Free flowing 100-500 micrometer size spherical crystals of common salt and process for preparation thereof

Abstract

The present invention provides spherical shaped crystals of common salt having size distribution in the range of 100-500 μm and a process for preparation of the said salt. The above mentioned size range is relevant for many important edible and non-edible applications of salt. The salt is demonstrated to have superior flow properties compared to the cubic counterpart. The free flow property of the salt can be further enhanced through surface treatment with additive such as glycine. The salt is produced by cost-effective mechanical means and such production is amenable to scale up.

1 . Spherical shaped crystals of common salt having size in the range of 100-500 μm and purity up to 99.9% on dry basis, obtained from saturated brine without using any habit modifier. 
     
     
         2 . Spherical shaped crystals according to  claim 1 , wherein the salt exhibits improved flow characteristics having an angle of repose in the range of 19.5°-20.5° without use of additives, with moisture content ranging between 0.3-0.5% (w/w). 
     
     
         3 . Spherical shaped crystals according to  claim 1 , wherein the salt exhibits improved flow characteristics having an angle of repose in the range of 17°-18° using glycine as a surface modifier with moisture content ranging between 0.3-0.5% (w/w). 
     
     
         4 . A process of preparing free flowing spherical shaped crystals of common salt without using any habit modifier, wherein the said process comprising the steps of:
 (a) charging saturated brine optionally along with an anti-solvent into a thermally insulated stainless steel vessel fitted with heating tape on the external wall of the vessel and with a mechanical stirrer of appropriate dimension and configurations with a stirring rate ranging between 200-275 r.p.m;   (b) raising the temperature of the brine as obtained from step (a) gradually while stirring up to the pre-set point in the range of 40° C.-80° C.,   (c) holding the temperature of the brine as obtained from step (b) at temperature of 58±2° C. and allowing the brine to evaporate under mechanical stirring at speed of 220±10 r.p.m;   (d) discharging the salt slurry as obtained from step (c) from the bottom when the pre-set point of evaporation is being reached and the brine volume being reduced to 25-35% (v/v) of the initial volume taken;   (e) charging the slurry as obtained from step (d) into a centrifuge followed by washing the charged slurry with a spray of saturated solution of sodium chloride;   (f) charging the wet salt as obtained from step (e) in a shallow stainless steel tray, followed by drying in the open sun to reduce the moisture content up to 0.5% (w/w) while minimizing agglomeration;   (g) sieving the dry salt as obtained from step (f) through progressively finer sieves ranging between 35-60 mesh size to fractionate the salt crystals so as to attain the desired size distribution;   (h) recycling the residual glycine solution in the subsequent batches after topping up with additional glycine if required.   
     
     
         5 . A process according to  claim 4 , wherein the brine used is selected from the group comprising synthetic brine or natural brines from sea, sub-soil, lake and plant sources. 
     
     
         6 . A process according to  claim 4 , wherein the anti-solvent used is selected from the group consisting of methanol, ethanol or propanol, more preferably ethanol and being in the concentration ranging between 0.3-0.5% (v/v). 
     
     
         7 . A process according to  claim 4 , wherein the stainless steel vessel used is having a cylindrical geometry with internal diameter of 0.1-1.0 m, height of 0.5-1.0 m for a volume capacity of 100 L. 
     
     
         8 . A process according to  claim 4 , wherein the bottom of the stirrer is being placed at a depth of 0.10 m-0.2 m within the vessel having dimensions according to  claim 7 . 
     
     
         9 . A process according to  claim 4 , wherein the mechanical stirrer used is having propeller type of blade having an inclination angle of 10°-20° and blade diameter being in the range of 0.1-0.5 m. 
     
     
         10 . A process according to  claim 4 , wherein the stirring rate maintained more preferably in the range of 245-255 r.p.m. and is a critical parameter to obtain the desired morphology of the said crystals. 
     
     
         11 . A process according to  claim 4 , wherein the effective stirring requires a minimum initial free volume in the range of 20-40% to be maintained to ensure proper stirring of the entire volume of brine. 
     
     
         12 . A process according to  claim 4 , wherein the centrifuged product is alternatively dried in a fluidized bed drier or in a forced air oven at 35±5° C. without significant caking. 
     
     
         13 . A process according to  claim 4 , wherein the sieving has been performed using 35 mesh size sieves for separating the crystals larger than 500 μm and 45-60 mesh size sieves for separating fine salt crystals. 
     
     
         14 . A process according to  claim 4 , wherein the product obtained from the said process is packaged in air tight plastic bags for storing for at least up to 3 months. 
     
     
         15 . Spherical shaped crystals of salt according to  claim 1 , wherein the said crystallized salt is useful as edible salt, more preferably as table salt with or without incorporating micronutrients, additives including free flow agents and also being useful in applications where conventional cubic salt of similar size is normally used.