Cooling aqueous alkali metal hydroxide liquors by vacuum evaporation with subsequent solids precipitate removal

Apparatus for treating aqueous caustic soda liquor to reduce the temperature and sodium chloride concentration thereof, said apparatus comprising:

• an initial cooling stage which includes an evaporating chamber, means for feeding an aqueous caustic soda liquor-solids slurry to a body of said slurry contained in said evaporating chamber, means for maintaining said evaporating chamber at a subatmospheric pressure, means for circulating said body of slurry in said evaporating chamber in a predetermined flow pattern, said evaporating chamber being at least partially enclosed to define a zone into which water vapor is evolved from said slurry body, said initial cooling stage also including a direct-contact condenser in communication with said zone, supply means for supplying a stream of relatively solids-free aqueous caustic soda liquor as a condensing medium to said direct-contact barometric condenser whereby water vapor evolved from said body will be contacted with said stream of relatively solids-free aqueous caustic soda liquor to condense at least a portion of said vapor, means for withdrawing a cooled caustic soda liquor-solids slurry stream from said evaporating chamber, means for feeding said withdrawn cooled caustic soda liquor-solids slurry stream to a second cooling stage, a liquid-solids separation means in communication with said second cooling stage, means for withdrawing a stream of further cooled caustic soda liquor-solids slurry from said second cooling stage, means for feeding said withdrawn stream of further cooled caustic soda liquor-solids slurry into said liquid-solids separation means, first outlet means for removing a relatively solids-free cooled caustic soda liquor from said liquid-solids separation means, second outlet means through which the solids constituents removed from said slurry by said separation means are discharged, and means communicating said first outlet means with said supply means for feeding at least a portion of said relatively solids-free caustic soda liquor to said direct-contact condenser means.

The process of cooling an aqueous alkali metal hydroxide liquor-solids slurry to reduce the temperature of said slurry and remove solids impurities which include alkali metal salts from said slurry, comprising the steps of:

• feeding said slurry to an initial cooling stage which includes an evaporating chamber maintained at a subatmospheric pressure wherein water vapor is evolved from said slurry;
• withdrawing a stream of cooled alkali metal hydroxide liquor-solids slurry from sid evaporating chamber;
• feeding said cooled slurry to a second cooling stage, further cooling said cooled slurry in said second cooling stage, feeding said further cooled slurry into a liquid-solids separation device wherein a substantial portion of the solids constituents in said slurry are removed to produce a relatively solids-free alkali metal hydroxide liquor;
• removing said relatively solids-free alkali metal hydroxide liquor from a first outlet in said liquids-solids separation device, said liquids-solids separation device also including a second outlet through which the solids constituents removed from said slurry are discharged;
• and, contacting the water vapor evolved from said evaporating chamber with at least a portion of said cooled relatively solids-free alkali metal hydroxide liquor removed from said first outlet of said liquid-separation device in a vessel which is in communication with said evaporating chamber, said cooled relatively solids-free alkali metal hydroxide liquor having a vapor pressure below the absolute pressure of said water vapor, whereby said water vapor condenses upon contact therewith.

Apparatus for cooling an aqueous alkali metal hydroxide liquor-solids slurry comprising:

• an evaporating chamber, an inlet for feeding said slurry to said evaporating chamber, said evaporating chamber being at least partially enclosed to define a zone into which water vapor from said slurry body is evolved, vacuum means for maintaining a subatmospheric pressure in said zone, direct-contact condenser means in communication with said zone for receiving water vapor evolved from said slurry body, supply means for supplying a stream of relatively solids-free alkali metal hydroxide liquor to said direct-contact condenser means, means for withdrawing a stream of cooled alkali metal hydroxide liquor-solids slurry from said evaporating chamber, cooling means for further cooling the cooled alkali metal hydroxide liquor-solids slurry withdrawn from said evaporating chamber, means for feeding said cooled alkali metal hydroxide liquor-solids slurry to said cooling means, a liquids-solids separation device, means for feeding said further cooled alkali metal hydroxide liquor-solids slurry stream from said cooling means to said separation device, said device including means for removing a substantial portion of the solids constituents in said slurry to produce a relatively solids-free alkali metal hydroxide liquor, first outlet means in said separation device for removing said relatively solids-free alkali metal hydroxide liquor from said separation device, second outlet means in said separation device through which the solids constituents removed from said slurry are discharged, and means communicating said first outlet with said supply means.