Abstract:
A method and apparatus are provided for depleting sodium thiocyanate (NaSCN) from washing solutions to obtain pure sodium thiocyanate. By adding an anti-solvent such as ethanol, which acts as anti-solvent for the main part of the accompanying salts in the solution, the major part of these accompanying salts can be separated by precipitation in a crystallization and removed, for example, by filtration, whereas the NaSCN remains in the solution. After separating the anti-solvent from the remaining aqueous solution in a rectification, a very pure crystallized NaSCN is produced in a NaSCN crystallization. After washing in a counterstream washing apparatus, the obtained crystallized NaSCN product has a purity of at least 99%.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to a method for depleting sodium thiocyanate (NaSCN), as well as an apparatus for carrying out this method, and a crystallized NaSCN obtained by this method.  
       BACKGROUND OF THE INVENTION  
       [0002]     When coal is hydrogenated to produce fuel, the resulting product usually contains a certain amount of sulphur that has to be removed. This can be done with a washing process in which a washing liquor is circulated. For example, the so-called Sulfolin process of Linde and the Stretford process provide suitable means for this. However, salts accumulate in the alkaline washing liquor. These are mainly sodium sulphate and sodium thiocyanate from the process reactions. Whereas a method for depleting sodium sulphate in the circulating solution has long been known and is applied relatively free of problems, greater difficulties for removing the NaSCN accumulating considerably more slowly are encountered. To avoid further enrichment and to comply with a maximum specified concentration, the possibility of discarding a part of the circulating solution is employed to a considerable extent. However, this is extremely undesirable for reasons of environmental protection.  
         [0003]     The U.S. Pat. No. 3,965,243 discloses a method for obtaining NaSCN from the circulating washing liquor of a Stetford process for removing hydrogen sulphide. For this purpose a part stream of contaminated washing liquor is fed into an extraction stage in which an aliphatic solvent containing 3 to 8 carbon atoms is added to the washing liquor as extraction agent. Preferentially n-butanol (C 4 H 9 OH) is utilised as solvent that has only limited miscibility (about 20%) with water. The extraction of the NaSCN is performed, for example, in a continuous counterstream extraction with four theoretical stages, whereby preferentially 1.5 parts by volume of n-butanol and 1 part by volume of washing liquor are used. In the extraction stage the solvent removes a considerable fraction of the NaSCN and other contaminating substances out of the washing liquor, which is then subjected to a steam stripping process to remove the residual solvent.  
         [0004]     The extract drawn out of the counterstream extraction contains mainly the solvent and a fraction of the water, as well as in particular the main fraction of the NaSCN. To separate the solvent from the aqueous phase, the extract is treated in a distillation apparatus where the n-butanol is driven out as low boiling azeotropic mixture with 42.5% of water. The condensate formed therefrom is decanted and the resulting butanol-rich fraction (80% n-butanol) is returned to the counterstream extraction as solvent, whereas the water-rich fraction (approx. 6% n-butanol) is returned to the distillation. A practically solvent-free aqueous solution of NaSCN can be drawn from the bottom of the distillation apparatus. From this a crude salt containing about 88% NaSCN is then obtained by evaporation concentration, filtration, decolorisation with active charcoal and evaporation to dryness. Pure white NaSCN is produced as marketable salt product by dissolving the crude salt in pure n-butanol, filtering the solution and then re-crystallizing. Utilisation of alternative solvents such as 2-propanol that are well miscible with water is considered to be technically feasible but not economically justified.  
         [0005]     The EP 0074278 A1 discloses a method for recovering anthraquinone disulfonate (ADA) and vanadate from the discarded fraction of a circulating liquor of a Stretford process for gas scrubbing. Thereby the solution is brought into contact with active charcoal for separating the anthraquinone disulfonate. The vanadate is separated by contact with an anion exchanger resin. The anthraquinone disulfonate and the vanadate are thereafter washed out of the active charcoal or out of the anion exchanger resin, respectively, with an alkaline aqueous solution that is preferably heated to 25-100° C., and returned into the circulation.  
         [0006]     The task of the present invention is to specify a method for depleting NaSCN out of circulating liquors, containing further accompanying salts, of a washing process for removing sulphur, that gives highest possible yield of a crystallized NaSCN as marketable pure product with least possible plant and operating expenditure; thereby the important chemicals for the washing process to remove sulphur, in particular the substances anthraquinone disulfonate (ADA) and sodium vanadate that act as catalysts, are to be recovered to the greatest possible extent. Furthermore, a plant for carrying out this method is to be specified.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a method for depleting NaSCN out of circulating liquors, containing further accompanying salts, of a washing process for removing sulphur, that gives highest possible yield of a crystallized NaSCN as marketable pure product with least possible plant and operating expenditure; thereby the important chemicals for the washing process to remove sulphur, such as the substances anthraquinone disulfonate (ADA) and sodium vanadate that act as catalysts, are to be recovered to the greatest possible extent. Furthermore, an apparatus for carrying out this method is to be specified.  
