrane; and

including pore diame-ter, porosity, membrane thickness and bend-ing factor.

BackgroundThe use of membrane distillation for the desali-nation of sea water was first proposed at the beginning of the 1960s by Findley in the USA, and Haute and Henderyckx in Europe.[1]

DCMD was first used in desalination by Weyl in 1964, but the small water-flux limit-ed its application. Following the introduction of Gore-Tex membranes for use in desali-nation, membrane distillation technology developed rapidly. Made from polytetrafluor-oethylene these membranes enabled a sharp increase in water flux to be achieved.

Two sets of pilot-plant test units for desali-nation were built on Hono Island (Sweden) during the late 1980s. These produced pure water and their operation was quite stable.

Membrane distillation plants for desalina-tion – with an output of 25 tonnes/day and 10 tonnes/day – came on stream in Japan during the early 1990s.

The main advantage of using membrane distillation in desalination applications is that it is easy to operate continuously under atmospheric pressure and at low temperatures. However, this technology has not been able to compete with reverse osmosis in desalination applications because it requires heat.

Application of membrane distillation in the metallurgical industryWith the development of membrane distilla-tion, metallurgists started to study the use of this technology in condensing waste acid, base or salt solutions, with a concentration of about 1 M, for applications involving treatment proc-esses to which it was not possible to apply other types of membrane-based systems.

Membrane distillation attracted metallurgists because of its good operating properties, includ-ing high condensation and low temperatures. The metallurgical industry strives to be energy-efficient, and a large amount of wasted heat needs to be recycled. In addition, solutions in hydro-metallurgical processes frequently needed to be concentrated. Therefore, the industri-alisation of membrane distillation can greatly

promote the progress of technology in the metal-lurgical industry.

Some successful uses of membrane distil-lation, involving the concentration of typical acidic or caustic solutions, have been accom-plished by the Metallurgical Separation Science and Engineering Institute of Central South University (CSU) and are discussed by Li et al. (2001),[2] Tang et al. (2002 and 2005)[3,4,5] and Zhang (2004).[6]

Condensation of waste sulphuric acid from titanium oxide hydrolysis

A VMD experimental device is shown in Figure 2. Pure dilute H2SO4 solution was first used to test the system, which showed that the acid could be concentrated to 10.3 mol/l (65.5%), from 2.1 mol/l (18.3%).

At the beginning of the test, the temperature at the hot side and the pressure at the cold side were set to 70°C and 2.67 kPa, respectively. As the process proceeded, the water flux gradually decreased to about 0, when the acid was con-centrated to 6.23 mol/l (55.1%). The tempera-ture was then set to 80°C to increase the vapour pressure difference, which resulted in further condensation of H2SO4 solution to 65.5%. The results are shown in Figure 3.

Based on these results, the waste TiO2 hydrolysis sulphuric acid solution, contain-ing 2.11 mol/l H2SO4, 37.47 g/l Fe2+ and 4.12 g/l TiO2, was then concentrated by the VMD-based system. It was found that the acid can be only concentrated to 31–32%. This is because of the crystallisation and precipitation of FeSO4 by the ‘salting-out’ effect, with the increase in the H2SO4 concentration, which resulted in the loss of membrane hydrophobic-ity. A further study indicated that titanium in the waste acid had no effect on the process.

In order to solve the problem, the H2SO4 solution was first separated by diffusion

Figure 2. The schematic layout of the vacuum membrane distillation (VMD) process used for the condensation of waste sulphuric acid from titanium oxide hydrolysis – showing the temperature controller (1); cycling tank (2); heater (3); cycled pump (4); membrane distillation device (5); manometer (6); condenser (7); flask (8); and vacuum pump (9).

Figure 3. The concentrating degree of H2SO4 alone using membrane distillation.

FEATURE

Membrane Technology May 20108

dialysis, followed by solvent extraction using tri-iso-octylamine (TIOA). The obtained H2SO4, with a concentration of 1.12 mol/l and recovery of 91.4%, was then concen-trated to 10.3 mol/l (65.1%) under operating conditions of a 80°C at hot side and 5.64 kPa at the cold side.

Recovery of hydrochloric acid from rare earth chloride solutions

In the solvent extraction process for rare earth solutions, a highly concentrated hydro-chloric acid (HCl) solution was used as the stripping reagent, which results in 2–5 mol/l HCl in the stripped liquor. The HCl is generally neutralised by a large amount of NH3·H2O or (NH4)2CO3, or recovered by diffusion dialysis.

