DESC:Field of the invention:
The present invention relates to a method for extracting syngenite [K2Ca(SO4)2H20] from alkali bypass dust coming out from cement kilns. The invention also relates to a method for extracting syngenite [K2Ca(SO4)2H20] and potassium chloride [KCl] sequentially from alkali bypass dust.

Background of the invention:
Separation of Potassium and Sodium halide especially KCl and NaCl from mixed salt solution has always been a tedious task. Both these metals belong to 1A group of periodic table and have nearly similar physical and chemical properties. Thus, it is difficult to separate these two salts by carrying out chemical or extraction technique. However, both these salts show different behavior in hot water. Potassium chloride has better solubility than sodium chloride in hot water.
Alkali bypass dust, produced in a kiln during the production of cement clinker, is a particulate mixture of partially calcined and unreacted raw feed, clinker dust and ash, enriched with alkali sulfates, halides and other volatiles. The chemical composition of the bypass dust varies depending on the raw feed, kiln, exhaust gases and the operating conditions. These particulates are captured by the exhaust gases and collected in particulate matter control devices such as cyclones, bag houses and electrostatic precipitators.
Every day, large quantities of bypass dust is produced by plants throughout the world. The dust is often not suitable for direct return to the cement-producing process as a feed owing to their high alkali and chloride content. Hence, bypass dust is often dumped in fills. The dumping of bypass dust, however, is undesirable as it not only poses environmental threats but also involves wasting commercially viable materials such as alkali metal salts.
Thus, various attempts have been made to re-utilize the alkali bypass dust. In particular, a lot of research has been undertaken to develop treatment methods for this bypass dust in order to extract commercially viable products therefrom. Some prior known processes are based on chemical methods which enable selective precipitation of desired salts whereas others are based on fractional crystallization. The most popular approach amongst such prior known processes is using an additional ingredient in the form of solid, liquid or gas to cause precipitation of impurities or products.
For instance, US 3925534 teaches dissolving the alkali metal chlorides from bypass dust in water and simultaneously carbonating the same to convert calcium hydroxide in solution to insoluble calcium carbonate. The slurry thus formed is treated in a second stage with additional water at an elevated temperature and sufficient time as to dissolve substantially all the remaining alkali metal sulphates, and simultaneously carbonating the same to convert calcium hydroxide in solution to the insoluble calcium carbonate, filtering the slurry from the second stage, and recovering potassium chloride and potassium sulphate from the filtrate.
US 4,031,184 relates to a process of reclaiming cement kiln dust and recovering the alkali content thereof which comprises leaching the cement kiln dust at elevated temperatures with an aqueous solution of potassium chloride, treating the leached slurry of cement kiln dust with a relatively small amount of oil and a fatty acid to flocculate, and preferably pelletize, the solids, in the aqueous phase, extracting the flocculated or pelletized dust from the aqueous phase, desirably lightly washing or rinsing the flocculated or pelletized material to still further reduce the alkali content, cooling the leaching solution to throw down or precipitate potassium chloride crystals by crystallization, and removing the crystallized material therefrom.
US 6,613,141 relates to a method and apparatus for recovering calcium sulfate di-hydrate (gypsum) from a precipitation reaction between cement kiln dust and sulfuric acid solution.
US 8,623,304, proposes treating of dust with SO2 gas to convert CaO and Ca(OH)2 to CaSO4.
US 8,721,785, describes the use of a fluidized bed reactor in the carbonation of a solid, inorganic and alkaline particulate material which contains alkaline metal salts.
US 3,647,395, relates to a process for recovering alkali metal salts wherein vapors of alkali metal salts contained in the gases emitted are condensed and then added to water with the dust from the flue gas. The aqueous solution obtained is separated from the insoluble particles. The latter are then subjected to a succession of dissolving and separating steps. The aqueous solution finally obtained is subjected to a crystallization of the soluble salts. However, this process, which requires a large number of dissolving and separating steps, is complex and does not make it possible to separately regenerate desired soluble salts.
AT511410A1, describes addition of HCl solution while contacting cement kiln dust with aqueous phase to obtain homogenous slurry followed by separation of solid and aqueous phase and then recovery of heavy metals and calcium from the separated filtrate by carrying out chemical treatment and then extraction of pure salts by subjecting the processed brine to fractional crystallization.
Aforesaid processes suffer from one or more disadvantages such as complexity, high raw material cost and additional capital investment because of modifications in system configuration. This limits their applicability in industries. It is thus desirable to devise a process for extracting commercially viable materials from alkali bypass dust which is not only simple and inexpensive to carry out, but also decreases raw material costs. It is also desirable that the process, after extracting the commercially viable materials, reduces the amount of bypass dust that needs to be disposed of, and preferably results in a reusable material.