         [0008]     A Sulfolin washing liquor after sulphate recovery typically has the following composition, where the maximum concentrations should not be exceeded:  
                                                                                 Component   Unit   min   max                                        Na 2 CO 3  + NaHCO 3     g/l   18   43           Na 2 SO 4     g/l   40   70           NaSCN   g/l   80   200           NaVO 3     g/l V   1.0   1.8           ADA (sodium salt of   g/l   0.2   0.5           anthraquinone sulphonic acid)           pH value       7.7   8.8                      
 
         [0009]     Sodium carbonate is the utilised absorption agent for desulphurisation and is consumed in the washing process. Instead of Na 2 CO 3  usually NaOH is fed to the washing solution; the NaOH reacts with CO 2  resulting in Na 2 CO 3  and partly NaHCO 3 . NaVO 3  and ADA are catalysts, Na 2 SO 4  and NaSCN are reaction products that can accumulate in the washing liquor. The Na 2 SO 4  is usually removed from the liquor by cooling crystallization of Glauber&#39;s salt, so that its concentration in the liquor is limited by this means to the maximum value or less.  
         [0010]     Surprisingly it has now been found that by adding certain alcohols to the washing liquor, e.g. to a Sulfolin solution or to the mother liquor of a Glauber&#39;s salt crystallization of the washing liquor, a very strong depletion of all salts except for the NaSCN can be achieved by drowning-out crystallization. These certain alcohols are those with at most 2 carbon atoms and have an anti-solvent effect on the major fraction of the salts that accompany the NaSCN in the washing liquor. Unlike the aliphatic alcohols (C3-C8) utilised in the process known from U.S. Pat. No. 3,965,243 these certain alcohols are completely miscible with the aqueous solution; the liquid phase is completely homogeneous. The miscibility to only some extent is essential to the known process in order to separate the NaSCN dissolved in the separate liquid organic phase, having at least 3 carbon atoms, from an aqueous phase by decanting the organic phase. In contrast to the known process all salts of the washing liquor, except for the NaSCN that has good solubility in the anti-solvent, have so small saturation concentrations in the anti-solvent/water mixture according to the invention, that a NaSCN solution only slightly contaminated with the accompanying salts is obtained. Only the ADA is an exception, in that a certain fraction thereof is not precipitated in the drowning-out crystallization.  
         [0011]     It was furthermore found that through a preliminary concentration by evaporation crystallization with only subsequent addition of the anti-solvent to the obtained suspension, the ratio of the NaSCN to the other inorganic salts can be further improved to a considerable extent without resulting co-crystallization of NaSCN.  
         [0012]     The suspension obtained in this manner is partitioned by known separating steps into crystallized mass and solution, whereby the crystallized mass can be returned to the washing process for removing sulphur. The NaSCN solution contains such a small concentration of the accompanying salts that in a subsequent NaSCN crystallization the NaSCN crystallizes selectively and thus can be obtained directly in a specifically pure form without requiring a re-crystallization. Before the NaSCN crystallization the anti-solvent can first of all be removed again from the solution and returned to the drowning-out crystallization. For this purpose the recovery of an anti-solvent/water mixture is fully sufficient.  
         [0013]     Suitable anti-solvents include methanol and ethanol.  
         [0014]     These and other objects, advantages, purposes and features of the present invention will become apparent upon review of the following specification in conjunction with the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a chart of the solubility of Na 2 SO 4  in various solvents;  
         [0016]      FIG. 2  is a chart of the solubility of NaSCN in various solvents;  
         [0017]      FIG. 3  is a chart of the percentage of purity of NaSCN, depending on the concentration of ethanol;  
         [0018]      FIG. 4  is a process schematic of a method according to the present invention;  
         [0019]      FIG. 5  is a schematic of an apparatus according to the present invention;  
         [0020]      FIG. 6  is a mass balance of an embodiment of the present invention; and  
         [0021]      FIG. 7  is a mass balance of a further embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     Referring now to the drawings and the embodiments illustrated therein, from  FIG. 1  it is evident that Na 2 SO 4  has only a very small solubility in the specified alcohols. The same is true for Na 2 CO 3  and NaVO 3  (not depicted). In contrast thereto, as shown in  FIG. 2 , the solubility of NaSCN in these alcohols is much greater. This is especially true for ethanol (2 carbon atoms) that clearly shows the highest solubility values at a temperature above about 55° C. Methanol (1 carbon atom) shows the highest values at temperatures below about 50° C. The combination of the two diagrams characterises ethanol (EtOH) as a well suited anti-solvent within the scope of the present invention, because it is able to dissolve very little Na 2 SO 4  but a particularly large amount of NaSCN. In fact, the ADA also dissolves in ethanol, but it does not attain saturation in the mother liquor of the NaSCN crystallization when a small amount of the solution is purged to the crystallization.  
