Phosphohemihydrate Process for Purification of Gypsum

1. Field of the Invention

The present invention relates to the production of phosphoric acid, and more particularly, refers to an improved wet process for converting phosphate rock into a concentrated phosphoric acid with concomitant production of an improved calcium sulfate.

In conventional wet methods of producing phosphoric acid, finely divided phosphate rock is digested with mixtures of phosphoric acid, sulfuric acid and water causing a reaction between the calcium phosphate in the rock and the acid to produce phosphoric acid and a precipitate of calcium sulfate. Most wet processes are directed to producing the calcium sulfate product in the dihydrate form, or phosphogypsum. In recent years, interest has increased in the hemihydrate, or hemihydrate-dihydrate processes, because of potential for higher overall recovery of P.sub.2 O.sub.5 and refinements for a by-product calcium sulfate of particle sizes and qualities more useful for use in making gypsum plaster, gypsum wallboard and use as a setting moderator for Portland cement.

These modified processes differ from the more conventional dihydrate processes in that the calcium sulfate is first formed as the hemihydrate during decomposition of the phosphate rock at elevated temperatures (90.degree.-100.degree. C. in the Mitsubishi process and 80.degree.-110.degree. C. in the Nissan process). The hemihydrate is obtained as large coarse crystals and then recrystallized to dihydrate in separate reaction vessels to allow optimum process control. In the recrystallization, or hydration sections, the slurry of metastable hemihydrate is cooled to induce crystallization of the dihydrate, mixed with cooled, recycled dihydrate slurry from the last tank of the hydration section to provide fine gypsum seed crystals and to maintain uniform calcium ion concentration in the slurry. Under these conditions, solubility of the hemihydrate rises rapidly, and the calcium sulfate recrystallizes as dihydrate on the surface of the dihydrate seed crystals present, while a part of the dissolved hemihydrate forms new dihydrate nucleii from the supersaturated solution. The slurry is finally cooled to 50.degree. C. or lower, and fully hydrated gypsum crystals are separated. The resulting dihydrate crystals, in comparison to those of the usual dihydrate processes, are larger and thus more readily filtered, retain less P.sub.2 O.sub.5 in the crystal lattice and have lower contents of some other impurities. It is believed that about 25 picoCuries per gram of calcium sulfate (25 pCi/g) of radium-226 remain in the calcium sulfate from these processes. Typical phosphogypsum from the Prayon and modified Prayon dihydrate processes also contain about 25 pCi/g of radium-226. These contaminations have become of increasing concern.

2. Description of the Prior Art

Prior attempts for reducing radioactivity of calcium sulfate from the wet process production of phosphoric acid have focused upon treatment of phosphogypsum obtained from the Prayon and modified Prayon processes. U.S. Pat. No. 4,146,468 discloses a process for reducing radioactive contamination of phosphogypsum by slurrying it with a dilute sulfuric acid containing barium sulfate and separating the solids resultant therefrom into fine and coarse fractions. It discloses that the fine fraction predominates in the radioactive contamination. It does not describe how much radioactivity may be removed in the coarser fractions, and there is no disclosure of how much of the starting phosphogypsum is recoverable, but losses could be considerable. U.S. Pat. Nos. 3,949,047 and 4,282,192 disclose treatment of the monocalcium phosphate solution prior to precipitation of the calcium sulfate. In the former patent, the solution is treated by addition of barium compounds to reduce radium contamination; and in the latter patent, the solution is treated with a sequestering agent, and a first phosphogypsum precipitate high in radium is discarded.

There is still a need in the art for effective and economical means for removing substantial portions of the radium that originates in the phosphate rock. There is a need in the art for making calcium sulfate products that have acceptable levels of radium such that they may be utilized in gypsum wallboard and other industrial and construction materials. In addition, there is a need to provide improved processes for obtaining higher P.sub.2 O.sub.5 content phosphoric acid products; and further, to provide processes which convert previous "waste" calcium sulfate by-products into material acceptable for commercial utilization.

