Treatment for reducing impurities in aqueous liquor

The invention relates to a method of treating an aqueous liquor, such as effluent liquor formed during coal gasification. The method comprises subjecting the liquor to dephenolation and ammonia stripping treatment to remove phenolic compounds and "free" ammonia from the liquor and then subjecting the resulting liquor, which still contains ammonium compounds and thus "fixed" ammonia, to reverse osmosis treatment to produce a permeate which is substantially free from impurities, including "fixed" ammonia.

The present invention relates to the treatment of aqueous liquors, such as 
effluent liquors formed during coal gasification processes. 
In known coal gasification processes for producing hydrogen and methane 
containing product gases, hot crude gas from the gasifier may also contain 
tar, entrained solids, steam, phenols and chlorine- nitrogen- and sulphur- 
containing compounds. The crude gas is cooled to produce a condensed gas 
phase comprising, on the one hand, hydrocarbons, basically tars and oil 
and, on the other hand, aqueous liquor containing most of the other 
impurities. Before such aqueous liquor can be discharged to a waterway or 
reused within the gasification plant it has to be treated or purified. 
A known treatment route previously used by the present Applicants basically 
comprises the following stages: 
Stage 1 -- subjecting the aqueous liquor (after separation from the 
condensed hydrocarbons) to solvent extraction treatment to remove phenolic 
compounds therefrom, for example by known liquid/liquid extraction 
methods; 
Stage 2 -- removing from the dephenolated liquor, `free` ammonia and 
dissolved gases such as hydrogen sulphide and carbon dioxide by steam 
`stripping`; 
Stage 3 -- adding alkali to the liquor resulting from stage 2 to convert 1 
fixed ` ammonia (present principally as ammonium chloride) to `free` 
ammonia and removing the latter by steam `stripping`; 
Stage 4 -- biologically treating the liquor resulting from stage 3 to 
oxidise and thereby degrade remaining organic compounds; and 
Stage 5 -- using activated carbon to adsorb from the liquor resulting from 
stage 4 residual compounds, mainly organics, and thereby substantially 
remove the remaining pollutants in the liquor. 
Applicants found that the resulting upgraded aqueous liquid effluent 
produced by the above treatment route could still contain sufficient 
amounts of chlorides which would have to be removed in one or more 
additional stages before the upgraded effluent could be reused as, say, 
cooling water make-up, boiler feed water or released to an inland 
waterway. Where necessary such chloride content of the liquor could be 
substantially reduced by various further methods, such as, evaporation, 
use of multi-stage flash desalination equipment or mechanical vapour 
recompression. Such methods may be energy intensive and may add greatly to 
the cost of the treatment of the aqueous liquor. 
Applicants have now investigated the use of reverse osmosis to replacing 
one or more stages in the above treatment route. 
Applicants investigations, surprisingly, showed that reverse osmosis 
treatment could satisfactorily replace above-mentioned stages 3, 4 and 5 
(and any further chloride-removal stage). Thus, whilst `free` ammonia, 
that is dissolved ammonia per se, is removed from the liquor in stage 2, 
the liquor which is subjected to reverse osmosis still contains `fixed` 
ammonia, that is ammonia forming part of an ammonium compound, such as 
ammonium chloride. 
The ease by which a separation may be carried out by reverse osmosis is 
dependent partly upon the natural osmotic pressure of the liquor being 
treated and the applied pressure in the reverse osmosis apparatus. The 
natural osmotic pressure of the liquor is dependent on the dissolved 
material content of the liquor and it was considered that the earlier the 
liquor was extracted from the known treatment route described earlier, the 
higher the dissolved solid content and therefore the higher the osmostic 
pressure would be, which would tend to disfavour the reverse osmosis 
process. With the knowledge in mind, the Applicants unexpectedly found 
that the quality of purity of the permeate from the reverse osmosis 
treatment when the latter replaced stages 3, 4 and 5 (and any additional 
stage) was substantially the same or very similar to that found when the 
same reverse osmosis treatment process replaced stage 5 or stages 4 and 5 
(and any additional stage). 
Applicants believe that there are various reasons or factors which 
contribute to the unexpected result. In the applicants novel process, 
alkali is not added to the aqueous liquor, as in stage 3 of the known 
treatment route where such addition of alkali increases the dissolved 
solid content of the liquor to disfavour the reverse osmosis process. 
Thus, the absence of the addition of alkali and avoidance of increased 
dissolved solid content favours the reverse osmosis process. Also, after 
`free` ammonia stripping, the `fixed` ammonia is present in salt form, 
such as ammonium chloride, which has a higher molecular weight compared to 
free ammonia which can be present to some extent in the biologically 
oxidised liquor or the dephenolated and ammonia (free and fixed) stripped 
liquor of the known treatment route. The presence of ammonia in the higher 
molecular weight salt form has been found to favour the rejection of the 
salt bound ammonia by the reverse osmosis membrane whilst it has been 
found that lower molecular weight `free` ammonia tends to pass more 
readily or straight through a semi-permeable membrane. Moreover, after the 
`free` ammonia stripping process the natural process condition of the 
liquor has a relatively low pH, for example pH3 or pH4, and is relatively 
hot, for example about 60.degree. C., and both these conditions have been 
found to favour the reverse osmosis process, that is to favour the 
rejection of impurities and favour permeate flux or flow through the 
semi-permeable membrane. 
According to the invention a method of treating an aqueous liquor 
containing impurities comprising phenolic compounds, ammonia, ammonium 
compounds, dissolved gases and residual organic materials, comprises 
subjecting the liquor to dephenolation and ammonia stripping treatment and 
thereafter subjecting the liquor resulting from the dephenolation and 
ammonia stripping treatment (such resulting liquor still containing 
ammonium compounds) to reverse osmosis treatment to produce a)- a permeate 
which is substantially free from, or contains substantially reduced 
amounts of, the impurities and b)- a concentrate containing the remaining 
amounts of the impurities. 
The dephenolation and ammonia stripping treatment may comprise initially 
dephenolating the liquor and then subjecting the dephenolated liquor to 
the ammonia stripping treatment. However, much of the phenols can be 
removed by stripping with steam, so an alternative arrangement could be a 
single steam stripping stage to remove or substantially reduce the 
dissolved gases and to remove some of the phenols, the remaining phenols 
being removed in the reverse osmosis stage. 
The aqueous liquor containing the impurities may comprise aqueous effluent 
separated from gas condensate resulting or obtained from the cooling of 
crude product gas produced from a carbonaceous material, such as a coal 
gasification process. 
Conveniently, the aqueous liquor or gas condensate is filtered prior to the 
reverse osmosis treatment. 
When subjected to the reverse osmosis treatment, the aqueous liquor is 
preferably at a temperature in the range 30.degree. C. to 70.degree. C., 
and more preferably in the range 40.degree. C. to 60.degree. C. 
Preferably, when the aqueous liquor is subject to the reverse osmosis 
treatment it has a pH of between 3 and 7, and more preferably between 4 
and 6. 
The concentrate resulting from the reverse osmosis treatment may be 
disposed of by a) direct release to the environment, b) release after 
evaporation and crystallisation or c) incinerated. The solid residue from 
b) may also be regarded as a by-product finding use for say road de-icing.