         [0023]      FIG. 3  shows that with a mass percentage concentration of ethanol of 60-70% in the mother liquor, the NaSCN fraction in the dissolved substances already asymptotically approaches the limit value of 95-97% NaSCN. This makes it possible to utilise an EtOH/water mixture as anti-solvent that lies significantly below the composition of the azeotropic mixture (approx. 95% EtOH). Compared therewith the azeotropic mixture of methanol and water contains about 60% of methanol. These properties of EtOH simplify the recovery of the EtOH in this procedure considerably, because an EtOH/water mixture containing, for example, 60-70% of EtOH can already be returned without any problems, so that it is not necessary to break an azeotropic mixture in the recovery procedure.  
         [0024]      FIG. 3  shows that with a mass percentage concentration of ethanol of 60-70% in the mother liquor, the NaSCN fraction in the dissolved substances already asymptotically approaches the limit value of 95-97% NaSCN. This makes it advantageously possible to utilise an EtOH/water mixture as anti-solvent that lies significantly below the composition of the azeotropic mixture (approx. 95% EtOH). Compared therewith the azeotropic mixture of methanol and water contains about 60% of methanol. These properties of EtOH simplify the recovery of the EtOH in this procedure considerably, because an EtOH/water mixture containing, for example, 60-70% of EtOH can already be returned without any problems, so that it is not necessary to break an azeotropic mixture in the recovery procedure.  
         [0025]     On the basis of this surprising interplay of the different solubilities it is possible to construct a process according to the present invention that is able to deplete NaSCN continuously in the washing process for removing sulphur, and at the same time to obtain NaSCN as industrial product with a purity that had not been achieved previously.  
         [0026]     The basic action sequence of the process according to the invention is shown in  FIG. 4  in the form of a schematic block diagram, commencing with the take over of a part or partial stream of washing liquor, for example from a (not depicted) Sulfolin process. The dot-dash line frames contain the respective three main functional blocks, namely:  
         [0027]     a. the separation of the product solution  
         [0028]     b. the recovery of the anti-solvent  
         [0029]     c. the product crystallization  
         [0030]     The separation of the product solution, that is the NaSCN solution, from the main fraction of the accompanying salts takes place by addition of anti-solvent in a crystallization that is preceded by a preliminary crystallization in which most of the water is removed from the solution. The anti-solvent recovery comprises stripping and rectification of the anti-solvent from the NaSCN solution (both achievable in a single process unit), whereby in the depicted case the anti-solvent is not separated in completely pure state, but instead as anti-solvent/water mixture containing x % of anti-solvent. The third main functional block is the product crystallization including a stage for increasing the concentration of the solution.  
         [0031]     The basic design of a plant for carrying out the process according to the invention is shown in  FIG. 5 . This depicts a continuously operating cleaning plant  2  (e.g. a Sulfolin plant) for removing sulphur from a material  1  containing sulphur (e.g. petrol) that leaves the cleaning plant  2  again as desulphurised material  3 . The washing circuit of the cleaning plant  2  is designated with 4. Via a purge line  5  a part stream of the alkaline washing liquor is continuously taken from the washing circuit  4  and fed to a Glauber&#39;s salt crystallization  6  in which the Na 2 SO 4  crystallizes and is taken out as Glauber&#39;s salt stream  7 . A part of the mother liquor of the Glauber&#39;s salt crystallization  6  goes via a line  8  into a preliminary crystallization  10 , whereas the other part of the Glauber&#39;s salt depleted mother liquor goes via a mother liquor return line  9  back into the washing circuit  4 .  
         [0032]     In the preliminary crystallization  10  that is operated at a temperature in the range of 60 to 130° C., optionally in the range of 80 to 100° C., most of the water fraction of the mother liquor is evaporated and removed through a water vapour exhaust line  11 . Downstream of the preliminary crystallization  10  follows directly a crystallization  12 , in which an anti-solvent/water mixture is added to the suspension obtained from the preliminary crystallization  10  coming through a line  24 . With EtOH the anti-solvent concentration in the mother liquor is appropriately held in the range from 40 to 90 mass %, optionally from 60 to 70 mass %, for example, at 65 mass %. The crystallization  12  may be implemented as cooling crystallization and may be carried out with an operating temperature in the range of 10 to 60° C., optionally in the range of 25 to 35° C. The anti-solvent precipitates the major fraction of the accompanying salts.  