It is an object and advantage of the present invention to provide a process for providing a calcium sulfate product of greatly reduced radium-226 levels. Another object and advantage is the production of a phosphocalcium sulfate dihydrate which may be converted to gypsum products useful in the construction and building materials industry. Still another object is to provide a process for economically and efficiently producing increased yields of strong phosphoric acid. Strong phosphoric acid is defined as phosphoric acid of at least about 35% P.sub.2 O.sub.5 (48% H.sub.3 PO.sub.4).

Broadly speaking, the present invention resides in modifying either the hemihydrate or the conventional dihydrate wet processes so as to obtain a small hemihydrate particle, as a finely grained particle in contrast to the conventional large, coarse particles by operating in Region II for less time, e.g. 10 minutes to 1 hour. A portion of the hemihydrate particles is converted to dihydrate in the presence of large proportions of coarser dihydrate seed crystals of calcium sulfate into very coarse, virtually radiation free calcium sulfate dihydrate (gypsum) particles in Region I. The fine hemihydrate contaminated with radioactive material is easily separated from the high purity, low radiation (e.g. less than 5 pCi/g of radium-226) coarse gypsum product. Surprisingly, an extremely high purity gypsum product, containing less than 1 pCi/g of radium-226, results.

FIG. 1 is a plot derived from information in Phosphoric Acid, Vol. 1, Part 1 edited by A. V. Slack of phosphoric acid concentrations (in the further presence of approximately 1.5% sulfuric acid) versus temperature showing the states of hydration of the calcium sulfate, and the area of interest in the present invention (Region II) in heavier lines.

FIG. 2 is a labelled flow sheet diagrammatically illustrating the various vessels, all of which are conventional, employed in a preferred form of the process applied to a modified Nissan process.

FIG. 3 is a corresponding labelled flow sheet diagrammatically illustrating a second preferred form of the process applied to a modified Prayon process.

FIGS. 4a and b are scanning electron microphotographs at different magnifications of phosphohemihydrate produced by the present invention.

FIGS. 5a and b are scanning electron microphotographs at different magnifications of purified gypsum produced by the present invention.

Referring to FIG. 1, it is theorized that all phosphoric acid wet processes are carried out under conditions wherein an insoluble anhydrite is the thermodynamically stable form of calcium sulfate. However, in most commercial wet processes the crystal states actually precipitated are the metastable varieties; the hemihydrate and dihydrate. In Region I, where most commercial processes operate, the dissolution of finely divided phosphate rock takes place through the dissolving action of phosphoric acid, and to some degree sulfuric acid. The calcium ion that is brought into solution in this region combines with sulfate ion to precipitate the unstable hemihydrate. It, in turn, dissolves and recrystallizes as the dihydrate, particularly if dihydrate seed crystals are available for surface precipitation. Above the practical 28-32% P.sub.2 O.sub.5 concentrations and 75.degree.-85.degree. C. area in FIG. 1 for the dihydrate process, the calcium sulfate dihydrate becomes unstable and increasing amounts of the metastable hemihydrate are formed, especially if large amounts of hemihydrate seed crystals are present. The rate of calcium sulfate crystal growth is proportional to the supersaturation level of a high concentration of reacting calcium and sulfate ions. The solids deposited per unit of time is proportional to the available crystal surface area, or for a given crystal seed, its specific surface; a high temperature; and a low water content in the slurry (i.e., a high P.sub.2 O.sub. 5 phosphoric acid concentration). The rate of precipitation is dependent upon the temperature, solution agitation, specific surface of seed crystal, solids content of the mixture, sulfuric acid concentration, and the time allowed for phosphate rock acidulation and hemihydrate recrystallization.

