Process for production of phosphoric acid by crystallization...

Chemistry of inorganic compounds – Phosphorus or compound thereof – Oxygen containing

Reexamination Certificate

Rate now

  [ 0.00 ] – not rated yet Voters 0   Comments 0

Details

Reexamination Certificate

active

06814949

ABSTRACT:

The invention relates to a process for producing simultaneously food-grade and fodder-grade phosphoric acid by crystallizing phosphoric acid hemihydrate, H
3
PO
4
×0.5H
2
O, from a prepurified feed acid.
Phosphoric acid can be prepared by the wet process by allowing a mineral acid, usually sulfuric acid, to react with a calcium phosphate concentrate, whereby a dilute phosphoric acid, containing approximately 30% P
2
O
5
, and a calcium sulfate precipitate are formed. After filtration the acid contains anionic impurities of many kinds, such as sulfate and fluorine compounds, as well as cationic impurities, of which the most significant are iron, aluminum, magnesium and calcium, and organic impurities. The quantity and type of the impurities are dependent above all on the crude phosphate used as the raw material.
In a typical wet process, the sulfuric acid reacts with the calcium phosphate concentrate. When the formed gypsum is separated by filtration, there is obtained an acid containing approximately 30% P
2
O
5
, which is further concentrated to a concentration of approximately 50-55% P
2
O
5
. Most of the impurities of the phosphate concentrate remain in the phosphoric acid, and therefore this acid is used typically only as a fertilizer raw material.
Many kinds of purification processes have been developed for the purification of wet phosphoric acid in order to lower the impurity concentrations to such levels that the acid could be used for purposes even other than the preparation of fertilizers. The known purification processes include solvent extraction, precipitation by means of a solvent, indirect purification, and ion exchange methods.
The prevailing method of preparing a pure food-grade or industrial-grade phosphoric acid is to separate the impurities from a wet-process acid by extraction with an organic solvent (Davister, A., Martin, G., From wet crude phosphoric acid to high purity products, Proc. Fertilizer Soc., (1981), No. 201).
Phosphoric acid is also prepared by a thermal furnace process, wherein elemental phosphorus is prepared from crude phosphate and carbon, and the elemental phosphorus is burned to phosphorus pentoxide and is hydrolyzed to pure phosphoric acid. This process is expensive and energy-intensive.
There are also known methods for purifying phosphoric acid by crystallization. In many of these processes there is first prepared an intermediate, such as urea phosphate (DE-A-25 11 345) or aniline phosphate (WO 97/00714), which in the further reaction forms phosphoric acid and a recyclable reagent or byproduct. These processes are better suited for the preparation of phosphate salts, e.g. ammonium phosphate, than for the preparation of phosphoric acid.
For improvement of these processes there have been developed processes in which phosphoric acid itself is crystallized, either as hemihydrate, H
3
PO
4
×0.5H
2
O, (EP-B1 0 209 920) or in anhydrous form, H
3
PO
4
(GP-A 2 078 694). In these processes the purification result has not been very good, or a very strong acid (68% P
2
O
5
) has been required. A Russian patent (RU-C1-2059570) discloses a crystallization process in which a sufficiently pure food-grade phosphoric acid is obtained only through four crystallizations. There are also known processes (JP application 3-237008 and JP application 3-193614) in which an electronics-grade acid is prepared by phosphoric acid crystallization from a food-grade phosphoric acid.
Fodder-grade phosphoric acid is commonly prepared from a wet-process acid by precipitating by various precipitation processes the ions detrimental in terms of the targeted use and by separating the precipitates from the acid. When necessary, the acid is concentrated to the desired concentration.
In extraction or crystallization processes there is required a pretreatment removing solids and ions poorly removable in the process. There is often the problem of a separate process step and an impurity-containing side stream for which it is necessary to find some targeted disposal.
From the literature there are known a number of processes by which individual ions (e.g. As, SO
4
) are precipitated from phosphoric acid.
There are many methods for removing magnesium from phosphoric acid (FI patent 95905, U.S. Pat. No. 3,642,439, U.S. Pat. No. 4,299,804, U.S. Pat. No. 4,243,643 and U.S. Pat. No. 3,819,810). The methods require separate processes and in general produce a byproduct. It has also been proposed that magnesium be removed from a concentrate by, for example, preliminary leaching (Chemical Engineering, September 1992, p. 23). In this case, also, a separate multiple-step process is required. According to Becker (Pierre Becker, Phosphates and Phosphoric acid, Raw Materials, Technology and Economics of the Wet Process, 1983, p. 432), however, there is no easy and economical method for removing magnesium from phosphoric acid.
There are a number of problems involved in the preparation of purified phosphoric acids. The thermal process has the disadvantages of a high energy consumption and additional equipment for preventing detrimental environmental emissions. The extraction requires a large unit in order for the complicated process to be profitable. Many new processes have had problems in achieving a sufficiently high quality.
The purification processes typically produce problematic side streams containing large quantities of impurities, and these streams need to be treated separately to produce byproducts or be taken to a dump.
In processes using a recyclable extraction or precipitation chemical the spent reagent regularly needs to be replaced with fresh chemical. In addition, there is always the possibility that toxic or otherwise detrimental residues are left in the product from the chemical.
In phosphoric acid crystallization processes, the purification has required several steps (4-5) for the achievement of the food grade. Crystallizations of anhydrous phosphoric acid, H
3
PO
4
, have additionally had the disadvantage of the high concentration (>68% P
2
O
5
) required of the feed, which strongly increases the viscosity of the impurity-containing acid.
The object of the present invention is to provide a novel process wherein, through a development of the wet process, there is produced a sufficiently pure strong phosphoric acid, from which there are obtained both a food-grade phosphoric acid and a phosphoric acid suitable for the production of animal fodders.
The above-mentioned objects of the invention have now been achieved by a process of a novel type, which includes a pretreatment step wherein the magnesium, arsenic and sulfate are adjusted to levels suitable for the end products. If the impurity levels, for example, the levels of Mg and As ions, are already sufficiently low, the pretreatment with respect to these can be omitted. The magnesium level is preferably adjusted in connection with the concentrating. The arsenic and sulfate levels are adjusted in connection with the wet process. In the concentration, the fluorine evaporates to the necessary level when the acid is sufficiently pure and the molar proportion F/Si<6. Before the acid is fed to the phosphoric acid crystallization, the gypsum precipitated from it in the treatment is removed; this improves both the working of the crystallization and the quality of fodder-grade acid.
The final product acids are obtained by crystallization by a phosphoric acid hemihydrate process, and the product recrystallized therein is suitable for use in food and the mother liquor of the process is suitable for use in animal fodder.
The main characteristics of the process according to the invention are given in accompanying claims
1
-
10
.
Thus, according to the invention, there is provided a process for producing food-grade phosphoric acid by crystallizing phosphoric acid hemihydrate, H
3
PO
4
×0.5H
2
O, from a prepurified feed acid, in which process the feed acid used for the crystallization is purified with the help of the following steps,
a) after the step of froth-flotation of phosphate concentrate, the concentrate is direct

LandOfFree

Say what you really think

Search LandOfFree.com for the USA inventors and patents. Rate them and share your experience with other people.

Rating

Process for production of phosphoric acid by crystallization... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Process for production of phosphoric acid by crystallization..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for production of phosphoric acid by crystallization... will most certainly appreciate the feedback.

Rate now

     

Profile ID: LFUS-PAI-O-3299182

  Search
All data on this website is collected from public sources. Our data reflects the most accurate information available at the time of publication.