Process for the recovery of soda ash

Abstract

A process for the production of soda ash by withdrawing an aqueous mining solution containing dissolved sodium carbonate and at least about 1 wt % sodium bicarbonate from an underground alkali source; stripping CO 2 gas from the withdrawn aqueous mining solution, to convert sodium bicarbonate dissolved therein to sodium carbonate; co-crystallizing sodium carbonate monohydrate and sodium sesquicarbonate by evaporation of water from the CO 2 -stripped aqueous mining solution, without co-crystallization of anhydrous sodium carbonate, to form a slurry of crystalline solids in an aqueous liquor; recovering crystalline solids from the slurry; and calcining the recovered crystalline solids to produce soda ash.

What is claimed is:  
     
         1 . A process for the production of soda ash which comprises: 
 (i) withdrawing an aqueous mining solution containing dissolved sodium carbonate and at least about 1 wt % sodium bicarbonate from an underground alkali source;    (ii) stripping CO 2  gas from the withdrawn aqueous mining solution, to convert sodium bicarbonate dissolved therein to sodium carbonate;    (iii) co-crystallizing sodium carbonate monohydrate and sodium sesquicarbonate from the CO 2 -stripped aqueous mining solution, without co-crystallization of anhydrous sodium carbonate, by evaporation of water at a temperature of at least about 50° C. to form a slurry of crystalline solids in an aqueous liquor;    (iv) recovering crystalline solids from the slurry; and    (v) calcining recovered crystalline solids to produce soda ash.    
     
     
         2 . The process of  claim 1  which further comprises separating the calcined crystalline solids into at least two different crystal size fractions.  
     
     
         3 . The process of  claim 1  which further comprises separating large crystals of sodium carbonate monohydrate, by crystal size separation, from small crystals of sodium carbonate monohydrate and from small crystalline sodium sesquicarbonate in the crystalline solids mixture.  
     
     
         4 . The process of  claim 3  which further comprises calcining the recovered large sodium carbonate monohydrate crystals to produce soda ash.  
     
     
         5 . The process of  claim 1  wherein the alkali source is selected from the group consisting of trona, nahcolite and wegscheiderite.  
     
     
         6 . The process of  claim 1  wherein the aqueous mining solution contains a total alkali content, expressed as Na 2 CO 3 , of sodium carbonate and sodium bicarbonate of at least about 8 wt % Na 2 CO 3 .  
     
     
         7 . The process of  claim 6  wherein the aqueous mining solution contains at least about 6 wt % sodium carbonate and from about 2 wt % to about 8 wt % sodium bicarbonate dissolved therein.  
     
     
         8 . The process of  claim 6  wherein the aqueous mining solution further contains sodium chloride dissolved therein, in an amount of up to about 8 wt %.  
     
     
         9 . The process of  claim 1  wherein the CO 2  stripping step is operated at a temperature of from about 50° C. to about 140° C.  
     
     
         10 . The process of  claim 1  wherein CO 2  gas is stripped from the aqueous mining solution by countercurrent multistage contact of the aqueous mining solution with a water vapor gas stream.  
     
     
         11 . The process of  claim 10  which further comprises recovering a CO 2 -rich gas from the CO 2  stripping step, by condensing and separating water from a CO 2 -containing exit gas stream from the CO 2  stripping step.  
     
     
         12 . The process of  claim 1  which further comprises concentrating the withdrawn aqueous mining solution, prior to the co-crystallization step, by evaporation of water at a temperature of at least about 50° C., without crystallization of sodium carbonate or sodium bicarbonate dissolved therein.  
     
     
         13 . The process of  claim 12  wherein the concentration step and CO 2  stripping step are carried out as continuous sequential operations in which the CO 2  stripping step is carried out before the concentration step.  
     
     
         14 . The process of  claim 13  wherein CO 2  gas is stripped from the aqueous mining solution by countercurrent multistage contact of the aqueous mining solution with a water vapor gas stream from the concentration step.  
     
     
         15 . The process of  claim 12  wherein the concentration step and CO 2  stripping step are carried out as a single step.  
     
     
         16 . The process of  claim 1  wherein the CO 2  stripping step and the co-crystallization step are carried out in a single vessel.  
     
     
         17 . The process of  claim 1  wherein the co-crystallization of sodium carbonate monohydrate and sodium sesquicarbonate is carried out at a temperature of about 70° C. to about 100° C.  
     
     
         18 . The process of  claim 1  wherein sufficient CO 2  is stripped from the aqueous mining solution to provide, in the subsequent co-crystallization step, a mixture of crystalline sodium carbonate monohydrate and crystalline sodium sesquicarbonate having a total alkali content in which sodium carbonate monohydrate is at least about one-quarter of the total alkali content in the crystallized solids.  
     
     
         19 . The process of  claim 18  wherein the mixture of crystalline sodium carbonate monohydrate and crystalline sodium sesquicarbonate has a total alkali content in which sodium carbonate monohydrate is at least about one-half of the total alkali content in the crystallized solids.  
     
     
         20 . The process of  claim 12  wherein sufficient CO 2  is removed from the aqueous mining solution during the CO 2  stripping and concentration steps to provide, in the subsequent co-crystallization step, a mixture of crystalline sodium carbonate monohydrate and crystalline sodium sesquicarbonate having a total alkali content in which sodium carbonate monohydrate is at least about one-quarter of the total alkali content in the crystallized solids.  
     
     
         21 . The process of  claim 20  wherein the mixture of crystalline sodium carbonate monohydrate and crystalline sodium sesquicarbonate has a total alkali content in which sodium carbonate monohydrate is at least about one-half of the total alkali content in the crystallized solids.  
     
     
         22 . The process of  claim 1  wherein sufficient CO 2  is stripped from the aqueous mining solution to convert at least about 10% of the sodium bicarbonate in the aqueous mining solution to sodium carbonate prior to co-crystallization of sodium carbonate monohydrate and sodium sesquicarbonate.  
     
     
         23 . The process of  claim 1  wherein sufficient CO 2  is stripped from the aqueous mining solution to convert at least about 20% of the sodium bicarbonate in the aqueous mining solution to sodium carbonate prior to co-crystallization of sodium carbonate monohydrate and sodium sesquicarbonate.  
     
     
         24 . The process of  claim 1  which further comprises introducing the soda ash into an aqueous medium to recrystallize the soda ash as sodium carbonate monohydrate, recovering the crystalline sodium carbonate monohydrate, and calcining the recovered sodium carbonate monohydrate to produce a dense soda ash product.  
     
     
         25 . The process of  claim 1  which further comprises introducing the soda ash into an aqueous suspension containing crystalline sodium carbonate monohydrate as a sole stable solid phase in equilibrium with the aqueous liquor of the suspension, to effect solubilization and conversion of the soda ash into crystalline sodium carbonate monohydrate, recovering crystalline sodium carbonate monohydrate from the suspension, and calcining the recovered sodium carbonate monohydrate to produce a dense soda ash.  
     
     
         26 . The process of  claim 1  which further comprises converting the soda ash to sodium carbonate monohydrate in a hydrator and thereafter calcining such sodium carbonate monohydrate to produce a dense soda ash.