EXAMPLE 
Crude gas produced by a coal gasification process using Pittsburgh 8 coal 
carried out in a slagging gasifier, is cooled to produce a gas liquor 
condensate comprising hydrocarbons and an aqueous liquor. The gas liquor 
is filtered to remove solids and the hydrocarbons and aqueous liquor 
fractions are then separated. The aqueous liquor is dephenolated using a 
ketone solvent (methyl iso butyl ketone). The dephenolated liquor is steam 
stripped of free ammonia, hydrogen sulphide and other easily removed 
dissolved gases, e.g. carbon dioxide. The resulting dephenolated and 
stripped liquor (still containing `fixed` ammonia) is filtered and then 
subjected to reverse osmosis in a reverse osmosis unit. 
In an alternative procedure, the dissolved gas stripping and partial 
dephenolation may be effected in a single stage by stripping the liquor 
with steam. Dephenolation treatment is then completed by the reverse 
osmosis stage. 
The unit used in the example was a tubular type manufactured by Paterson 
Candy International Ltd. with a ZF99 polyamide type semi-permeable 
membrane. 
The reverse osmosis treatment produced a permeate which was substantially 
free from or containing substantially reduced amounts of the original 
impurities and which premeate was suitable for direct (i.e. without 
further treatment) discharge into an inland waterway or for reuse as an 
aqueous source in the gasification process. A concentrate of impurities 
was also produced by the reverse osmosis and this was subjected to 
incineration treatment. 
The dephenolated, free ammonia stripped liquor subjected to the reverse 
osmosis process was at a temperature of about 40.degree. C. and had a pH 
of about 5.7. 
The volume concentration factor was about 5, whilst the flux at this 
concentration factor was 38l/m.sup.2.hr. 
The applied pressure on the liquor in the reverse osmosis unit was 60 barg. 
The concentrations of components or impurities in the raw liquor (i.e. the 
aqueous liquor as separated from the `tar` condensate), the dephenolated 
free ammonia stripped liquor and the permeate from the reverse osmosis 
unit are shown in Table 1. 
TABLE 1 
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CONCENTRATION IN LIQUOR (mg/l) 
Dephenolated Permeate 
Free Ammonia From RO 
COMPONENT Raw Liquor Stripped Unit 
______________________________________ 
Ammonia 7687 1510 79 
Chloride 1079 1079 98.5 
Phenol 5392 -- -- 
Sulphate 287 52 0.5 
Thiocyanate 
823 1059 83.0 
Carbon (TOC) 
5500 1174 150 
Chemical 19776 3960 272 
Oxygen Demand 
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As can be appreciated from Table 1, the concentration of ammonia in the 
crude gas liquor is reduced to approx. 20% by the dephenolating and free 
ammonia stripping processes and to approx. 1% in the permeate resulting 
from the reverse osmosis treatment. It can also be seen how effective the 
reverse osmosis treatment is in lowering the concentration of chloride 
which is reduced to about 9% of the chloride concentration in the crude 
liquor. The effectiveness of the reverse osmosis process in removing 
sulphate is also clearly apparent. 
The replacement of the final three stages (and any subsequent stage) in the 
known treatment route by the reverse osmosis treatment step can have 
important financial and technical implications. Thus, the total operating 
and capital costs may be reduced, whilst the biological treatment stage, 
which is believed to be the least guaranteeable or reliable stage of the 
earlier treatment route, has been completely eliminated.