Brief Description of Drawings
Figure 1 illustrates the washing step involving contacting alkali bypass dust with the aqueous medium in the disclosed process;
Figure 2 illustrates the process for generation and filtration of (i) syngenite followed by (ii) potassium chloride;
Figure 3 illustrates the X-ray Diffraction (XRD) of syngenite phase of the isolated product;
Figure 4 illustrates the XRD phase of isolated potassium chloride.

Summary of the Invention:
The present disclosure relates to a process for selective extraction of alkali metal salts from mixed salt solutions. In particular, the present disclosure relates to a process for extraction of syngenite and potassium chloride from alkali bypass dust.
The term “alkali bypass dust” as used in the present disclosure is defined as the flue gas or gaseous effluent arising from alkali metal extraction processes.
According to an embodiment of the invention, there is provided a method for extracting syngenite [K2Ca(SO4)2H20] from alkali bypass dust comprising the steps of:
a. contacting a stream of alkali bypass dust with a counter-current stream of water, the ratio of alkali bypass dust to water being 1:1 to 1:1.5, to obtain an aqueous suspension;
b. separating out any insoluble constituents of the aqueous suspension to obtain an aqueous solution comprising soluble salts from the alkali bypass dust in a concentration ranging from 5% to 20%; and
c. concentrating the aqueous solution of step (b) at 70oC and then heat-filtering the solution to yield syngenite of at least 65% by weight,
wherein step (a) is repeated in at least three separate reactors arranged sequentially such that the first stream of bypass dust arrives in the first reactor and the bypass dust is then sequentially re-contacted with water in subsequent reactors, the bypass dust in each reactor being contacted with a stream of brine coming in from the next sequential reactor, the last reactor in the sequence using a stream of water for contacting the sequentially washed alkali bypass dust.
According to another embodiment of the invention, there is also provided a method of obtaining KCl from the alkali bypass dust, the process comprising the steps mentioned above and then finally concentrating the filtrate obtained in step (c) at 60oC and then cooling the filtrate to obtain crystals of KCl having over 95% purity.