In one preferred embodiment, a Nissan hemihydrate process is modified as shown in FIG. 2, the steps and conditions to the left of the vertical dotted line being generally conventional in operation of the Nissan process, producing phosphoric acid product having a P.sub.2 O.sub.5 content of about 42-50%. In the digestion attack zone, as shown in FIG. 2, generally from about 2-10 mixing cells are utilized for a slurry of about 20-50% solids. At substantially lower solids content, more phosphate ion is co-precipitated with the calcium sulfate, causing phosphate loss in the process; while substantially above 50% solids, the slurry is difficult to mix. The attack/digestion zone should be operated at a temperaure between 60.degree. C. and 110.degree. C. preferably 80.degree. C. and 110.degree. C., preferably about 95.degree. C. At substantially lower i.e. below 80.degree. C. temperatures, increased cooling capacity is needed and sufficient phosphogypsum may be formed to interfere with the process; while temperatures substantially higher are undesirable as requiring additional heat to maintain the reaction. Further, the slurry will have in the attack zone about 1-4% with about 1.5-2% being optimum, sulfuric acid concentration in excess over stoichiometric (excess, or free sulfuric); and about 62-73% total phosphoric acid and sulfuric acid contents combined as shown in FIG. 2. At total acid concentration below about a combined total of 43% P.sub.2 O.sub.5 and 4% free sulfuric acid, free water becomes available for phosphogypsum formation and tends to move the operation too far toward Region I of FIG. 1; while above a combined total of about 50% P.sub.2 O.sub.5 and 1.5% free sulfuric acid, viscosity becomes sufficiently high that increasing amounts of phosphate ion are co-precipitated with the hemihydrate causing phosphate losses in the process. After digestion, the hemihydrate phosphoric acid slurry is filtered to give a 42-50% P.sub.2 O.sub.5 phosphoric acid and a hemihydrate filter cake.

In accordance with the present invention, the hemihydrate filter cake is re-slurried with an about 10-30%, with 10-23% being preferred, P.sub.2 O.sub.5 (e.g. filtrate) and optionally passed through a first hydroclone to remove silica sands and any large particles of hemihydrate e.g., greater than about 30 micrometers average particle diameter. The oversize hemihydrate particles may be sent to the conventional hydration station, which now becomes a waste product hydrator for processing waste material to disposal. The fine particle hemihydrate phosphoric acid slurry, and optionally 1-4% free sulfuric acid, is transferred to the purified gyspum hydrator; where it is mixed with a recycle stream of purified gypsum seed crystals for conversion of the hemihydrate to gypsum and for crystal growth. The hydrator is operated preferably at about 60.degree. C., more generally about 55.degree. to 80.degree. C. The recycle gypsum seed crystals are metered in to provide preferred weight ratios of about 1-4:1 of fine hemihydrate particles:coarse gypsum seed crystals. After approximately 25% to 50% of the hemihydrate has been converted to gypsum of greater than 50 micrometers size (in about 10-60 minutes) the slurry is passed through a second hydroclone to remove the purified gypsum product. Considerable small gypsum may be generated during recycling and carried along for total hemihydrate to gyspum conversions of 60-90% without affecting purification. The recovered gypsum will be large acicular particles with diameters that may range from 50-100 micrometers or more. The purified gypsum product is filtered, washed and dried. The overflow from the second hydroclone is passed through a third hydroclone where smaller sized gypsum crystals (such as about 30 to 50 micrometers) are recycled to the purified gypsum hydrator to act as a new source of gypsum seed crystals. The overflow from the third hydroclone containing the radioactive radium-226 is passed to the "conventional" hydration station, where it is converted to a waste gypsum product, filtered, washed and passed to disposal.

In accord with the modified Nissan process diagrammed in FIG. 2, ground phosphate rock and 60% sulfuric acid were metered with a recycle stream to obtain a 45% P.sub.2 O.sub.5 (62.1% H.sub.3 PO.sub.4) phosphoric acid and 1.5% excess sulfuric acid reaction mixture at slightly above 95.degree. C. to produce a hemihydrate slurry. Ten minutes after they were added, the slurry was screened through two stainless steel sieves with 0.42 millimeter (420 micrometers or 40 U.S. Standard mesh) and 0.05 millimeter (53 micrometers or 270 U.S. Standard mesh) openings to remove silica sand of +40 mesh, and unreacted phosphate rock and coarse particles of hemihydrate of +270 mesh.

The fine fraction, smaller than 53 micrometers, was washed into a hydration tank containing purified gypsum seed crystals (greater than 53 micrometers size and in about 1.5:1 weight ratio of hemihydrate to gypsum seed) with wash water proportioned to give a hydration solution containing 15% P.sub.2 O.sub.5 and 1.5% H.sub.2 SO.sub.4.

