Patent Application
20120039785
This invention relates to any processes that involve reacting or mixing two or more substances to form one or more new substances under low supersaturation. However, the specific process used to exemplify the present invention is the “wet process” phosphoric acid production, where phosphate ore is reacted with sulfuric acid to form the product phosphoric acid and the byproduct gypsum. The byproduct gypsum is then separated from the product phosphoric acid through filtration.
Depending on the reaction conditions, there are mostly two types of gypsum crystals formed in the “wet process” phosphoric acid production: the dihydrate gypsum (CaSO4.2H2O), and the hemihydrate gypsum (CaSO4.½H2O). Currently the majority of the “wet process” phosphoric acid is produced through the dihydrate route even though the hemihydrate route generates a significantly higher concentration of phosphoric acid.
With the advancements of more effective process controls, more efficient slurry cooling, and especially the realization of high product slurry re-circulations, modern plants can easily produce more than 2000 tons of P2O5 daily in a single line. However, two seemly formidable problems still face the industry: the high P2O5 losses in the byproduct and the frequent process upsets causing a lower filtration rate and even higher P2O5 losses. The phosphate-containing gypsum is being piled up at or near the production sites, which imposes potentially dangerous environmental hazards. The pile site is also very costly in both construction and maintenance.
Normally there is more than 4% P2O5 value left in the gypsum byproduct of which only a small amount is undigested phosphate ore caused by poor digestion that is environmentally “inert”. The majority of it is either the citrate-soluble P2O5, which occurs when the phosphate ion substitutes the sulfate at the gypsum crystal lattice during the gypsum precipitation, or the water-soluble P2O5, which is the result of incomplete washing of gypsum cake. The citrate and water soluble phosphates will gradually leach out of the gypsum pile and enter into either a “pond” or underground waters if no impermeable membrane is lined underneath.
It is therefore desirable to eliminate the presence of phosphate in the byproduct gypsum during phosphoric acid production. Not only is more P2O5 value recovered, but environmentally friendly gypsum is also produced, which can be better used for other products.
In the traditional “wet process” phosphoric acid production, the formation of the lost P2O5 occurs at two places: where the phosphate ore is digested and where the sulfuric acid is added. Due to the fast phosphate ore dissolution kinetics and the presence of sulfate in the recycled slurry, a significant amount of gypsum is precipitated when the ore is introduced. These precipitates usually take place in the vicinity of the dissolving phosphate ore particles. The highly viscous liquid medium with the presence of more than 30 wt % of gypsum crystals determines that only a laminar flow can occur in the slurry at the agitation intensity currently employed by the industry. In such a case, phosphate ions are incorporated into the gypsum lattices as precipitation occurs. Bigger and slower dissolving phosphate ore particles also have a chance to be “coated” by the precipitates, resulting in the occurrence of unreacted phosphate ore particles in the final byproduct of gypsum. The unreacted phosphate ore will generate a sulfuric acid surplus and thus causing the process to fluctuate, which in turn will produce more unreacted phosphate ore particles.
The dolomite and calcite impurities have their most significant effects on this concomitant gypsum precipitation. Their smaller particle size and faster dissolution kinetics, as compared to the phosphate ore particles, make them the more effective contributors of calcium ions. As a result, a higher supersaturation is achieved faster, resulting in gypsum fines being produced and more phosphate ore particles being coated. More significant sulfate concentration fluctuations will then occur. Other impurities like iron and aluminum will also have more pronounced effects on gypsum precipitate due to its continuous concomitant formations during the entire prolonged phosphate ore dissolution process. These impurities would have a much smaller influence over the crystallization process if there was no phosphate ore dissolution involved (when present in the ionic form).