The neutralisation consumes a large amount of reagent and treatment by diffusion dialysis is too slow. For these reasons, the use of VMD was inves-tigated as a possible way of recovering HCl from rare earth chloride solutions on a laboratory scale.

It seems that HCl with a high concentra-tion cannot be recovered from the rare earth chloride solution by membrane distillation because it is an azeotrope. However, because of the ‘salting-out’ effect of RECl3, and using the partial pressure calculation of H2O and HCl, based on the solution system contain-ing SmCl3 and hydrochloric acid, it was found that the pressure of HCl increased with the concentration of SmCl3, while that of H2O decreased.

The gas–liquor equilibrium in the solution with different concentrations of SmCl3 (at 25°C) is shown in Figure 4. As can be seen, the molar ratio of HCl/ H2O increases with the concentration of SmCl3. In the experimental process, the concentration of RECl3 gradually increases, and the temperature is much higher than 25°C, which benefits the increase of HCl concentration in the gas phase.

The stripped liquors of medium rare earth, with CRE = 0.6–0.9 mol/l, CHCl = 2–2.5 mol/l, and of heavy rare earth, with CRE = 0.2–0.4 mol/l, CHCl = 4.5–5.5 mol/l were treated every 5 l by membrane distillation to recover HCl under the conditions of a feed solution at 62°C and 8–10 kPa in the cold side and a flow-rate of 5.4 cm/s. The test results that are shown in Tables 1 & 2 demonstrate the theory.

Recovery of sulphuric acid from rare earth sulphate solutions using integrated membrane technologyDiffusion dialysis (DD) has been used to inves-tigate the recovery of sulphuric acid from rare earth sulphate solutions.

The results show that the technology can be used to effectively separate sulphuric acid and

Figure 5. A schematic diagram of the continuous vacuum membrane distillation process used for the condensation of spent liquor derived from the production of Al2O3 – showing the temperature controller (1); cycling tank (2); heater (3); cycled pump (4); flowmeter (5); vacuum membrane distillation device (6); condenser (7); receiving flask (8); vacuum pump (9); manometer (10); high-position tank (11); overflow cap (12); and measuring receiving tank (13).

Time(h)

Concentrated solution

Distilled solution

Recovery of HCl(%)

Rejection of rare earth

(%)CHCl(mol/l)

CRE(mol/l)

Volume(ml)

CHCl(mol/l)

CRE (mol/l)

2.00 2.497 0.873 735 0.693 0.0145 4.5 99.7

3.37 2.728 1.000 1295 0.841 0.0153 9.7 99.5

5.27 2.770 1.190 1800 1.249 0.0160 20.0 99.2

6.77 2.728 1.348 2235 1.619 0.0162 32.2 99.0

8.27 2.431 1.579 2640 2.047 0.0169 48.1 98.8

9.77 1.957 1.896 3290 2.410 0.0172 64.8 98.6

11.77 1.493 2.412 3445 2.568 0.0180 78.8 98.3

Table 1. Test results of membrane distillation for recovering HCl from the stripped liquid of medium rare earth solutions.

Figure 4. The gas–liquor equilibrium in the solution with different concentrations of SmCl3, at 25°C (SmCl3 concentration (mol/kg): the five curves, from the bottom upwards, represent 0; 0.4; 0.8; 1.2; and 1.4, respectively).

FEATURE

May 2010 Membrane Technology9

the rare earth component. However, it is not practical for industrial plants to use DD tech-nology for the recovery of sulphuric acid from a large amount of a rare earth sulphate solution, containing no more than 0.5 mol/l of H2SO4, since it is not economically feasible and hard to achieve the water balance in the process.

Therefore, a combination of VMD and DD technology was applied in order to study the condensation and recovery of H2SO4 from a rare earth sulphate solution.

The solution was first treated by VMD, for the concentration phase, followed by DD to recover H2SO4. Because the feed solution used in the DD process was the concentrated liquid that had been produced by VMD, it greatly reduced the handling capacity for the DD process and the investment in fixed assets. Meanwhile, the concen-tration of recovered H2SO4 increased, compared with the process that used DD alone.

