Countercurrent, cooling crystallization and purification method for multi-component molten mixture

In a countercurrent, cooling crystallization and purification method for a multi-component molten mixture for separating a component of the mixture, wherein the purification of the molten mixture is effected by countercurrent contact of crystals formed with a mother liquor, wherein the improvement comprises, in a system comprising a plurality of cooling type crystallizers each having, at their respective upper portions, a clarifying portion containing a liquid, and a vertical purifier having, at its upper portion, a clarifying portion and, at its lower portion, heating means, feeding said molten mixture into one of said crystallizers, effecting crystallization in each of the crystallizers at different temperatures, said purifier being operated at a temperature higher than any of the crystallizers, feeding a slurry containing crystals formed in each of the crystallizers to the clarifying portion of another of the crystallizers wherein the crystallization is effected at a higher temperature and feeding a slurry containing crystals formed in the crystallizer wherein the crystallization is effected at the highest temperature among the crystallizers to the clarifying portion of the purifier, allowing the crystals in the purifier to move downwardly therein and melting said crystals by said heating means, withdrawing a portion of the resulting melt as a product and allowing the remainder of said melt to move upwardly as a reflux in said purifier to wash the descending crystals, feeding the clarified liquids from the clarifying portions of the purifier and the crystallizers directly to the liquids in the respective clarifying portions of the crystallizers of sequentially lower temperature so that crystals formed in each of the crystallizers of sequentially lower temperature are washed and the clarified liquid fed into each crystallizer from the crystallizer of sequentially higher temperature or from the purifier, respectively, is cooled by the liquid of lower temperature in the crystallizer of sequentially lower temperature to be recrystallized, and discharging the clarified liquid in the crystallizer of the lowest temperature from the system.

In a method for purification of a molten mixture to separate a component of said mixture in substantially pure form, including a step of contacting crystals of said component with a countercurrently flowing stream of a mother liquor from which said crystals were crystallized, the improvement which comprises:

• feeding said molten mixture into a lower cooling portion of an upright crystallizer and flowing said molten mixture upwardly in said crystallizer toward a first upper clarifying portion containing a liquid located above said lower cooling portion and in direct communication therewith;
• cooling said molten mixture as it flows upwardly in said cooling portion of said crystallizer to form a slurry of crystals of said component in said mother liquor;
• feeding said slurry into an upper clarifying portion of a purifier so that said crystals flow downwardly toward a lower heating portion which is located below said upper clarifying portion of said purifier and is in direct communication therewith, said second upper clarifying portion of said purifier being at a higher temperature than said crystallizer and said lower heating portion of said purifier being at a higher temperature than said second upper clarifying portion of said purifier;
• allowing said crystals to settle in said upper clarifying portion of said purifier so that said crystals descend into said lower heating portion of said purifier;
• heating said crystals in said heating portion so that said crystals become molten and form a melt;
• withdrawing a portion of said melt as a substantially pure product and flowing the remainder of said melt upwardly in said purifier as a reflux stream in contact with the descending crystals so that a clarified liquid is formed in said upper clarifying portion of said purifier;
• and
• feeding the clarified liquid from said upper clarifying portion of said purifier directly to the liquid in said first clarifying portion of said crystallizer, so that said clarified liquid directly contacts and is mixed with the liquid in the first upper clarifying portion of said crystallizer, the crystals in the liquid in the first upper clarifying portion of said crystallizer are washed and the clarified liquid fed from the purifier is cooled by the liquid in the first upper clarifying portion of said crystallizer so as to be recrystallized.

A method for purification of a molten mixture to separate a component of said mixture in substantially pure form, including the steps of:

• feeding said molten mixture into a first cooling zone at the lower end of a first crystallizer, said first crystallizer having a first clarifying zone at the upper end thereof and directly communicating with said first cooling zone for receiving first clarified liquid therefrom, said first crystallizer being operated at a first temperature;
• cooling said molten mixture while agitating said molten mixture in said cooling zone to obtain at the bottom of said first crystallizer a first slurry comprising crystals of said component and a mother liquor and to obtain a first clarified liquid in said first clarifying zone;
• feeding said first slurry from said first cooling zone to a second clarifying zone located at the upper end of a purifier and which is operated at a second temperature which is higher than said first temperature, said purifier having a lower heating zone which is in direct communication with said second clarifying zone and which is operated at a third temperature which is higher than said second temperature;
• flowing a melt of said component upwardly in said purifier as a reflux stream that is in contact with and is in countercurrent flow relationship with crystals that are descending from said second clarifying zone toward said lower heating zone, and agitating the mixture of said crystals and said melt in said purifier and obtaining a second clarified liquid in said second clarifying zone;
• heating crystals in said lower heating zone to form said melt;
• filtering solid impurities from said melt in said lower heating zone;
• recovering a portion of said filtered melt from said lower heating zone of said purifier as a substantially pure product;
• feeding said second clarified liquid at said second temperature from near the top of said second clarifying zone directly into said first clarified liquid located in said first clarifying zone in the upper portion of said first crystallizer and which is at said first temperature so that crystals in said first clarified liquid are washed and said second clarified liquid is cooled by said first clarified liquid to be recrystallized;
• feeding clarified liquid from near the top of said first clarifying zone to a third clarifying zone comprising an upper portion of a second crystallizer, said second crystallizer having a second cooling zone at the lower end thereof and being operated at a fourth temperature lower than said first temperature;
• countercurrently washing crystals of said component in said third clarifying zone with said clarified liquid by flowing said clarified liquid upwardly in said third clarifying zone while said crystals settle downwardly therein and enter said second cooling zone;
• cooling and agitating said clarified liquid and said crystals in said second cooling zone to obtain a second slurry enriched in said crystals;
• feeding said second slurry into said first clarifying zone of said first crystallizer;
• countercurrently washing crystals of said component in said first clarifying zone with said clarified liquid by flowing said clarified liquid upwardly in said first clarifying zone while said crystals settle downwardly therein and enter said first cooling zone;
• and
• removing clarified liquid near the top of said third clarifying zone from said second crystallizer.