Process for recovering thiocyanate

A safe, energy-Saving process for recovering (or purifying) thiocyanate from a large amount of waste liquid (or thiocyanate-containing aqueous solution) by the aid of a reverse osmosis membrane whose sodium chloride rejection is from 10% to 70%. The process performs recovery without decreasing the concentration of thiocyanate in feed.

The present invention relates to a process for recovering thiocyanate from 
a thiocyanate-containing aqueous solution by filtration through a reverse 
osmosis membrane. 
Waste liquid arising from desulfurization of coke oven gas contains 
ammonium thiocyanate, ammonium thiosulfate, ammonium sulfate, colored 
components (such as picric acid), tar, and solids. It is an inevitable 
by-product in coke production, and it cannot be discarded as such. 
Therefore, it needs adequate treatment for disposal, which involves the 
separation and recovery of thiocyanate as a useful material. 
There are several known processes of separating and recovering ammonium 
thiocyanate from waste liquid arising from desulfurization of coke oven 
gas. They include the one which utilizes the difference of solubility 
(Japanese Patent Laid-open Nos. 7825/1982, 25699/1973, and 17421/1982), 
the one which utilizes an organic polar solvent which selectively extracts 
ammonium thiocyanate (Japanese Patent Laid-open No. 26699/1973), and the 
one which re-sorts to distillation under reduced pressure (Japanese Patent 
Laid-open Nos. 75489/1974 and 58000/1975). 
Also, there is a known process for separating and recovering thiocyanate 
from a thiocyanate-containing aqueous solution. It utilizes gel filtration 
by the aid of a polymeric gel for separation (Japanese Patent Laid-open 
Nos. 106494/1974, 38695/1975, and 139600/1976). 
According to the process disclosed in Japanese Patent Laid-open No. 
26699/1973, waste liquid arising from treatment of coal carbonization gas 
is discolored by activated carbon, with or without pretreatment for the 
removal of precipitates by stirring with hot air and then evaporated to 
dryness, and the residues are extracted with an organic polar solvent 
which selectively dissolves ammonium thiocyanate. Example 1 in it 
indicates that precipitates to be removed by stirring with hot air are 
mostly sulfur. 
According to the process disclosed in Japanese Patent Laid-open Nos. 
7625/1982 and 17421/1982, waste liquid arising from desulfurization of 
coke oven gas is discolored and then oxidized by blowing oxygen at lower 
than 200.degree. C. while keeping the waste liquid slightly acidic or 
alkaline. It is claimed that the process yields a solution in which 
substantially all of ammonium thiosulfate is converted into ammonium 
sulfate and ammonium thiocyanate remains intact. It is also claimed that 
the solubility of ammonium sulfate dissolves in the mixed solution of 
ammonium sulfate and ammonium thiocyanate is by far lower than that in 
water but the solubility of ammonium thiocyanate in the mixed solution 
does not greatly differ from that in water, and hence it is possible to 
separate ammonium sulfate by crystallization and to recover ammonium 
thiocyanate from the filtrate. 
The separation of thiocyanate by gel filtration varies from one process to 
another. The one disclosed in Japanese Patent Laid-open No. 106494/1974 
consists of supplying an impure aqueous solution of thiocyanate to a layer 
of crosslinked dextran, thereby causing it to capture impurities and 
thiocyanate, and eluting impurities and then thiocyanate. 
The process disclosed in Japanese Patent Laid-open No. 38695/1975 employs 
crosslinked dextran to separate thiocyanate from its aqueous solution, 
like the aforesaid process, but it differs in that the first eluate is a 
solution containing a salt of oxygen acid of inorganic sulfur and the 
second eluate (before the elution of a colored solution) is a 
thiocyanate-containing solution. The process also involves treatment with 
activated carbon prior to gel filtration. 
The process disclosed in Japanese Patent Laid-open No. 139600/1976 employs 
a crosslinked polymer of acrylamide or a derivative thereof for gel 
filtration. 
The above-mentioned conventional processes suffer their respective 
disadvantages. The process that utilizes the solubility difference needs a 
large amount of energy for cooling. The process that resorts to solvent 
extraction also needs a large amount of energy for solvent distillation. 
The process that resorts to distillation under reduced pressure needs a 
large amount energy and gives off toxic thiocyanate gas, although it 
yields a pure thiocyanate. The process that employs gel filtration needs a 
step for the concentration of eluate, although it yields a pure 
thiocyanate. 
