Method and apparatus for recovering an acid from an acid-containing waste liquor

An apparatus for recovering an acid from an acid-containing waste liquor, which is a diffusion dialytic cell having a plurality of anion exchange membranes disposed to alternately form feed liquor compartments to which the acid-containing waste liquor is supplied and recovery compartments to which water is supplied, wherein a cooling compartment defined by a water-impermeable membrane is provided adjacent to such feed liquor compartments or recovery compartments.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to novel method and apparatus for efficiently 
recovering a free acid from an acid-containing waste liquor, especially 
from an acid-containing waste liquor having a high acid concentration. 
2. Discussion of Background 
As a method for recovering an acid from an acid-containing waste liquor, 
diffusion dialysis employing anion exchange membranes has been proposed 
and has been practically employed to some extent. However, when such a 
method is practically put in operation, heat of diffusion is likely to 
accumulate, and the temperature rise takes place, thus leading to problems 
such as a deterioration of the ion exchange membranes and heat 
deformations of parts constituting the diffusion dialytic cell. Thus, it 
has been difficult to conduct a consistent operation for a long period of 
time. It has also been proposed to preliminarily cool the acid-containing 
waste liquor and water for recovery before their supply in order to 
control the temperature level within such a range that no problem will be 
created even when heat of dilution has accumulated in the diffusion 
dialytic cell. However, it has been found that the heat accumulation is 
unexpectedly so high that the preliminal cooling of the waste liquor or 
water must be great. As the result, the recovered acid also becomes cold 
and, it is necessary to reheat it when the acid is used at fairly high 
temperature. Thus, this method has been found to be ineffective from the 
viewpoint of the energy consumption. Further, a multistage dialytic method 
has been proposed as an idea to solve the problem by reducing the heat 
generation by dilution per unit dialytic cell. However, such a method has 
a problem that the installation cost increases. Thus, the commercial 
operation of such a method has been found to be difficult. 
There has been no effective method other than the ion exchange membrane 
method. Thus, in spite of high concentration of a valuable acid content, 
the waste liquor has had to be disposed. Yet, it is required to neutralize 
such a waste liquor with an alkali before disposition to avoid pollution, 
and the cost required for such treatment has been substantial. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide novel method and 
apparatus, whereby a free acid in such an acid-containing waste liquor of 
high concentration can be efficiently recovered. 
The present inventors have measured the temperature distribution in a 
diffusion dialytic cell when an acid-containing waste liquor is subjected 
to diffusion dialysis against water by means of anion exchange membranes 
and have found the following facts. Namely, it has been found that a very 
high peak temperature appears at about the central portion in the vertical 
direction, and the temperature rise may reach a level as high as from 
50.degree. to 60.degree. C. depending upon the operational condition. 
However, at the same time, it has been found that by changing the 
operational condition e.g. the ratio of the flow rate of the 
acid-containing waste liquor to the flow rate of the water for recovery, 
the position of this peak temperature can be moved up or down, and the 
peak temperature itself can be lowered. This indicates that the position 
and the temperature of the peak temperature are determined by the relation 
between the heat capacity of the acid-containing waste liquor as the 
ascending stream and the heat capacity of the water for recovery as the 
descending stream. Thus, the present inventors have found it possible to 
easily control the peak temperature by introducing a third heat transfer 
medium into the diffusion dialytic cell to destroy the balance of the heat 
quantities of the ascending and descending streams and thereby conduct a 
consistent operation for a long period of time. 
Thus, the present invention resides in a method for recovering an acid from 
an acid-containing waste liquor in a diffusion dialytic cell having a 
plurality of anion exchange membranes disposed therein, wherein the 
temperature rise in the diffusion dialytic cell is controlled by supplying 
a third heat transfer medium (cooling water) as shown in FIG. 2. 
Namely, the present invention provides an apparatus for recovering an acid 
from a acid-containing waste liquor, which is a diffusion dialytic cell 
having a plurality of anion exchange membranes disposed to alternately 
form feed liquor compartments to which the acid-containing waste liquor is 
supplied and recovery compartments to which water is supplied, wherein a 
cooling compartment defined by a water-impermeable membrane is provided 
adjacent to such feed liquor compartments or recovery compartments. 
