Dialysis for Acid Recovery
R. J. KEATING, Manager Special Applications
ROBERT DVORIN, Manager Industrial Waste Treatment
Graver Water Conditioning Company
New York, New York
Dialysis, a separation method utilizing diffusion as its driving force, may
be used for the economical recovery of acids. The driving force is created by
separating the process liquor (or acid) from water with a semi-permeable
membrane. The water and acid attempt to come to equilibrium, i.e. , the
acid and metal salts diffuse into the water. The membrane pores are small
and constructed so that the smaller and the more active molecules, such as
acid, travel at a high rate relative to the metal salts.
The work done to move the acid from the spent liquor to the water is
provided by natural diffusion. It would take place if no membrane were
there. The function of the membrane is to screen the dissolved solids and
to hold back the unwanted molecules. Electro-dialysis, another method of
separating or removing dissolved solids from liquids, uses electrical energy as
the driving force. Dialysis and electro-dialysis equipment are quite different
in construction, cost, and operation, and therefore, must be considered as two
separate processes.
Dialysis is not a new process. The first published literature describing
dialysis was by Thomas Grahma of England in 1854, and the process remained
a laboratory tool until well into this century. The growth of the viscose rayon
industry in this country in the 1930's justified the first commercial application
of dialysis. Enormous quantities of caustic soda, containing colloidal hemi
cellulose, are a waste product of this industry. Dialysis provided the means
for recovering the bulk of this wasted caustic. Eighty-five hundredths lbs. of
caustic were recovered per lb of rayon produced, sufficient to have a real effect on the production cost.
These early dialysis units used parchment paper as the membrane. This
was, and is, a relatively fragile and delicate material and membrane life
might be anything from several days to several weeks. Frequent shut-downs
for replacement of the membrane resulted in high maintenance and replacement costs. Despite this handicap this process proved valuable and is still in
use in the rayon industry. High strength, acid resistant, plastic membranes
introduced in 1958 increased the application of dialysis to the field of acid
recovery.
Figure 1 shows a cross-section ofthe membrane. The membrane thickness is actually about 10 x as shown, (it is a split cross-section). The horizontal paths are the pores. The membrane is about four mils thick and the
pore size is about 50-80 Angstroms. Because of the long and tortuous path the
molecule must travel to get through the membrane, it is easy to see why very
large molecules or collids would get blocked while many very small ones got
through.
DIALYSIS PRINCIPLES
The reason dialysis takes place is the drive for equilibrium between the
water side and liquor side. Figure 2 shows a single dialysis cell. The concentration gradient across the membrane determines to a large degree, the
rate  of transport  of a given molecule.    In order to maintain a maximum
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