Process for manufacture of sodium hydroxide and sodium chloride products from waste brine

Abstract

The present invention provides a process for the manufacture of sodium hydroxide and sodium chloride products from a waste brine stream containing sodium chloride, and at least sodium carbonate or sodium bicarbonate. The method of this invention comprises converting the sodium carbonate and/or sodium bicarbonate to sodium hydroxide through addition of lime to the brine, while simultaneously converting impurities to less soluble forms to facilitate their removal. The causticized brine is separated from unreacted lime, calcium carbonate and solid impurities to produce clean brine with reduced levels of impurities. The brine is then concentrated by evaporation, forming relatively pure sodium chloride crystals, which are extracted and rinsed to reduce alkali contaminants. The process continues until sodium hydroxide solution strength of about 46-50% is reached, with cooling of the solution and separation of crystallised salts further reducing impurities to a comparable level to diaphragm cell derived industrial grade sodium hydroxide.

What is claimed is: 
     
         1 . A process for manufacture of sodium hydroxide and sodium chloride products from a brine stream containing at least sodium chloride, and sodium carbonate or sodium bicarbonate (or a mixture of sodium carbonate and sodium bicarbonate), which comprises at least the following steps:
 a) converting (causticizing) the sodium carbonate and sodium bicarbonate present to sodium hydroxide through addition of lime to the sodium chloride rich brine;   b) separating the causticized brine from unreacted lime and calcium carbonate formed;   c) concentrating the causticized brine via evaporation of water such that sodium chloride crystals form;   d) separating the sodium chloride crystals from the saturated solution;   e) continuing the evaporation/crystallisation process until only a concentrated sodium hydroxide solution remains.   
     
     
         2 . A process according to  claim 1 , which includes the additional steps of:
 f) recycling all or most of the unreacted lime and calcium carbonate recovered at (b) for reuse in (a) by burning in a kiln to convert calcium carbonate back to calcium oxide;   g) reducing unwanted impurities in the brine through their co-precipitation during (a), without or with addition of mineral/chemical reagents, and separation of precipitated impurities together with the unreacted lime and calcium carbonate at (b);   h) bleeding off a portion of the unreacted lime, calcium carbonate and impurities of (g), or a portion of the burnt lime of (f), as required to control the build-up of impurities in the lime cycle of (f).   
     
     
         3 . A process according to  claim 2 , which includes the additional steps of:
 i) cooling the concentrated sodium hydroxide from (e) and crystallising out and separating a portion of the remaining sodium chloride and sodium carbonate impurities so as to produce an industrial grade sodium hydroxide product;   j) combining the crystallised sodium chloride and sodium carbonate from (i) with the sodium chloride crystals from (d), either directly or following treatment with hydrochloric acid; or recycling the crystallised sodium chloride and sodium carbonate from (i) back to sodium chloride rich brine at (a).   
     
     
         4 . A process according to  claim 3 , which includes the additional steps of:
 k) washing the sodium chloride crystals from (d) with a small amount of clean water recovered from evaporation at (c) and (e) so as to displace residual sodium hydroxide from the surface of the salt crystals, hence minimising sodium hydroxide contamination, with the washings returned to the evaporator/crystallisers so as to maximise sodium hydroxide recovery;   l) optionally, washing the salt crystals recovered from (d) and/or (i) in a sodium chloride saturated brine solution dosed with hydrochloric acid so as to convert the residual alkalis (mostly Na 2 CO 3 ) to sodium chloride, followed by separating out the purified salt crystals from the saturated brine solution which is recirculated back to the washing/acid treatment process.   
     
     
         5 . A process according to  claim 4 , which includes the additional step of:
 m) purification of the causticized brine from (b), for example by membrane ultra-filtration or electro-coagulation to remove unwanted impurities, prior to evaporation/crystallisation at (c).   
     
     
         6 . A process according to  claim 2 , which includes the additional steps (k) and (l). 
     
     
         7 . A process according to  claim 2 , which includes the additional step (m). 
     
     
         8 . A process according to  claim 6 , which includes the additional step (m). 
     
     
         9 . A process according to  claim 3 , which includes the additional step (m). 
     
     
         10 . A process according to  claim 1 , which includes the additional steps (i) and (j). 
     
     
         11 . A process according to  claim 10 , which includes the additional steps (k) and (l). 
     
     
         12 . A process according to  claim 10 , which includes the additional step (m). 
     
     
         13 . A process according to  claim 11 , which includes the additional step (m). 
     
     
         14 . A process according to  claim 1 , wherein the conversion of sodium carbonate and sodium bicarbonate to sodium hydroxide is carried out in the presence of an unusually high concentration of chloride ions relative to causticizing reactions in the pulp (Kraft) and alumina refining (Bayer) processes, for example four times higher than in pulp mills operating with relatively high chloride levels. 
     
     
         15 . A process according to  claim 14  where optimising the lime dosage in the causticizing step at above 100% of the stoichiometric CaO requirement enables attainment of an unusually high conversion efficiency of at least 90%, and preferably 95-99%, such that sodium carbonate contamination of the sodium chloride and sodium hydroxide products is minimised. 
     
     
         16 . A process according to  claim 2  where causticizing of the brine and subsequent separation of precipitated calcium carbonate/unreacted lime at step (g) reduces calcium, magnesium and silica contaminant levels in the brine by more than 40%, hence reducing the level of these contaminants in the NaCl and NaOH products, and significantly reducing the potential for scale deposition in the evaporation/crystalliser equipment. 
     
     
         17 . A process according to  claim 3  where the sodium hydroxide produced is suitable for a range of industrial applications, being of comparable quality to that produced by the diaphragm cell process, having a preferred NaOH concentration of 45-51% by mass, and containing less than 1.5% NaCl and 0.3% Na 2 CO 3  contaminants by mass. 
     
     
         18 . A process according to  claim 4  where the sodium chloride produced is of suitable quality for a range of applications, including the chlor-alkali industry, containing less than 500 ppm Ca and 300 ppm Mg, and less than 0.8% sodium carbonate, which can optionally be reduced through the hydrochloric acid wash process (l). 
     
     
         19 . A process according to  claim 1  where the primary evaporation step to bring the causticized brine close to saturation in sodium chloride is carried out in evaporators which provide a high rate of heat transfer and high energy efficiency (e.g. falling film multiple effect or falling film MVR evaporators) and where the secondary evaporation/crystallisation of saturated brine occurs in forced circulation evaporators capable of growing sodium chloride crystals suitably sized for efficient separation and washing, while preventing salt build-up on heat exchange surfaces. 
     
     
         20 . A process according to  claim 1  where the brine stream processed is a waste derived from the treatment of water co-produced during coal seam gas extraction.