Further, the present invention provides a method for recovering an acid 
from an acid-containing waste liquor by diffusion dialysis in a diffusion 
dialytic cell having a plurality of anion exchange membranes disposed to 
alternately form feed liquor compartments to which the acid-containing 
waste liquor is supplied and recovery compartments to which water is 
supplied, which comprises supplying a cooling medium to a cooling 
compartment defined by a water-impermeable membrane and provided adjacent 
to such feed liquor compartments or recovery compartments, to conduct the 
diffusion dialysis while suppressing the temperature rise in the diffusion 
dialytic cell. 
In the present invention, the temperature of the dialytic cell may be 
represented by either the temperature of the acid-containing waste liquor 
in the feed liquor compartments or the temperature of the water in the 
recovery compartments, since the heat exchange in the dialytic cell should 
sufficiently be conducted by ion exchange membranes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the present invention, the principle of recovering sulfuric acid in an 
acid-containing waste liquor in a diffusion dialytic cell is illustrated 
in FIG. 1. In FIG. 1, A indicates an anion exchange membrane. Anion 
exchange membranes are disposed as shown in the Figure to form a plurality 
of partitioned compartments. As an apparatus having such a construction, 
various types may be used. However, it is particularly preferred to employ 
a so-called filter press type dialytic cell wherein a plurality of anion 
exchange membranes are disposed between clamping frames with opened center 
portions to form dialytic compartments by means of compartment frames 
having liquid supply and discharge mechanisms and spacers, and the entire 
assembly is clamped to form a cell (as disclosed in e.g. Japanese Examined 
Patent Publication No. 34119/1973 and Japanese Unexamined Patent 
Publication No. 141803/1981). 
As the anion exchange membranes for the present invention, from weakly 
basic type to strongly basic type anion exchange membranes can be used. 
Preferably, a styrene-divinylbenzene copolymer or a 
vinylpyridinedivinylbenzene copolymer having a base of a quaternary 
ammonium salt as ion exchange groups, may be used. Such anion exchange 
membranes preferably have an ion exchange capacity of from 2.0 to 5.0, 
particularly from 3.0 to 4.0, meq/g dry resin for efficient recovery of 
acid. 
Referring to FIG. 1, the acid-containing waste liquor x is supplied to 
alternate dialytic compartments I.sub.1, I.sub.2, I.sub.3 and I.sub.4, as 
shown in the Figure, at a rate of from 0.5 to 2.0 l/hr.multidot.m.sup.2, 
while water y is supplied to recovery compartments II.sub.1, II.sub.2, 
II.sub.3 and II.sub.4 adjacent to the respective dialytic compartments 
partitioned by anion exchange membranes, at substantially the same flow 
rate as the above acid-containing waste liquor. The acid-containing waste 
liquor and water are preferably supplied counter-currently rather than in 
a concurrent flow fashion, so that a substantial gradient in the 
concentration of acid is always maintained with the anion exchange 
membranes disposed therebetween. 
Thus, the acid-containing waste liquor and water will face each other as 
partitioned by the anion exchange membranes. The treated solution z having 
the acid removed by the dialysis, is then discharged out of the system. 
In the present invention, the following dialytic cell is preferably 
employed to control the temperature of the dialytic cell to a level lower 
than the above-mentioned prescribed temperature. Namely, in the 
above-mentioned dialytic cell, a cooling compartment defined by 
water-impermeable membranes is provided adjacent to the above-mentioned 
feed liquor compartments or recovery compartments. The water-impermeable 
membrane is preferably a film having preferably a water absorptivity (ASTM 
D-570, 24 hours) of not higher than 0.1%, preferably less than 0.01% and 
having preferably a thickness of from 50 to 300 .mu.m, preferably from 100 
to 250 .mu.m. The size of the film may be the same as the size of the ion 
exchange membranes. Eventually, the cooling compartment preferably has the 
same thickness and size as the feed liquor compartment or recovery 
compartment. 
FIG. 2 illustrates an arrangement of the membranes of a diffusion dialytic 
cell having cooling compartments, constructed as described above. In FIG. 
2, the same symbols as used in FIG. 1 indicate the same members. In FIG. 