BRIEF SUMMARY OF THE INVENTION 
The present invention was completed to address the above-mentioned 
problems. It is an object of the present invention to provide a process 
for recovering thiocyanate in a safe manner with a less amount of energy 
from a large amount of waste liquid.

DETAILED DESCRIPTION 
The present invention is based on the finding that thiocyanate permeates 
through a membrane greatly differently from coexisting impurities and 
hence it is possible to selectively recover a thiocyanate by the aid of a 
reverse osmosis membrane if the membrane, waste liquid, and operating 
conditions are properly selected. 
The present invention is embodied in a process for recovering thiocyanate 
from a thiocyanate-containing aqueous solution, said process comprising 
filtering the solution through a polymeric reverse osmosis membrane whose 
sodium chloride rejection is from 10% to 70%. 
A detailed description of the invention follows. 
Typical examples of the thiocyanate-containing aqueous solution include 
waste liquid arising from desulfurization of coke oven gas and its 
concentrate. It takes on a dark red color and contains ammonium 
thiocyanate, ammonium thiosulfate, ammonium sulfate, colored components 
(such as picric acid), tar, and solids. Their content greatly varies as 
shown below. 
Ammonium thiocyanate 20-30 wt. % 
Ammonium thiosulfate 5-25 wt. % 
Ammonium sulfate 3-10 wt. % 
Colored components 0.01-1 wt. % 
Tar 0.01-1 wt. % 
Solids 2-10 wt. % 
Water 40-60 wt. % 
Some manufacturing process may yield sodium salts in place of ammonium 
salts. 
Needless to say, the process of the present invention may be applied not 
only to waste liquid arising from desulfurization of coke oven gas but 
also to any thiocyanate-containing aqueous solution. An example of the 
latter is an aqueous solution recovered from the solvent used for the 
spinning of acrylonitrile polymer. 
The polymeric reverse osmosis membrane used in the present invention should 
be one which whose sodium chloride rejection is from 10% to 70%, 
preferably from 30% to 60%. With lower than 10%, the membrane permits 
ammonium thiosulfate and ammonium sulfate and low-molecular-weight colored 
components to permeate, with the result that the recovered thiocyanate has 
a low purity. With higher than 70%, the membrane needs a high pressure if 
a large amount of liquid is to be filtered. Moreover, it prevents the 
passage of impurities as well as thiocyanate (to be recovered), with the 
result that the filtrate has a low concentration of thiocyanate. There are 
no restrictions on the material of the reverse osmosis membrane so long as 
the above-mentioned requirements are satisfied. Examples of the material 
include polyolefin, polysulfone, polyamide, and acetylcellulose. 
Incidentally, the sodium chloride rejection of the reverse osmosis membrane 
is measured under the condition that a 0.2% aqueous solution of sodium 
chloride permeates under a pressure of 10 kg/cm.sup.2 until 30% of it is 
recovered. 
The operation may be carried out under the conditions recommended by the 
manufacturer for the particular reverse osmosis membrane. In practical 
operation, the pressure and flow rate should preferably be 2-20 
kg/cm.sup.2 and 2-60 L/m.sup.2 -hr, respectively. Although waste liquid 
needs no pretreatment, its temperature and pH should be properly adjusted 
to the reverse osmosis membrane. Further, when the waste liquid contains 
foreign matter it is sometimes necessary to remove the same by filtration. 
The permeability of ions through a polymer membrane is determined by the 
chemical potential of the particular ions in structureless water (having 
its structure disordered by the membrane material). The lower the chemical 
potential, the higher the permeability. The order of chemical potential 
coincides with the lyotropic series, and thiocyanate ions are at the 
lowest. Therefore, thiocyanate is stabler than other salts in 
structureless water. This is the reason why thiocyanate can be separated 
from less permeable impurities and recovered without appreciable decrease 
in concentration in the filtrate. 
The invention will be more clearly understood with reference to the 
following examples; however, they are intended to illustrate the invention 
and are not to be construed to limit the scope of the invention. 
EXAMPLE 1 
Recovery of ammonium thiocyanate was carried out according to the flowsheet 
shown in FIG. 1, in which there are shown a feed tank A, a feed pump B, 
and a reverse osmosis membrane module C. Feed is admitted through 1, 
concentrated solution is discharged from 2, and filtrate (product) is 
discharged from 3. 
