Perspective
The Florida phosphate industry has always had a problem with unacceptable high levels of metallic impurities in the phosphate rock used in the manufacture of phosphoric acid. This problem is becoming increasingly critical as mining has moved south from Polk County and the average total impurity content of the phosphate rock has increased. The three impurities that are of most concern are iron, aluminum, and magnesium. While high levels of any one of these metals creates specific operating problems, a combined total that exceeds a certain level makes it impossible to make on grade DAP (fertilizer grade diammonium phosphate).
Phosphoric acid producers also have to contend with scale formation in all piping and equipment due to the very nature of the process and the phosphoric acid itself. This piping scale is controlled today by having two parallel pipe lines so that one can be washed with water to remove the scale while the other one is used for the acid.
This project was to develop an economical process for removing iron from phosphoric acid by means of magnetic separation. A second possibility that would be investigated was aluminum removal by paramagnetism. The other half of the project effort was directed toward eliminating scale formation in the phosphoric acid piping by magnetic means.
The project demonstrated that it is possible to partition the iron in the acid and lower the iron in a portion of the acid. However it was not possible to formulate a practical economical application for this technology at this time.
The project also demonstrated that it is possible to dissolve the scale using a magnetic field but it will require additional research before a practical application for this technology can be proposed.
A practical iron removal scheme has much to offer to the industry since it would allow the production of on grade DAP without the need for adding supplemental nitrogen. It could also allow the use of lower grade phosphate rock, reducing the quantity of rock that has to be discarded as unusable, and thereby reducing the number of acres that must be mined each year.
Elimination of and/or control of scale formation could equate to better operating rates and this increased efficiency would better position the industry to compete in the world phosphate fertilizer market.
EXECUTIVE SUMMARY
This research was directed toward the appropriate use of magnetism to effect a separation of unwanted materials from phosphoric acid. Two types of magnetism are involved:(l) paramagnetic substances have one or more unpaired electrons, and these substances will be attracted into a magnetic field; and (2) diamagnetic substances have no unpaired electrons and these substances will be repelled, weakly, by a magnetic field. The goal of this research was to use diamagnetics to prevent or minimize sodium fluorosilicate scale formation in phosphoric acid and to use paramagnetism to reduce the concentration of iron and/or chromium in phosphoric acid.
Sodium fiuorosilicate, a by-product of the production of phosphoric acid, is a nuisance material because of the tendency to build up in pipes and the need to remove the material from pipes. Samples of scale from the industry were analyzed and were tested in the presence and absence of magnetic fields (1200 and 2000 gauss). Enhanced solubility was demonstrated in the presence of a magnetic field. The effect is ascribed to an increase in entropy as a result of the field, which in turn results in an increase in the solubility of the scale. At room temperature, the solubility of fluorosilicate scale was 1.70 g/100 g, and at 50° C in the presence of a magnetic field it was increased by about 25-28%. The results were obtained at flow rates of about 100 mL/sec, and it is anticipated that the diamagnetism effect would be enhanced at higher flow rates. The control should be effected by the use of one 2,000-gauss magnet for each inch in diameter of stainless steel pipe.
The second phase was concerned with removing iron and/or chromium from phosphoric acid using paramagnetism. Some magnetic attraction designs were tested using this technique. The best approach was a two-step process applied to 54% industrial-grade phosphoric acid with 1.37 % iron, and 81.9 ppm chromium: a hydrocyclone was used as a first -stage separation of solids, and in the second stage, the supernatant was passed through a magnetic field to effect further separation. A total of 78 % iron was removed in the two-stage process without discernable loss of phosphoric acid (no statistically significant difference between the initial and final acid sample). In addition about 13 % of the chromium was removed, again without discernable loss of phosphorus.
The designs that have been described here could easily be adapted to a phosphoric acid plant to minimize scale formation and have the material precipitate at a later stage that would be more convenient for removal by hydrocyclones. The design for removing iron and chromium probably would need little modification to effect the separation, but design features such as recycling would need to be factored into the process.
Dean F. Martin et al., University of South Florida. October 1997.