The experimental conditions were as fol-lows: CRE = 0.070 mol/l and CH2SO4 = 0.468 mol/l in the feed solution, a tempera-ture of 60°C at the hot side, a pressure of 12.7 kPa at the cold side and a flow-rate of 5.8 cm/s, in the VMD process; and a tem-perature of 28–29°C, a flow-rate of 150 ml/h and the flow ratio of 1, in the DD process. The results are shown in Table 3.

Condensation of spent liquor from an Al2O3 production process

The spent liquor derived from the production of Al2O3 generally contains a large amount of Na2CO3, a certain amount of NaOH and a small amount of Al2O3 and SiO2. In treat-ment methods that are currently used the spent liquor is concentrated by evaporation and returned to the step of raw slurry making which, in terms of energy usage, is not effi-cient and can be costly.

Based on the consideration that the latent heat in spent liquor can be regarded as an energy resource for membrane distillation, a continuous experiment was carried out to study the feasibil-ity of using membrane distillation in a concen-trating process for spent liquor. A schematic diagram of this process is shown in Figure 5.

A 2 l pre-concentrated Na2CO3 solution, with a concentration of 244 g/l, was placed in the cycling tank (2). The feed solution of 122 g/l Na2CO3 flowed from the high tank (11) down to the cycling tank. The concentrated solution – pro-duced by VMD – was obtained from the measur-ing tank (13) as an overflow from the top of the cycling tank. Meanwhile, the distilled solution was obtained from the receiving tank (8).

Figure 6 shows the fluxes of the concentrated and distilled solution as a function of time. The pH of the concentrated solution was stable at around 13, which indicated the rejection of base was very high. The base concentration in the concentrated solution, as a function of time, is shown in Figure 7. As illustrated, the base concentration of the concentrated solu-tion can be maintained at twice that of the feed solution under stable operation.

The examples presented here undoubtedly show the potential that membrane distil-lation offers for the creation of novel and energy-saving metallurgical processes.

Problems and future development of membrane distillation

Problems Although there are some exceptional advantages of using membrane distillation compared with other membrane-based systems, there are still some problems associated with the technology. These are highlighted in literature by Yu et al. (1991),[7] Yan and Ma (2001),[8] Ma (2003),[9] Wu (2003)[10] and Zhang (2004),[6] and include those listed below.

The water flux of mem-brane distillation generally cannot compete with that of reverse osmosis, although in some excellent membrane distillation systems it can be as high as 75 kg/m2·h. Moreover, it gradu-ally decreases during the operation of the process. This is because over time pollution and moisture have a negative effect on the membrane. Pollution can be caused by bac-teria growth on the membrane’s surface and concentration polarisation. Concentration polarisation not only impairs the driving force in the membrane distillation process, but it also can damage the hydrophobicity of membrane when the concentration is suf-

Time(h)

Concentrated solution

Distilled solution

Recovery of HCl (%)

Rejection of rare earth

(%)CHCl(mol/l)

CRE(mol/l)

Volume(ml)

CHCl(mol/l)

CRE (mol/l)

2.00 5.259 0.353 675 5.018 0.0011 13.1 99.9

3.50 5.069 0.390 1140 5.341 0.0015 23.6 99.8

5.15 4.919 0.451 1680 5.535 0.0019 36.0 99.7

6.65 4.723 0.529 2145 5.718 0.0020 47.4 99.7

8.15 4.383 0.629 2605 5.824 0.0022 58.7 99.6

9.65 3.840 0.776 3055 5.949 0.0024 70.3 99.5

11.65 3.163 1.131 3635 5.964 0.0029 83.8 99.3

Table 2. Test results of membrane distillation for recovering HCl from the stripped liquid of heavy rare earth solutions.

Figure 6. The relationship of flux versus time for the Al2O3 production process shown in Figure 5.

FEATURE

Membrane Technology May 201010

ficiently high. It is also regarded as the most serious form of pollution of the membrane.

The heat efficiency of membrane distillation is normally around 30%, with nearly 70% of heat loss occurring during heat transmission. This is caused by temperature polarisation which means that not all of the heat is used to vaporise the feed solu-tion, resulting in an impairment of the driving force and a decrease in flux.

Research covering the manufacture of membranes and components that are used exclusively for membrane distillation has been neglected. Understanding the properties of a membrane material is key to the development of the membrane distillation process.