The complexity of the concomitant phosphate ore digestion and gypsum precipitation resulted from the effects of above mentioned factors is further aggravated by the variations in the chemical compositions and the reactivity of the phosphate ore, its particle size and size distribution, as well as agitation, acid concentration, solids loading, residence time, rate of product slurry recirculation, and temperature. The process has actually become diffusion controlled and the availability of water at the vicinity of the dissolving phosphate ore particles evolves to be a significant factor in determining the filterability of gypsum crystals. The thermodynamics of the reactions on the other hand appears to play a lesser role toward the process efficiencies. So far, the effects of the above mentioned factors on the instantaneous nucleation and the subsequent crystal growth have never been fully understood for such a complicated reaction system. Up to date, because of all of these complexities, the “wet process” phosphoric acid production is greatly mystified and its control still relies on proprietary knowhows to a great extent.
In regard of the sulfuric acid addition, even with the high intensity of agitation, it is not homogenized fast enough to avoid localized supersaturations. Although short lived, such supersaturation will lead to instantaneous crystallization given the high viscosity of the slurry. Phosphate ions are more likely to be incorporated into the gypsum crystals and more gypsum fines will be produced.
The localized high degree of supersaturations, at places where the phosphate ore is added and the sulfuric acid is introduced, will not only incur high degrees of P2O5 losses, but also produces more amount of finer crystals. These fine crystals, even present in a small amount, can significantly reduce the gypsum cake filtration efficiencies, causing losses of water-soluble P2O5 value.
It is therefore ideal to eliminate gypsum precipitation taking place at both where the phosphate ore is added and where the sulfuric acid is introduced. P2O5 losses will then be minimized while the filterability of gypsum crystals is enhanced. A new process for phosphoric acid production through the “wet process” is invented. This process is composed of three separate stages: (1) digesting the phosphate ore and dispersing the sulfuric acid separately and simultaneously with the gypsum slurry that has roughly stoichiometrically balanced calcium and sulfate ions in the solution and is also at considerably low supersaturation to generate the calcium containing slurry and the sulfate containing slurry respectively; (2) mixing the calcium containing slurry, i.e., the slurry carrying the digested phosphate ore, with the sulfate containing slurry, i.e., the slurry with dispersed sulfuric acid, to form the gypsum slurry that has roughly stoichiometrically balanced calcium and sulfate ions in the solution and is also at considerably low supersaturation. Part of this slurry is subsequently used to digest the phosphate ore and part of it to disperse the sulfuric acid. The remainder part is recycled back as a dilution medium for the mixing of the calcium containing slurry with the sulfate containing slurry; and (3) filtering part of the sulfuric acid dispersed slurry from stage (1) to produce the product phosphoric acid.
The flow rate of the slurry used for digesting the phosphate ore is controlled at a level that the P2O5 contained in this slurry is about 5-40 times more than from the phosphate ore feed. The flow rate for the slurry to disperse the sulfuric acid on the other hand is maintained at such a level that the sulfate content in this slurry is appropriate for its filtration. And the amount of the slurry recirculated for the mixing of the calcium containing and the sulfate containing slurries is about 10-50 times of the sum of the calcium containing and the sulfate containing slurries to further diminish the gypsum supersaturation.
The temperatures at the phosphate ore digestion and the slurry mixing compartments are kept about 2-10° C. higher than at the sulfuric acid dispersion region. The phosphoric acid concentration in the gypsum slurry that has roughly stoichiometrically balanced calcium and sulfate ions in the solution and is also at considerably low supersaturation is about 1-5% P2O5 lower than in the final slurry to be filtered.
The key differences of the present invention from all the previous efforts and practices are:
The slurry containing digested phosphate ore is kept at a higher temperature than the slurry with dispersed sulfuric acid. Due to the higher operating temperature possible for the phosphate ore digestion in the present invention, the total amount of heat to be removed from the system is less than the current practices. Less amount of slurry entrainment in the cooling system is therefore expected.
This invented process can be employed for both the dihydrate process and the hemihydrate process. All the existing plants can easily be converted into this process with minimal modifications to redirect the slurry flows in the reactor. Furthermore, a dihydrate production using this process can be freely converted into a hemihydrate production with no significant equipment modifications.