Detailed Description:
In the context of the present disclosure, the alkali bypass dust is a bypass dust obtained from cement kiln, blast furnace or exhaust gases. The main components of bypass dusts are CaO, Al2O3, SiO2, Fe2O3, Na2O, K2O, SO3 and chlorides.
The disclosed process causes in-situ generation of the first product comprising syngenite. Syngenite is a double salt of potassium and calcium sulphate with formula K2Ca (SO4)2•H2O. Upon hot filtration, syngenite is separated from the aqueous solution which can then be used as slow release fertilizers. The remaining filtrate essentially comprises of sodium chloride (NaCl) and potassium chloride (KCl). This filtrate is subjected to partial evaporation until it is saturated with respect to potassium chloride. The filtrate is then cooled to crystallize potassium chloride therefrom. The potassium chloride crystals are separated using any known technique of filtration. Potassium chloride recovered in this manner is substantially free of sulfate contamination.
In accordance with an aspect, a third product comprising a mixture of potassium chloride and sodium chloride is obtained upon filtration of potassium chloride crystals from the filtrate.
In accordance with an aspect, the insoluble constituents of the bypass dust constitute treated bypass dust, having chlorine content ranging between 0.01% to 1 %. Said treated bypass dust may be used for soil stabilization, waste stabilization/solidification, Portland cement replacement, fly ash replacement, asphalt pavement, and construction fill.
In accordance with an embodiment, the aqueous medium is selected from a group consisting of tap water, hard water, processed water, reverse osmosis purified water and demineralized water. When alkali bypass dust is brought into contact with the aqueous medium, soluble components of the bypass dust such as oxides, sulfates and chlorides, are leached into the aqueous medium. In accordance with yet another embodiment, the ratio of the aqueous medium to bypass dust ranges between 1 to 5 parts of water by weight to one part of bypass dust by weight, with the preferred ratio being 1 to 1.5.
In accordance with an embodiment, contacting the aqueous medium with the bypass dust in a countercurrent manner is carried out using multi reactor system. Preferably, the multi-reactor system consists of three reactors. In accordance with an embodiment, the alkali bypass dust is contacted with the aqueous medium for 5 minutes to 1 hour, and preferably 30 minutes at room temperature. Figure 1 illustrates the washing step wherein the aqueous medium is contacted with the bypass dust. Washing of dust is done in multiple stages wherein the dust to be processed is either washed with brine or fresh water. In stage 1, the fresh dust is contacted with brine obtained after separation from stage 2 and having chloride content between 1 to 4%. Similarly, in stage 2, the separated dust obtained after stage 1 is then contacted with brine which is obtained from stage 3 and having chloride content in the range of 0.7% to 1.5%. Similarly, in stage 3, the separated dust obtained after stage 2 is then contacted with water. . Separation of dust with aqueous solution is done either by notch filter or press filter or centrifuge in all the stages.
In accordance with an embodiment, the water used in stage 3 of the sequence may be selected from fresh tap water, processed water, reverse osmosis purified water, demineralized water or a combination thereof.
In accordance with an embodiment, the partial evaporation of the aqueous solution in step (b) is carried out at a temperature in a range of 50 to 100 °C, and preferably at 70°C under 50 to 200 mm Hg vacuum. Preferably, partial evaporation is carried out such that 2/3rd of initial volume of water is removed which is then condensed and this demineralised water can be used for washing the alkali bypass dust in the last reactor of the series of reactors used in the sequential washing of step (a). In accordance with a related embodiment, the hot filtration of the aqueous solution subsequent to the partial evaporation is carried out at a temperature between 30 to 100°C, and preferably at 60 °C. The filtration may be carried out under vacuum in notch filter, Buckner filter or by centrifugation, to obtain the first product comprising syngenite. The first product comprises syngenite in a quantity preferably greater than 65% and most preferably greater than 80%. The balance comprises Calcium hydroxide (Ca (OH)2) in the range of 0.1 -5% and Potassium chloride (KCl) in the range of 15-35%.
In accordance with an embodiment, the filtrate obtained from hot filtration is subjected to partial evaporation such that supersaturated solution is obtained. Preferably, partial evaporation is carried such that 2/3rd of initial volume of water is removed which is then condensed and this demineralised water can be used for washing the alkali bypass dust in the last reactor of the series of reactors used in the sequential washing of step (a).. In accordance with a related embodiment, the resulting solution is cooled to temperatures between 0 to 40° C, preferably to temperatures between 5 to 30° C, and most preferably between 5 to 25 °C to allow crystallization of potassium chloride. The mixture containing the crystals is then subjected to filtration to obtain the second product comprising potassium chloride. The filtration may be carried out under vacuum in notch filter, Buckner filter or by centrifugation. The second product comprises potassium chloride in a quantity greater than 85% and preferably greater than 95%. Figure 2 provides an overview of the process for generation and filtration of (i) syngenite followed by (ii) potassium chloride.
The following experimental examples are illustrative of the invention but not limitative of the scope thereof. In all the experimental examples, the composition of the alkali bypass dust was estimated to be as follows:
CaO: 51.73%, SO3: 4.9%, Al2O3: 4.48%. MgO: 2.28%, Fe2O3: 2.54%, SiO2: 11.0%, K2O: 6.58%, Na2O: 3.09%, Cl2: 5.07% and loss on ignition 7.11%