2, reference numerals III.sub.1 and III.sub.2 indicate cooling 
compartments defined by water-impermeable membranes B. The cooling 
compartment may be adjacent to the feed liquor compartments I.sub.1 and 
I.sub.2 as shown by III.sub.1, or may be adjacent to the recovery 
compartments II.sub.2 and II.sub.3 as shown by III.sub.2. Into the cooling 
compartments, a cooling medium H such as purified water, city water or the 
acid-containing feed liquor is supplied. The temperature of cooling 
compartment is preferably lower by from 2.degree. to 10.degree. C., 
preferably from 2.degree. to 4.degree. C. than the temperature of the feed 
liquor compartments or the recovery compartments, although such may vary 
depending upon the desired temperature of the dialytic cell. 
The number of cooling compartments in a dialytic cell varies depending upon 
the temperature of the dialytic cell to be controlled. However, it is 
usually preferably from 0.5 to 0.02, preferably from 0.15 to 0.1 unit per 
unit number of the feed liquor compartment or recovery compartment. The 
cooling compartments may not necessarily be provided with regular 
intervals or distances. However, in order to maintain the dialytic cell, 
the feed liquor compartment or the recovery compartment at a uniform 
temperature, preferably less than 60.degree. C., especially 40.degree. C., 
they are preferably provided with predetermined intervals or distances. 
There is no particular restriction as to the material for the 
water-impermeable membrane forming the cooling compartment. However, from 
the viewpoint of the corrosion resistance and costs, polyvinyl chloride, 
polyethylene or polypropylene is preferably employed. The cooling 
compartment may not necessarily be defined by water-impermeable membranes 
at both sides, and only one side may be defined by a water-permeable 
membrane, an anion exchange membrane or a cation exchange membrane. 
However, in order to avoid the mixing with the dialytic feed liquor or 
with the recovered solution, it is preferred that both sides are defined 
by the water-impermeable membranes. 
In a present invention, the waste liquor containing an acid such as 
sulfuric acid, hydrochloric acid, nitric acid, acid mixture thereof can be 
treated. When the concentration of free acid in the waste liquor is 
usually at least 3N, especially 5N, the present invention is effectively 
applied. 
In a case of a waste liquor from an electrolytic etching step of aluminum 
foils for capacitors, the waste liquor contains from 20 to 30% of an 
aluminum component and an acid mixture of sulfuric acid and hydrochloric 
acid at a free acid concentration of from 5 to 7N. According to the 
present invention, it is possible to recover the acids of from 5 to 7N 
containing no substantial aluminum component consistently over a long 
period of time without a deterioration of ion exchange membranes or 
without a problem such as heat deformations of parts constituting the 
dialytic cell. 
The titanium sulfate waste liquor to be treated by the present invention is 
the one discharged from the process for producing titanium by a sulfuric 
acid method. The waste liquor usually contains from 5 to 20 g/l of a 
titanium component (as TiO.sub.2, the majority is dissolved in the form of 
TiO--SO.sub.4) and from 200 to 400 g/l of sulfuric acid. According to the 
present invention, up to 90% of sulfuric acid in such a titanium sulfate 
waste liquor can be recovered as pure sulfuric acid (concentration: 200 to 
400 g/l) for the first time on an industrial scale. Therefore, the method 
of the present invention is extremely useful. 
Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted to such specific Examples. 
EXAMPLE 1 
A waste liquor of an acid mixture of sulfuric and hydrochloric acids from 
the etching step of aluminum foils (containing 7.05N of free acids and 17 
g/l of Al) was introduced from below to the feed liquor compartments of 
the diffusion dialytic cell as shown in FIG. 2 at a flow rate of 791 l/hr, 
while water was introduced from above to the recovery compartments of the 
diffusion dialytic cell at a flow rate of 673 l/hr. 
In the diffusion dialytic cell, 880 sheets of anion exchange membrane (a 
strongly basic styrene-divinylbenzene copolymer) and 576 sheets of 
polyvinylchloride (water absorptivity: less than 0.1%, thickness: 200 
.mu.m) were incorporated, and cooling compartments were regularly disposed 
so that the cooling compartments were 0.2 compartment per unit number of 
the feed liquor compartment (or the recovery compartment). 
On the other hand, cooling water at a temperature of 25.degree. C. was 
supplied from below to the cooling compartments at a rate of 3,500 l/hr. 