         [0033]     The resulting suspension comprising NaSCN solution and a crystallized mass of the accompanying salts is fed through a suspension output line  13  into a separating device that is designed, for example, as a filter  14 . The filtrate of the filter  14  (NaSCN solution) is taken through a filtrate output line  21  to a first rectification  23 , that may be equipped with an indirect sump heater, for stripping the main fraction of the anti-solvent contained in the solution. The first rectification  23  can be operated such that an anti-solvent/water mixture can be drawn out of it, with a concentration of anti-solvent higher than given in the drowning-out crystallization  12 . This mixture can be returned to the crystallization  12  through an anti-solvent/water mixture line  24 .  
         [0034]     The NaSCN solution is almost completely stripped from the anti-solvent (for example, ethanol) when it leaves the rectification  23  and can be fed through a line  25  into a facility for increasing the concentration, optionally up to close to the saturation point. This may be accomplished with a falling film evaporator  26 . With a considerable fraction of water, the very small fraction of remaining anti-solvent in the concentrating device is driven out and drawn off via a vapour line  27 . It is possible as option to condense these vapours and feed the condensate completely or partly (depicted as broken line) into a condensate feed line  17  that leads to a dissolving unit  16 . The filter cake of the filter  14  is also brought via a filter cake feed  15  into this dissolving unit  16  and dissolves completely in the condensate. Of course, this dissolution could also be performed with other provided water. The resulting solution of accompanying salts, that also contains the ADA and NaVO 3  (the catalysts of the Sulfolin process) is taken, for recovering the anti-solvent as completely as possible, via a solution feed line  18  into a second rectification  19  that is optionally equipped like the first one with an indirect sump heater. Whereas the stripped solution of the accompanying salts, that is largely depleted of NaSCN, is returned from the second rectification  19  via a mother liquor return line  20  into the washing circuit  4 , the anti-solvent/water mixture obtained by the stripping in the second rectification  19  can be appropriately fed via an anti-solvent/water mixture line  22  into the first rectification  23 . By this means the return of the anti-solvent into the drowning-out crystallization  12  can be implemented with the desired concentration from a single plant unit.  
         [0035]     Via a NaSCN solution feed line  28  the concentrated NaSCN solution can be taken from the falling film evaporator  26  into the NaSCN crystallization  29  that can optionally be operated at a temperature range of 35 to 60°, for example, at 40° C. The mother liquor of the NaSCN crystallization  29  is returned via a mother liquor return line  30  into the drowning-out crystallization  12 , so that also the fractions of ADA and NaVO3 still contained come back into the drowning-out crystallization  12 . For an almost complete recovery and return of these valuable catalysts it is therefore of no significance that in the drowning-out crystallization  12  in each case only a fraction of these catalyst materials can be separated together with the other accompanying salts out of the mother liquor.  
         [0036]     In order to obtain also with regard to colour a crystallized NaSCN  40  that is as pure white as possible, the crystallized mass drawn out of the NaSCN crystallizer  29  through a crystallized mass draw-off line  31  is fed to a counterstream washer  32 , because usually the mother liquor is coloured. From the counterstream washer  32  the crystallized NaSCN is fed through a crystallized mass feed line  33  into a separating device that may be designed as centrifuge  34 . The crystallized mass freed from adhering washing liquor in the centrifuge is then taken through a crystallized mass feeder  35  into a dryer  36  and dried there. The resulting NaSCN product  40  has a purity of typically better than 99.0%, such as 99.5% or 99.9%.  
         [0037]     Saturated NaSCN solution is conveniently utilised as washing liquor for the counterstream washer  32 , and for this purpose a fraction of the washed crystallized NaSCN is dissolved in water in a salt dissolver  39 . In  FIG. 5  the water feed to the salt dissolver  39  is designated with 41 and the crystallized mass feeder is designated with 38. Of course, instead of the depicted feed-in of a fraction of the centrifuged crystallized mass, a purge of a fraction out of the crystallized mass line  23  to the centrifuge  34  could also be provided.  
         [0038]     It is advisable to feed the washing liquor separated in the centrifuge  34  through a drain line  36  also into the salt dissolver  39 , from which a washing liquor feed line  42  feeds the washing liquor into the counterstream washer  32 . The withdrawal of the washing liquor takes place through a washing liquor drain line  43  conveniently into the drowning-out crystallization  12 , so that practically no NaSCN is lost.  
         [0039]     The  FIGS. 6 and 7  show mass balances with regard to the main functional blocks of the process according to the invention, for two embodiment examples under different operating conditions. In  FIG. 6  the anti-solvent is recovered from the vapours of the NaSCN crystallization whereas  FIG. 7  is based on the process scheme of  FIG. 5 . In both cases EtOH was utilised as anti-solvent, and the purity of the obtained crystallized NaSCN was significantly better than 99.5%.  
         [0040]     Changes and modifications to the specifically described embodiments may be carried out without departing from the principles of the present invention, which is intended to be limited only by the scope of the appended claims as interpreted according to the principles of patent law including the doctrine of equivalents.