Forced circulation evaporative crystallizers exhibit exceptional versatility, catering to a broad spectrum of applications by manipulating the crystallizer’s geometry and accompanying components. These systems operate based on a fundamental set of principles, where supersaturation is induced by circulating a process slurry through the heat exchanger’s tubes. 

This controlled circulation mitigates the temperature differential between the heating medium and process slurry, elevating the fluid’s temperature. The liquid phase undergoes evaporation as the fluid reaches an area within the crystallizer body, characterized by a sufficiently low pressure known as the boiler surface. The resulting crystals are recovered and processed in the dewatering system, which involves washing and dewatering procedures to maintain the desired purity levels. The residual mother liquor is collected in a dedicated tank and reintroduced into the crystallizer, closing the loop. 

Fine-tuning of crystal purity is attainable through the strategic use of a purge stream, with increased purging yielding higher purity levels. Furthermore, purity enhancement can be achieved by incorporating an elutriation and wash leg to eliminate impurities from the solution before separation. 

Adding a slurry return draft tube inlet results in a larger average crystal size distribution and augments the washing efficiency, leading to higher product quality.  This is achieved by reducing supersaturation at the boiling surface by creating an upward flow, facilitating the transport of larger crystals to the boiling surface, thereby increasing the available surface area for crystal growth. This, in turn, reduces supersaturation and limits the nucleation of new crystals, commonly referred to as fines. The draft tube inlet also diverts supersaturation from the vessel walls toward the boiling surface, resulting in extended washout cycles. 

Incorporating an elutriation leg refines the crystal size distribution (CSD), narrowing its range by redirecting fine crystals back into the crystallizer for continued growth while providing an exit pathway for larger crystals to prevent excessive growth.  

The draft tube crystallizer is designed for crystal growth, facilitating the growth of 2-3 times larger crystals than forced circulation designs. This is achieved through the induction of supersaturation by lowering the solution’s temperature and concentrating the brine by flash evaporation.  

To achieve this controlled crystal growth, an internal baffle, accounting for approximately 10% of the active volume, diverts to a heat exchanger, which is heated to dissolve fines before reintroducing into the crystallizer. Upon returning, the heated brine re-enters the crystallizer and experiences a flash back to its saturated temperature, causing dissolved crystals to adhere to the existing crystal surfaces within the magma.

This system employs a dedicated barometric or surface condenser to maintain the vacuum pressure setpoint in conjunction with a vacuum jet ejector or vacuum pump system. External cooling water is utilized within the barometric or surface condenser for vapor condensation generated within the crystallizer. The condensed cooling water is collected in a hotwell or receiver and pumped out using a cooling water pump.

Crystals produced within the crystallizer undergo recovery in the dewatering system, where they are thoroughly washed and dewatered to sustain the desired purity level. The mother liquor is collected in a dedicated tank and recycled into the crystallizer. A small purge may be extracted from the process to ensure the crystal purity is maintained at the desired level. 

Vacuum crystallization is a versatile technology used to precisely control operating temperatures, harnessing solubility and speciation characteristics. While adaptable to various vessel configurations, it finds frequent application in forced circulation-style crystallizers.

The surface cooling crystallizer is a forced-circulation, draft tube inlet style crystallization technology designed to grow large crystals by increasing product concentration above the saturation point through subcooling the liquor. It operates continuously and maintains a consistent inventory of product crystals in slurry magma. It uses a mixed-suspension mixed product removal (MSMPR) approach to ensure uniform slurry density throughout the process. 

Concentrated liquid from the evaporator flows through precoolers before entering the cooling crystallizer. Precoolers use exiting mother liquor and cooling water to reduce the heat transfer duty of the cooling crystallizer/chiller circuit. Utilizing a draft-tube inlet style circulation design, the cooling crystallizer induces crystal precipitation by decreasing the bulk solution temperature, causing dissolved solids in the feed to precipitate and crystallize as they become a supersaturated solution.  

The slurry is pumped to the surface cooling, dissipating heat into a circulating coolant solution. A crystallizer slurry pump feeds the hydrocycline, which pre-thickens the crystal phase for the pusher centrifuge. Hydrocycline overflow goes to the mother liquor tank. The centrifuge dewaters the product crystals, which are washed with clean water to displace the remaining mother liquor in the dewater crystals. 

The washed crystals are discharged into a conveyor for further solids handling. A portion of the mother liquor is purged from the system to maintain the desired impurity endpoint, while the rest of the balance is returned to the crystallizer.

The Oslo crystallizer is a specialized product crystallizer designed to produce large product crystals, typically 2-3 times bigger than those produced in a forced circulation crystallizer and slightly larger than a draft tube crystallizer. This is achieved by reducing secondary nucleation (crystal breakage) by recirculating highly concentrated mother liquor via the heater, allowing crystals to grow within the crystallizer. 

 A baffle gently removes brine at low velocity, slowing crystals to settle out, and redirects it to a forced circulation heat exchanger for heating. The heated brine is then pumped to a flask tank for evaporation, inducing supersaturation.  The supersaturated brine flows through a downcomer into the growth chamber, relieving supersaturation through precipitation onto the surface of the existing crystals.

Oslo crystallizers can have an elutriation leg for a more uniform crystal size distribution. The feed solution can serve as the suspending fluid in the elutriation leg, reducing impurities in crystal surface moisture. Solids exceeding a critical mass settle down the elutriation leg and are pumped out for dewatering before further growth.  

The reactants are pumped into the draft tube of the reactor to allow adequate mixing of the reactants with the existing crystals, allowing for surface growth rather than primary nucleation of new crystals.

The crystallizer system is a draft-tube baffle (DTB) reactive crystallizer designed to grow and classify crystals. The DTB crystallizer incorporates an internal battle that creates a quiescent zone for crystal classification and a draft tube that promotes the circulation of the crystal slurry, promoting crystal growth. Feeds to the crystalizer are introduced directly into the draft tube to facilitate immediate mixing and interaction between feeds.  

Combining baffles concentrates the crystallizer slurry, minimizing crystal loss during overflow purging. The slow velocity up the baffle ensures that only the lighted fines remain in the active crystallizer body. Further suspended solids can be recovered via a centrifuge or hydroclone connected to the overflow line. With minimized crystal loss, the spent mother liquor is removed from the system as purge for processing. 

Crystal retrieval takes place from the bottom of the crystallizer and is sent to a centrifuge for dewatering. A constant slurry feed rate to the centrifuge is maintained. For crystals with an average size of less than 100 microns, a specialized peeler batch centrifuge is utilized, accompanied by a slurry feed tank and product feeder for the drying process.