Example 1: Solvent selection for washing
Table 1 gives the list of solvent used for washing dust and % of chlorides present in the dust after multiple washings. Hard water, demineralized water and a combination of both were screened in order to get the optimum extraction of chlorides from dust after washing. The hard water used was sea water having 0.7 to 1.5% chlorides in the form of NaCl. The cake was then dried in oven and its chloride content were measured. The loss in the filtrate volume was then topped-up with fresh solvent and the recycled aqueous solution was then stirred with fresh dust. The same process was repeated up to three recyclings.
Table 1 List of solvent for chloride extraction and their extracted chloride % with each recycling
Solvent 1st run 1st recycle 2nd recycle 3rd recycle
Demineralized water (DM) 0.54% 0.78% 1.01% 1.11%
Hard Water: DM (50:50) 2.23% 2.32% 2.59% 3.03%
Hard Water: DM (75:25) 2.63% 2.37% 2.35% 2.50%
Table 1 shows the % of chlorides present in the washed dust after treating with different grades of solvent. Demineralized water itself has no chlorides so a greater amount of chlorides was extracted from the dust while using demineralized water. On the other hand, hard water is enriched in minerals especially sodium chloride. Table 1 Row 2 corresponds to only those chlorides present in the dust after washing with demineralized water. while Row 3 and Row 4 corresponds to the percentage of chlorides present in dust after washing with a mixture of hard water and demineralized water. Hence, the dust washed with demineralized water showed minimum % of chlorides in it as compared to the dust washed with hard water and DM water combination.
Example 2:
Step 1: Counter current approach
Alkali bypass dust was washed with water in a ratio of 1:1.5 (alkali bypass dust: water) at room temperature i.e. at 25 to 30 °C for about 30 minutes. The washing was carried out in counter current manner as shown in Figure 1 to obtain a slurry. Washing of dust was done in multiple reactors wherein the dust to be processed was washed with brine followed by fresh water. In stage 1 in the first reactor, the fresh dust was contacted with brine that was obtained after separation from stage 2 in the second reactor and having chloride content between 1 to 4%. Similarly, in the second reactor, the separated dust obtained after stage 1 was then contacted with brine which was obtained from stage 3 in the third reactor and having chloride content in the range of 0.7% to 1.5%. Similarly, in stage 3, the separated dust obtained after stage 2 was then contacted with water .Separation of dust with aqueous solution was done either by notch filter or press filter or centrifuge in all the stages.
The separated bypass dust as obtained after stage 3, was then subjected to sun drying. The chloride content of the treated bypass dust was found to be 0.67 %. Table 2 represents the composition of incoming alkali bypass dust and treated bypass dust gravimetrically.
Table 2: Gravimetric analysis of alkali bypass dust and treated bypass dust
Constituents Alkali Bypass Dust Separated Dust
%
CaO 51.73 59.34
K2O 6.58 1.03
SO3 4.9 4.02
Na2O 3.09 0.33
Al2O3 4.48 4.29
MgO 2.18 2.94
Fe2O3 2.54 3.03
SiO2 11 12.31
LOI 7.11 11.97
Cl 5.07 0.67

Step 2: Separation of insoluble constituents and aqueous solution
Said filtrate obtained after stage 1 washing in the first reactor was then filtered through notch filter under vacuum to obtain the aqueous solution comprising salts. The insoluble constituent comprising treated bypass dust was then taken forward to stage 2 washing in the second reactor. The aqueous solution was found to have a concentration of about 11 – 12% of dissolved salts, as determined by standard titration methods.

Step3: Isolation of Syngenite
The aqueous solution rich in chloride and small concentration of heavy metals and sulphates as obtained in stage 1 washing in the first reactor was subjected to partial evaporation to remove preferably 2/3rd of the initial volume of water at 70° C under 100 to 200 mm Hg vacuum. A thick white slurry was obtained which was then subjected to hot filtration at 60 °C in notch filter. The isolated product was subjected to sun drying. Example 2 was repeated multiple times and the isolated product was characterized and found to be comprising 65-85% syngenite having chemical formula K2Ca(SO4)2.H2O along with 0.1 to 5.0% of Ca(OH)2 and 15-35% of KCl. i.e. in one instance the isolated product was found to have 65% syngenite, 0.1% Ca(OH)2 and 34.9% KCl, in a separate replication of Example 2, the isolated product was found to have 70% syngenite, 5% Ca(OH)2 and 25% KCl, and in a third replication, the isolated product was found to have 85% syngenite, 0.1% Ca(OH)2 and 14.9% KCl. Figure 3 shows the XRD of syngenite in the isolated product along with its SEM-EDS image.

Step 5: Isolation of Potassium chloride
The filtrate obtained upon hot filtration in step 3 was subjected to partial evaporation under vacuum at 70 °C by evaporating 2/3rd of its initial volume till thick white slurry was obtained. The resulting mixture was then cooled to ambient temperature. The solid thus obtained was filtered under vacuum using notch filter. The filtered solids were found to contain 95% potassium chloride. The yield was found to be 75 % compared to the initial amount of potassium chloride in the salt solution. Fig.4 represents the XRD of isolated product comprising potassium chloride.

Recovering Salt mixture: The resulting filtrate was found to contain 5 to 20% of KCl and NaCl with traces of heavy metals.

