Crystallizers solar ponds

Recovery Process. The Texas Gulf, Cane Creek potash operation (60) of Moab, Utah produces KCl by solution mining (61—64). Brine is pumped from underground to 1.6 x 10 (400 acres) of solar ponds where a mixture of KCl and NaCl is crystallized in a salt mass called sylvinite. 

Production of KCl at the Wendover, Utah operation employs a large 7000 acre complex of solar ponds. Both shallow brine wells and deeper wells are used to pump brine into the pond complex. In the preconcentration ponds water is evaporated and sodium chloride is crystallized. Later the brine is transferred to production ponds where sylvinite is deposited. Brine is then transferred to other ponds where camaUite is crystallized. Sylvinite is removed from drained ponds with self-loading scrapers and taken to the plant were KCl is separated by flotation with an amine oil collector. The camaUite,... 

Recovery Process. Figure 5 shows a typical scheme for processing sodium chlodde. There are two main processes. One is to flood solar ponds with brine and evaporate the water leaving sodium chlodde crystallized on the pond floor. The other is to artificially evaporate the brine in evaporative crystallizers. Industrial salt is made from solar ponds, whereas food-grade salt, prepared for human consumption, is mosdy produced in the crystallizers. 

The first industrial crystallizers and crystallization processes came up about 150 years ago. The crystallization process became entirely independent of locations (e.g., solar ponds) or stationary energies and all product purities became educible. The further development led to different crystallizers adjusted to the respective crystallization processes and concentrated on the product quality aspect demanded by the market. Applying vacuum technology opened the possibility to choose operation apart from the atmospheric pressure and herewith the opportunity to precisely design the crystallization processes to the respective phase systems and the material properties. Today, these vacuum crystallization processes have become the common standard in the industrial continuous single mass crystallization from solutions. 

This leads to the replacement of the impure mother liquor by the wash solution, before the suspension becomes separated on the centrifuge. It has the advantage that the mother liquor in the crude crystallization, respectively the second crop, can be more concentrated that is, higher yields can be realized. Even solid impurities can be separated, if their settling rates are smaller than those of the product crystals. This is, for example, the case in the recrystallization of table salt from solar salt or in the case of the direct evaporative crystallization from solar pond concentrates, where... 

In many cases, therefore, salt from solar ponds is processed downstream by some purification steps. These are, for example, a combined grinding and washing process or even the complete recrystallization by the evaporative vacuum crystallization. A modern alternative is the combination of the energy-saving solar pond concentration with industrial crystallization. In such cases, the seawater... 

In order to make negligible any dilution by rainfall, the final solar pond brine is collected in 10 m deep tanks, before the brine is eventually used as feed stream for the crystallization plant constantly all over the year. The clear advantage of this alternative is the improvement in regard to salt quality and particle size distribution (uniformity and particle size as well) that helps to approach a more genera] market than before. 

The salt plant is fed with brine from the solar pond produced in the same way as earlier. However, unlike before, the brine is only concentrated near to saturation. Sodium chloride is not brought to crystallization in the fields anymore. By using the former crystallization fields instead also for brine concentration the solar pond capacity can be increased significantly. 

Some of the hydrocydone overflow is purged to balance the impurities, that is, to set the impurity concentration in the process to a constant level. This is executed by collecting the solution in an intermediate tank that is divided into two parts. One compartment is entirely filled and overflows into the other part, from where the solution is returned to the receiver tank B. From the filled compartment, the purge liquor is taken off back to the solar ponds, controlling the impurity level in the crystallizer stage 4. 

Preferably, the pusher-type centrifuge has two stages. The first stage is used to separate the solution from the crystal mass, and the second to wash the separated filter cake. In this way, the washing becomes significantly more effective. This is due to the washing only after the complete separation of the mother liquor (here solar pond brine) and due to the complete reformation of the filter cake before the washing. 

In modern equipment for crystallisation, on the other hand, any crystalline product can be produced with purities of about 99.9% and with almost any desired crystal size distribution (CSD). The specific rate of production in industrial crystaUisers is several thousand times larger than in solar ponds [1-4]. 

A plant of this type was erected for the treatment of 601 h concentrated liquor from a solar pond on the coast of the Mediterranean Sea. The plant is producing 2.5 th grain-size salt of >2 mm average crystal size and 6.51 h" of normal vacuum salt. Per hour it consumes lit of steam and evaporates 341 of water. By... 

Crystallization is one of the pristine unit processes. It may be assumed that our ancestors used sodium chloride found in crevices of the surface rocks after drying caused by the sun this process is still in use in modem solar ponds. 

Other salts crystallize. Lake Abijdata in Ethiopia has similar brines and solar ponds, but is a much smaller soda ash operation. The Sebka El Adhibate, Tunisia has about a 16 ppm Li concentration in a seawater-type brine, and after solar evaporation for potential potash production the end-liquor would contain 250-340 ppm Li (Hamzaoui et al, 2000). There are several other solar evaporation or mineral recovery projects throughout the world with end-liquors that might also be considered for potential lithium recovery. 

From the final pond the concentrated brine (Table 1.3) with a density of about 1.25 g/cc was pumped nearly 4.8 km (3 mi 1.5 mi in 1967, Gadsby, 1967) to the processing plant in the town of Silver Peak. The plant had been converted from a silver ore cyanide-leach plant that had operated there from 1864-1961. In the conversion all of the tanks and settlers were rubber lined to reduce iron contamination in the product, and considerable new equipment was added. The solar pond brine was first reacted with lime to remove most of the residual magnesium and some of the sulfate and borate ions, and then a small amount of soda ash was added to precipitate most of the calcium from the lime reactions. The slurry from these operations was settled and filtered, and the overflow solution sent to storage tanks. From there the brine was pumped through filter presses to be totally clarified, and then heated to 93°C (200°F lithium carbonate has an inverse solubility) and reacted with dry soda ash and hot wash and make-up waters to precipitate the lithium carbonate product. Extra water was added to prevent salt from crystallizing, since the pond brine was samrated with salt. The lithium carbonate slurry was thickened in a bank of cyclones, and the underflow fed to a vacuum belt filter where it was washed and dewatered. The cyclone overflow and filtrate were... 

In the solar ponds (Fig. 1.58) salt (halite) crystallized immediately as the brine evaporated, and with the low-sulfate brine utilized by SQM, much of the potassium... 

Lake Texcoco. Lake Texcoco, a few miles northeast of Mexico City, is in the lowest part of the Valley of Mexico. The lake is mostly dry and alkaH is recovered from brine weUs that have been drilled into the underlying stmcture. The brine is concentrated first in a spiral flow solar evaporation pond and further in conventional evaporators. This strong brine is carbonated and then cooled to crystallize sodium bicarbonate which is subsequently filtered and calcined to soda ash. Purity of this product is similar to Magadi material (9,29). 

---