The mixture was agitated with a motor driven stirrer at 60.degree. C. for 1 hour for 76% total conversion of coarse +270 mesh and fine -270 mesh hemihydrate to gypsum, as analyzed by Differential Scanning Calorimeter Thermogravimetric Analysis (DSC-TGA). At this time the reaction was quenched with 2 volumes of 2-propanol and the mixture screened through a 53 micrometer screen to remove the coarse purified gypsum product. The purified gypsum product, when washed with alcohol and acetone and dried at 40.degree. C., showed by X-ray diffraction analysis to be essentially all gypsum with only trace amounts of hemihydrate.

The hemihydrate obtained in the digestion stage and the coarse purified gypsum product were submitted to scanning electron microscopic examination, as shown in FIGS. 4 and 5. The relative particle sizes of the fine hemihydrate feed (FIGS. 4a and b) to the coarse purified gypsum (FIGS. 5a and 5b) is readily apparent. It is noted that FIG. 5b shows that the coarse purified gypsum product is virtually free of any dusting or sticking of contaminated unrehydrated hemihydrate. By control of the process parameters and apparently the inherent physical chemistry properties of the formed alpha hemihydrate particle, extremely clean separation of coarse purified gypsum from the contaminated hemihydrate is occurring.

Samples of the hemihydrate feed, purified gypsum product and waste fine hemihydrate were submitted to radioactivity determinations. The coarse purified gypsum product was found to contain less than 0.5 pCi/g of radium-226 calculated as an average of the 0.295, 0.352 and 0.609 MEV peaks of its lead-214 and bismuth-214 short lived progeny. These were measured at three weeks evaluation in equilibrium with the long-lived radium-226 precursor in the samples. Full analysis of the three week exposure sample was as follows:

This is a very considerable reduction in radioactivity for the product. The hemihydrate feed was found to contain an averaged 30 pCi/g radium-226, with full analyses as follows:

and the fine waste fraction contained an averaged 50 pCi/g radium-226, with analyses as follows:

In the second preferred embodiment, this process can be modified so that it can be used with conventional Prayon process for producing 30% P.sub.2 O.sub.5 phosphoric acid. The basic modification is to operate the first attack tank at about 80.degree.-105.degree. C. to produce in it an about 30% P.sub.2 O.sub.5 phosphoric acid slurry and hemihydrate instead of the conventional phosphogypsum. As set forth in FIG. 3, a 30% P.sub.2 O.sub.5 phosphoric acid slurry is extracted from the first digestion attack tank, and passed through a first hydroclone. The underflow, containing unreacted phosphate rock, silica sand and coarse particle size hemihydrate is passed back into the second digestion attack tank to continue on in the conventional Prayon process. The fines fraction of hemihydrate obtained from the first attack tank is passed to the gypsum hydrator for conversion to gypsum and crystal growth on the purified gypsum seed crystals at about 55.degree.-80.degree. C. Desirably, the product slurry is removed from the hydrator after about 25%-75%, preferably 50% conversion. The product slurry is sent from the hydrator to a second cyclone to remove, as an underflow, the purified gypsum product which is filtered and washed. The overflow from the second hydroclone is passed to a third hydroclone for separation into a smaller purified gypsum seed for recycle and to unreacted hemihydrate for return to the second attack tank.

In accord with the modified Prayon process diagrammed in FIG. 3, recycle acid, ground phosphate rock and 60% sulfuric acid can be metered into a 30% P.sub.2 O.sub.5 (42% H.sub.3 PO.sub.4) phosphoric acid solution containing 1.5% excess sulfuric acid; and the slurry held at above 95.degree. C. to precipitate calcium sulfate as hemihydrate. In this example, ten minutes after the last metered additions, a portion of the slurry was withdrawn to form a mixture of 30% P.sub.2 O.sub.5 phosphoric acid containing 1.5% sulfuric acid and gypsum seed crystals (coarser than 200 U.S. Standard mesh, greater than 74 micrometers) in the gypsum hydrator as shown in FIG. 3. The resulting slurry was stirred for 1/2 hour at 60.degree. C. to allow about 50% of the hemihydrate to convert to gypsum and precipitate on the gypsum seed crystals. The initial ratio of hemihydrate to gypsum seed crystals in the gypsum hydrator was 2:1 by weight. At this time the reaction was quenched with 3 volumes of 2-propanol filtered, washed with acetone and dried at 40.degree. C. The dried product was screened to give a purified gypsum product between 100 and 200 mesh (74-150 micrometers) average diameter of acicular particles. The purified gypsum product was analyzed by DSC-TGA and did not show any unreacted hemihydrate. The waste product was screened to a fine fraction passing 53 micrometers that showed a mixture of 33% hemihydrate and 65% gypsum. Scanning electron microphotographs of the feed hemihydrate and coarse purified gypsum product respectively showed considerable surface dusting of fine hemihydrate on the ostensibly purified gypsum product. Radiation analysis of this product showed it to be higher in retained radium-226, when the recrystallization was in 30% P.sub.2 O.sub.5 slurry in comparison to the material recovered in Example 3, with analyses as follows:

As a further optional embodiment, the Prayon process of FIG. 3 can be further modified so that the 30% P.sub.2 O.sub.5 phosphoric acid slurry extracted from digestion (attack tank No. 1) and the 22% P.sub.2 O.sub.5 phosphoric acid returning to No. 2 and 3 digester tanks can be blended together for hydration. As shown in FIG. 3 the P.sub.2 O.sub.5 phosphoric acid filtrate drawn from the product acid stream (weak acid recycle from filter) was blended with the slurry from attack tank 1, to obtain a 23% P.sub.2 O.sub.5 and 3% sulfuric acid slurry in the first hydroclone and passed directly into the gypsum hydrator. Gypsum seed was recycled to obtain a 2.5:1 weight ratio in the hydrator. The slurry was hydrated at 60.degree. C. for 3 hours. Conversion of the hemihydrate in these lower P.sub.2 O.sub.5 concentrations resulted in less phosphate impurity in the purified gypsum product. X-ray diffraction analysis showed the product to be virtually all dihydrate with only trace amounts of alpha quartz and hemihydrate. Radiation analysis was as follows:

The process as set forth in Example 3 was repeated except that the slurry from the first hydroclone was centrifuged to concentrate the hemihydrate solids entering the hydrator and water was added to the hydrator to obtain a 15% P.sub.2 O.sub.5 and 1.5% sulfuric acid slurry in the hydrator. The water to be added could be wash water from the purified gypsum filter not shown. Gypsum seed was recycled in a weight ratio of 2.5:1 in the hydrator. The slurry was hydrated at 60.degree. C. for 1 hour. Conversion of the hemihydrate in these lower P.sub.2 O.sub.5 concentrations resulted in less phosphate impurity in the purified gypsum product. X-ray diffraction analysis showed the product to be all dihydrate with a trace of hemihydrate. Radiation analysis was as follows:

In this evaluation highly effective radiation reduction was accomplished. The desired product portion of hydrated material had, within error limitations at the time of analysis, 0.4 pCi/g of radium-226 calculated as an average of the 0.295, 0.352 and 0.609 MEV values for the daughter products of radium-226 whereas the waste fraction contained 67 pCi/g of radium-226. It is noted in comparing this evaluation to the immediately preceeding ones that as the phosphoric acid concentration decreased in the hydrating steps from 30% to 23% to 15% the radium-226 count in the desired coarse hydrated fraction correspondingly decreased in greater than lineal fashion, e.g., about 7, about 2.4 and 0.4 pCi/g.

It will be appreciated from the foregoing description of preferred embodiments that various further modifications may be made within the scope of the invention. For example, the radioactivity may be further concentrated in the waste portion and the proportion of hemihydrate converted to purified gypsum increased, such as by sequentially repeating the hydration step. Thus the -53 micrometer waste fraction may be mixed with additional gypsum seed and dilute phosphoric acid to hydrate a further portion of the hemihydrate to purified gypsum and further concentrate the radiation in smaller amounts of smaller sized unhydrated hemihydrate. On sequential partial hydrations microhydrocyclones that separate particle sizes of 1-10 micrometers may be substituted for the regular hydrocyclones. In this manner total conversions of hemihydrate to purified dihydrate on the order of 90-99% may be achieved and the waste portion may be composed of 1-10 micrometer particles.