As a result, 6.95N of an acid mixture of sulfuric acid and hydrochloric 
acid was obtained as recovered acids at a rate of 605 l/hr, and the 
recovery rate reached 75%. The temperature of the dialytic cell at that 
time was 27.degree. C. at the upper portion of the recovery compartment 
and 30.degree. C. at the center portion. A continuous operation was 
conducted for about six months, whereupon the dialytic performance was 
stable, and the dialytic cell was disassembled to inspect the ion exchange 
membranes, whereby no abnormality was observed. 
COMATIVE EXAMPLE 1 
Diffusion dialysis was conducted under the same condition as in Example 1 
except that the supply of cooling water to the cooling compartments was 
stopped, whereby at the initial stage of the operation, 7.0N of an acid 
mixture of sulfuric acid and hydrochloric acid was obtained as recovered 
acids at a rate of 610 l/hr. The recovery rate was 77%, and the separation 
rate of Al was 96%. 
However, when the operation was continued, the performance decreased 
gradually. Upon expiration of about 3 months, the separation rate of Al 
decreased to a level of 81%, although the acid recovery rate was 77%. At 
that time, the temperature of the dialytic cell was at a level of from 
25.degree. to 26.degree. C. at the upper and lower portions, but as high 
as 65.degree. C. at the central portion, thus clearly indicating the heat 
accumulation. The ion exchange membranes were inspected, whereby a 
deterioration of the resin of the membranes was observed in a strip shape 
with a width of about 20 cm at about the 1/3 portion from the top of the 
dialytic cell, and this was found to be the cause for the decrease of the 
Al separation rate. 
EXAMPLE 2 
A titanium sulfate waste liquor (H.sub.2 SO.sub.4 : 291 g/l, Ti: 5 g/l) 
from the process for the production of titanium oxide by a sulfuric acid 
method, was introduced from below to the feed liquor compartments of the 
diffusion dialytic cell as shown in FIG. 2 at a flow rate of 1,162 l/hr, 
while water was introduced from above to the recovery compartments of the 
diffusion dialytic cell at a flow rate of 960 l/hr. 
In the diffusion dialytic cell, 880 sheets of anion exchange membrane (a 
strongly basic styrenedivinylbenzene copolymer) and 352 sheets of 
polyvinylchloride (water absorptivity: less than 0.1%, thickness: 200 
.mu.m) were incorporated, and cooling compartments were regularly disposed 
so that the cooling compartments were 0.4 compartment per unit number of 
the feed liquor compartment (or the recovery compartment). 
On the other hand, cooling water at a temperature of 25.degree. C. was 
supplied from below to the cooling compartments at a rate of 3,500 l/hr. 
As a result, 247 g/l of sulfuric acid was obtained as a recovered acid at a 
rate of 882 l/hr, and the recovery rate reached 71%. The temperature of 
the dialytic cell at that time was 25.degree. C. at the upper portion of 
the recovery compartment and 28.degree. C. at the center portion. A 
continuous operation was conducted for about one month, whereupon the 
dialytic performance was stable, and the dialytic cell was disassembled to 
inspect the ion exchange membranes, whereby no abnormality was observed. 
COMATIVE EXAMPLE 2 
Diffusion dialysis was conducted under the same condition as in Example 2 
except that the supply of cooling water to the cooling compartments was 
stopped, whereby at the initial stage of the operation 276 g/l of sulfuric 
acid was obtained as the recovered acid at a rate of 882 l/hr, and the 
recovery rate reached 72%. 
However, when the operation was continued, the performance decreased 
gradually. Upon expiration of about one month, the acid concentration 
decreased to a level of 230 g/l, and the recovery rate also decreased to a 
level of 60%. At that time, the temperature of the dialytic cell was at a 
level of from 25.degree. to 26.degree. C. at the upper and lower portions, 
but as high as 40.degree. C. at the central portion, thus clearly 
indicating the heat accumulation. The ion exchange membranes were 
inspected, whereby precipitation of titanium dioxide was observed in a 
strip shape with a width of about 20 cm at the 1/3 portion of the upper 
membrane portion on the side facing the feed liquor compartment, and this 
precipitation was found to be the cause for the deterioration of the 
performance.