Future development of membrane distillation

As a relatively new membrane technology, mem-brane distillation has not been largely industrial-ised worldwide. In China, further work needs to be carried out, covering the following areas:

Membranes made from polytetrafluoroeth-ylene and polyvinylidene fluoride are not able to meet the requirements of the mem-brane distillation process. There is an urgent need to produce and develop new hydro-phobic membranes – with hypodispersion of pore diameter, big porosity and a small

bending factor – to help promote the indus-trialisation of this technology.

A comprehensive understanding of the underlying mecha-nism is essential. Although some mechanism models of membrane distillation have been put forward, and studied, many parameters in the process are not reliable. These need to be further emended and refined.

Heat efficiency is an important technical index in membrane distillation. Low heat-efficiency is the single main factor that limits the industrialisation of this technology.

: Membrane distil-lation is now applied to the separation and concentration of aqueous solutions. The separation of an azeotrope, especially a mix-ture of organic solvents, is a promising area that needs to be studied in greater detail.

VMD has the highest flux of all four types of membrane distillation. The membrane cannot be easily damaged during operation and the resistance in receiver stream is relatively small.

With further development of membrane materials and the progress being made in manufacturing processes, the essential and use-ful properties of membranes will be improved while their cost will decrease. It is believed that membrane distillation technology will be devel-oped quickly and applied broadly in the metal-lurgical industry.

AcknowledgementThe authors would like to thank Dr Qixiu Zhang for reviewing this paper and providing valuable comments.

References1. Zhang, Y.J. and Wu, X.M., The applica-

tion and development of membrane distilla-tion technology, 11 (2001) p. 16.

2. Li, Q., Zhang, Q.X., Zhang, G.Q. and Zhou, K.G., Study on direct concentration of waste sulphuric acid from titanium oxide hydroly-sis by vacuum membrane distillation, Metals and Hard Alloy 146 (2001) p. 1.

3. Tang, J.J., Zhou, K.G. and Zhang, Q.X., Study on hydrochloric acid recovery from chloride solutions of rare earth by vacuum membrane distillation, and Technology 22(4) (2002) p. 38.

4. Tang, J.J., Zhang, W., Zhou, K.G., Li, R.X. and Zhang, Q.X., Sulphuric acid recovery from RE sulphate solutions by integrated membrane distillation, and Technology 25(3) (2005) p. 54.

5. Tang, J.J., Chen, J.J., Zhang, W., Zhou, K.G. and Zhang, Q.X., Study on sulphuric acid recovery from RE sulphate solutions by diffusion dialysis. Membrane Science and Technology 25(2) (2005) p. 50.

6. Zhang, Q.X., ‘Metallurgical Separation Science and Engineering’, Beijing, Science Press, 2004.

7. Yu, L.X., Liu, M.L. and Jiang, W.J., The study and development tendency of membrane distilla-tion, Chemical Engineering Progress 3 (1991) p. 1.

8. Yan, J.M. and Ma, R.Y, Study on the membrane pollution on the membrane distillation process,

21(3) p. 21.9. Ma, R.Y., Review and prospect of membrane

distillation technology, Journal of Tianjin Institute of Urban Construction 9(2) (2003) p. 100.

10. Wu, Y.L., Advance of membrane distillation technology and the application, Membrane Science and Technology 23(4) (2003) p. 67.

Contact:

Li Zeng, Metallurgical Science and Engineering School,

Central South University, Changsha 410083, China.

Email: yuanzhenxia_1104@yahoo.com.cn

Sample VMD for concentration DD for separation

Concentrated solution (mol/l)

Rejection of rare earth

(%)

Dialysis solution(mol/l)

Rejection of rare earth

(%)

Recovery of H2SO4(%)

CRE CH2SO4 CRE CH2SO4

1 0.070 0.468 — 0.0039 0.340 94.4 72.6

2 0.103 0.687 99.6 0.0064 0.500 93.8 72.9

3 0.140 0.932 98.9 0.0085 0.681 93.9 73.1

4 0.206 1.372 98.9 0.0132 1.003 93.6 73.1

Table 3. Test results of vacuum membrane distillation (VMD) and diffusion dialysis (DD) used for recovering H2SO4 from a rare earth sulphate solution.

Figure 7. The relationship of base concentration in a concentrated solution with respect to time for the Al2O3 production process shown in Figure 5.