Feed is waste liquid arising from desulfurization coke oven gas which has 
been filtered through a 5-.mu.m cartridge filter. It contains 23.2% 
ammonium thiocyanate, 7.0% ammonium thiosulfate, and 3.6% ammonium 
sulfate, and has pH 7.3. The reverse osmosis membrane module is NTR-7250 
made by Nitto Denko Corporation (equipped with a polyamide membrane whose 
sodium chloride rejection is 60%). Filtration is performed under a 
pressure of 15 kg/cm.sup.2. The resulting filtrate was found to contain 
23.1% ammonium thiocyanate, 0.35% ammonium thiosulfate, and 0.18% ammonium 
sulfate. Thus, thiocyanate was separated from impurities and recovered 
with almost no decrease in concentration. 
EXAMPLE 2 
The same procedure as in Example 1 was repeated except that NTR-7250 was 
replaced by NTR-1550 made by Nitto Denko (equipped with a cellulose 
acetate membrane whose sodium chloride rejection is 50%) and the operating 
pressure was changed to 20 kg/cm.sup.2. The resulting filtrate was found 
to contain 23.0% ammonium thiocyanate, 0.30% ammonium thiosulfate, and 
0.10% ammonium sulfate. This result proves the effective recovery of 
thiocyanate. 
EXAMPLE 3 
The same procedure as in Example 1 was repeated except that NTR-7250 was 
replaced by NTR-7410 made by Nitto Denko (equipped with a polyamide 
membrane whose sodium chloride rejection is 10%) and the operating 
pressure was changed to 10 kg/cm.sup.2. The resulting filtrate was found 
to contain 23.2% ammonium thiocyanate, 3.6% ammonium thiosulfate, and 2.0% 
ammonium sulfate. This result proves the recovery of thiocyanate without 
decrease in concentration, although the concentrations of impurities in 
the filtrate do not decrease as desired. 
EXAMPLE 4 
The same procedure as in Example 1 was repeated except that the feed was 
replaced by an aqueous solution recovered in the spinning of acrylic 
fiber. It contains 17.9% sodium thiocyanate, 0.3% sodium sulfate, 0.3% 
sodium chloride, and 0.4% sodium nitrate. The resulting filtrate was found 
to contain 17.9% sodium thiocyanate, 0.1% sodium chloride, 0.2% sodium 
nitrate, and none of sodium sulfate. This result proves the effective 
recovery of thiocyanate without decrease in concentration. 
COMATIVE EXAMPLE 1 
The same procedure as in Example 1 was repeated except that NTR-7250 was 
replaced by NTR-1698 made by Nitto Denko (equipped with an acetyl 
cellulose membrane whose sodium chloride rejection is 98%) and the 
operating pressure was changed to 35 kg/cm.sup.2. No filtrate was 
obtained. 
COMATIVE EXAMPLE 2 
The same procedure as in Example 1 was repeated except that NTR-7250 was 
replaced by SC-L100R made by Toray Industries Inc. (equipped with an 
acetyl cellulose membrane whose sodium chloride rejection is 85%). It was 
found that the concentration of ammonium thiocyanate in the filtrate was 
10.2%. This concentration is too low for the recovery process to be 
practicable. 
COMATIVE EXAMPLE 3 
The same procedure as in Example 4 was repeated except that NTR-7250 was 
replaced by NTU-3508 made by Nitto Denko (equipped with a polysulfone 
membrane whose sodium chloride rejection is 7% and whose fractionation 
molecular weight is 8000) and the operating pressure was changed to 2 
kg/cm.sup.2. The filtrate was found to contain sodium thiocyanate, sodium 
sulfate, sodium chloride, and sodium nitrate in the same concentrations as 
in the feed. In other words, the recovery of thiocyanate did not take 
place at all. 
As mentioned above, the process of the present invention employs a specific 
reverse osmosis membrane for the selective recovery of thiocyanate from a 
thiocyanate-containing aqueous solution (as feed). Therefore, it permits 
the desired thiocyanate to be selectively recovered without decrease in 
concentration. This eliminates the necessity of concentrating the 
filtrate, which leads to energy saving. In addition, it gives rise to no 
toxic gas detrimental to environment and health. It is easy to practice on 
an industrial scale for the recovery of thiocyanate from waste liquid 
arising from desulfurization of coke oven gas and from the solution for 
spinning of acrylic fiber. By virtue of these advantages, the present 
invention is of great industrial importance. 
FIG. 1 is a flowsheet of one embodiment of the present invention. Feed (1) 
is admitted to the feed tank (A) and then introduced under pressure to the 
reverse osmosis membrane module (C) by means of the feed pump (B). The 
concentrated liquid (2) and filtrate (3) are discharged for further 
processing.