Industrial Applicability
The disclosed process allows selective extraction of alkali metal salts from mixed salt solutions obtained from industrial process. The disclosed process is simple and inexpensive to carry out. The process does not require setting up expensive processing equipment. The disclosed process also eliminates the requirement of any additional ingredients which are usually required to precipitate one or more salts, thus reducing raw material costs. Additionally, the process allows re-circulation of approximately 65 % of the water (or aqueous medium) with minimum consumption of energy.
Said process enables extracting potash by means of syngenite and potassium chloride which is otherwise allowed to be wasted by dumping of mixed salt solutions. These products can then be used subsequently on a commercial scale without incurring extra costs as the purity of obtained products comprising syngenite and potassium chloride is greater than 85%. Also, negligible or very small percentage of potash values are9 lost during the process.
The disclosed process allows separation of potassium and sodium halide i.e. KCl and NaCl from mixed salt solution, which is otherwise a difficult task. Both the metals belong to group 1A of periodic table and have similar physical and chemical properties, making it difficult to separate the two salts by carrying out chemical or extraction technique. In the present process, potassium chloride separates from sodium chloride owing to the better solubility of potassium chloride in hot water than that of sodium chloride.
Using the disclosed process, it has been made possible to obtain treated bypass dust comprising reduced alkali and chloride content which could be recycled to cement-producing process. Thus, the process not only reduces the bypass dust which needs to be disposed off but also makes it suitable for reuse.
Thus, the disclosed process provides dual advantage of: (i) extracting commercially viable materials from alkali bypass dust, and (ii) providing treated bypass dust which could be recycled without the need of dumping and hence reducing environmental burden.
Three reactor assembly enables a continuous operation improving the process efficiency i.e. better yields and lower utilization of water, while a single stage washing would have led to incomplete removal of salts from the alkali bypass dust and additionally a single stage washing would have required excess water for chloride removal.

The above examples are non-limiting. The invention is defined by the claims that follow.

,CLAIMS:We claim:
1. A method for extracting syngenite [K2Ca(SO4)2H20] from alkali bypass dust comprising the steps of:
a. contacting a stream of alkali bypass dust with a counter-current stream of water, the ratio of alkali bypass dust to water being 1:1 to 1:1.5, to obtain an aqueous suspension;
b. separating out any insoluble constituents of the aqueous suspension to obtain an aqueous solution comprising soluble salts from the alkali bypass dust in a concentration ranging from 5% to 20%; and
c. concentrating the aqueous solution of step (b) at 70oC and then heat-filtering the solution to yield syngenite of at least 65% by weight,
wherein step (a) is repeated in at least three separate reactors arranged sequentially such that the first stream of bypass dust arrives in the first reactor and the bypass dust is then sequentially re-contacted with water in subsequent reactors, the bypass dust in each reactor being contacted with a stream of brine coming in from the next sequential reactor, the last reactor in the sequence using a stream of water for contacting the sequentially washed alkali bypass dust.
2. The method as claimed in claim 1 wherein, the separation in step (b) is a filtration or centrifugation.
3. The method as claimed in claim 1 wherein, the concentration of step (c) involves evaporating two-thirds of the water in the aqueous solution.
4. The method as claimed in claim 1 wherein, the water used in the last reactor of the sequence is fresh tap water, processed water, reverse osmosis purified water, , demineralized water or a combination thereof.
5. The method as claimed in claim 1 wherein, the stream of brine in the first reactor comprises 1 to 4 % chlorides, the stream of brine in the next reactor comprises 0.7 to 1.5 % chlorides and the stream of water in the last reactor comprises is free of chlorides.
6. A method for extracting syngenite [K2Ca(SO4)2H20] and KCl from alkali bypass dust comprising the steps of:
a. contacting a stream of alkali bypass dust with a counter-current stream of water, the ratio of alkali bypass dust to water being 1:1 to 1:1.5, to obtain an aqueous suspension;
b. separating out any insoluble constituents of the aqueous suspension to obtain an aqueous solution comprising soluble salts from the alkali bypass dust in a concentration ranging from 5% to 20%;
c. concentrating the aqueous solution of step (b) at 70oC and then heat-filtering the solution to yield syngenite of at least 65% by weight; and
d. concentrating the filtrate obtained in step (c) at 60oC and then cooling the filtrate to obtain crystals of KCl having over 95% purity,
wherein step (a) is repeated in at least three separate reactors arranged sequentially such that the first stream of bypass dust arrives in the first reactor and the bypass dust is then sequentially re-contacted with water in subsequent reactors, the bypass dust in each reactor being contacted with a stream of brine coming in from the next sequential reactor, the last reactor in the sequence using a stream of water for contacting the sequentially washed alkali bypass dust.
7. The method as claimed in claim 6 wherein, the separation in step (b) is a filtration or centrifugation.
8. The method as claimed in claim 6 wherein, the concentration of step (c) and step (d) involves evaporating two-thirds of the water in the aqueous solution.
9. The method as claimed in claim 6 wherein, the water used in the last reactor of the sequence is fresh tap water, processed water, reverse osmosis purified water, demineralized water or a combination thereof.
10. The method as claimed in claim 6 wherein, the stream of brine in the first reactor comprises 1 to 4 % chlorides, the stream of brine in the next reactor comprises 0.7 to 1.5 % chlorides and the stream of water in the last reactor is free of chlorides.