Membrane Separations for Green Chemistry - Process Intensification for Green Chemistry - page 324

of waste water from the metallurgical industry for the purification of water containing
heavy metals. They concluded that water purified by both nanofiltration and reverse
osmosis satisfied the state reutilization qualification, but nanofiltration is more suitable for
development at the industrial scale.
Electrodyalisis is another membrane process that appears very promising in heavy
metal waste water treatment. Generally this process is used for the production of
drinking water and to process brackish water and sea water, treat industrial effluents,
recover useful materials from effluents and produce salt. An electrodyalisis pilot plant
comprising a set of ion-exchange membranes was developed by Nataraj et al. [27] for the
removal of hexavalent chromium. The performance of this process was also evaluated,
with success, for the separation of Cu and Fe from solutions in copper electrowinning
operations. Lambert et al. [28] studied the separation of Cr (III) by electrodyalisis using
modified cation-exchange membranes, while Mohammadi et al. [29] studied Pb 2 þ
separation from waste water. In the latter case, increasing voltage and temperature
improved cell performance, while an increase of flow rate decreased the separation
percentage.
12.3.2 Membrane Operations in Acid Separation fromWaste Water
Membrane processes also find application in inorganic and organic acid separation. Inorganic
acids are commonly used in numerous industrial processes, such as steel treatment, surface
treatment, metal refining, the electronics and glass industries and pigment and chemical
production, while organic acids are increasingly used in the food, pharmaceuticals and
packaging industries. Recovery of inorganic acids provides economic and environmental
benefits as they are both valuablematerials and hazardouswaste. Organic acids are frequently
used as food or pharmaceutical additives, as active ingredients in some care products and as
monomer units in biodegradable polymer synthesis. The interest that exists in the recovery of
organic acids is because:
Some low-molecular-weight carboxylic acids appear as waste materials in different
industrial waste waters (petrochemical, pharmaceutical, polymer, etc.).
Organic acids are produced by fermentation and they need to be extracted from a
complex mixture in order to obtain a pure product.
Table 12.9 illustrates some interesting examples of inorganic and organic acids used for
different applications in industry and the sources from which they can be extracted.
The disposal of inorganic acids is usually carried out by a neutralization-precipitation-
discharge sequence that uses solvent extraction, stripping, precipitation and evaporation.
This makes the process more complicated and increases the cost of operation and the
accumulation of toxic solid waste.
The recovery of inorganic acids through simultaneous concentration and purification has
the advantages of waste disposal cost savings and the recovery of valuable materials that
can be reused in the production line. Membrane processes, such as electrodialysis and
diffusion dialysis, can be used to separate inorganic acids from industrial waste water,
while nanofiltration, reverse osmosis and membrane distillation can purify acids such as
phosphoric acid of cationic impurities. Innovative production schemes based on membrane
operations may be applied during organic acid production in fermenters and by conversion