Phosphate recovery from wet phosphoric acid purification process

The invention is a process for recovering phosphorus value from magnesium pyrophosphate containing filter media in the wet superphosphoric acid purification process. The invention comprises treating the filter cake with a salt to remove the adhering phosphoric acid without solubilizing magnesium acid pyrophosphate. The filter cake, after phosphoric acid removal, can be utilized as commercial or technical grade magnesium acid pyrophosphate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention is directed to a method of recovering valuable phosphoric 
acid from residues generated during the purification of wet process 
phosphoric acid. The invention is specifically directed to the recovery of 
phosphoric acid from a magnesium acid pyrophosphate filter cake. 
2. Related Art 
U.S. Pat. No. 4,443,421 to Hollifield et al. discloses a process for 
removing particulate impurities from aqueous phosphoric acid. In the 
process disclosed by Hollifield et al., after filtering the phosphoric 
acid as described in the reference, the filter cake can be contacted with 
a wash fluid, as for instance water, to remove entrained phosphoric acid 
from the filter cake. 
U.S. Pat. No. 4,409,194 to Petersen discloses a process for separating 
magnesium from wet process superphosphoric acid by filtration. This patent 
is incorporated herein by reference. 
Wet process phosphoric acid is conventionally prepared by reacting sulfuric 
acid and phosphate rock, followed by filtration to remove insoluble gypsum 
and other insoluble compounds. The resulting dilute, weak phosphoric acid 
containing about 26-30% P.sub.2 O.sub.5 by weight, is commonly known as 
"filter acid" and is a highly impure material containing the dissolved 
sulfates, fluorosilicates, and salts of iron, aluminum, magnesium, sodium 
and other metals. These impurities may precipitate and settle out in 
varying rates and amounts during storage or further processing of the 
dilute wet phosphoric acid. 
Concentrations of weak, wet-process phosphoric acid up to the 
superphosphate range (containing 64-72% P.sub.2 O.sub.5) is done in two 
steps. Preferably, this two-step concentration is done in separate 
equipment because of variations in temperature, corrosion and viscosity 
that occur over the total range. As a first step, it is common to 
evaporate a dilute or weak acid and to partially purify the acid by 
removal of precipitated impurities consisting of CaSO.sub.4, Na.sub.2 
SiF.sub.6, (Fe,Al).sub.3 KH.sub.14 (PO.sub.4).sub.8.4H.sub.2 O, and other 
salts to a concentration of about 38 to about 56 weight percent P.sub.2 
O.sub.5. This acid is known as "evaporator" acid, with about 54% P.sub.2 
O.sub.5 being a most common strength. 
As a second step, the partially purified evaporated acid (38-56 weight 
percent P.sub.2 O.sub.5) is further evaporated to superphosphoric acid 
containing about 64-72 weight percent P.sub.2 O.sub.5. Impurities that 
precipitate in the production of the superphosphoric acid consist of 
MgH.sub.2 P.sub.2 O.sub.7, FeH.sub.2 P.sub.3 O.sub.10, AlH.sub.2 P.sub.3 
O.sub.10 and other salts. 
Liquid ammonium phosphate fertilizer solutions are derived from purified 
wet process superphosphoric acid. Said solutions, commonly 10-34-0 grade 
(10 weight percent N, 34 weight percent P.sub.2 O.sub.5 and 0 weight 
percent K.sub.2 O), and other variations, are prepared either (1) by 
reacting superphosphoric acid containing 64%-72% P.sub.2 O.sub.5 with 
liquid and/or gaseous ammonia or (2) by reacting acid containing 54 to 60 
weight percent P.sub.2 O.sub.5 with gaseous ammonia. 
U.S. Pat. No. 4,409,194 discloses a process for removing magnesium 
impurities from the wet process acid. The process disclosed is to culture 
and precipitate magnesium from wet process superphosphoric acid in the 
form of singular, well-defined crystals, as well as some agglomerates. 
That process comprised aging wet process superphosphoric acid containing 
62%-72% by weight of P.sub.2 O.sub.5 with about 10%-45% of the P.sub.2 
O.sub.5 in the polyphosphate form, from about 4 to 80 hours with 
intermittent or no agitation. Thereafter, the aged acid from which 
magnesium has crystallized in the form of MgH.sub.2 P.sub.2 O.sub.7 is 
filtered to remove the acid therefrom. In this filtration process, as well 
as in the filtration process associated with removing magnesium impurities 
from wet process acid, a significant quantity of free phosphoric acid is 
retained in the filter cake. 
It is an object of the instant invention to provide a means for recovering 
the free phosphoric acid. It is also an object of the invention to provide 
a magnesium polyphosphate product suitable for use as a fertilizer or for 
other purposes. In addition, it is a further object of the invention to 
recover the free acid in a form suitable for use as a fertilizer or 
industrial chemical feed stock. 
SUMMARY OF THE INVENTION 
The invention comprises a process for recovering phosphoric acid from the 
wet process superphosphoric acid filtration media through suspension of 
the filter media in a sufficient salt solution to solubilize the free 
phosphoric acid contained therein, then recovering the free acid from the 
filter cake. 
DETAILED DESCRIPTION OF THE INVENTION 
The invention is directed to a process for recovery of phosphoric acid from 
the filter cake derived from the purification of superphosphoric acid 
derived from the wet phosphoric acid process. 
The filter cake treated in a manner as disclosed herein, can be any filter 
cake obtained in a process of purifying wet process superphosphoric acid 
to remove therefrom impurities comprising magnesium, iron, aluminum, and 
the like. In the process of the invention, wet process phosphoric acid is 
concentrated to about 70% P.sub.2 O.sub.5, and magnesium acid 
pyrophosphate is crystallized out of the solution. During the filtration 
process to remove this magnesIum salt, a significant quantity of free 
phosphoric acid is retained in the filter cake. 
The present invention answers the need to separate the liquid 
superphosphoric acid, adhering to the crystal surface of the magnesium 
acid pyrophosphate crystals, from the crystals. In the instant invention. 
the magnesium acid pyrophosphate is maintained in a crystalline state 
while substantially removing the adhering superphosphoric acid. 
The phosphoric acid is recovered as a usable alkali metaI or ammonium salt 
by suspending the phosphoric acid-containing wet filter cake in the alkali 
metal or ammonium salt solution with subsequent filtration and rinsing 
with the suspension solution. The alkali metal or ammonium salt used in 
the process of the invention removes the phosphoric acid but does not 
solubilize the magnesium acid pyrophosphate. The filter cake, after 
phosphoric acid removal and drying, is generally a grayish-white 
free-flowing powder which can be utilized as commercial or technical grade 
magnesium acid pyrophosphate. The filtrate solution comprising the alkali 
metal phosphate or ammonium phosphate solution can be utilized as an 
intermediate in chemical or fertilizer manufacture. 
Alkali metal salt suitable for use in the present invention can comprise 
sodium hydroxide, sodium carbonate, or its hydrates sodium bicarbonate, 
potassium hydroxide, potassium carbonate or potassium bicarbonate. 
Suitable ammonium salts for use in practicing the invention comprise aqua 
ammonia or ammonium hydroxide. The amount of the alkali metal hydroxide, 
carbonate, and bicarbonate or ammonium salt which can be utilized in the 
practice of the invention is limited only by economic and/or process 
constraints. Generally, the concentration of alkali metal or ammonium salt 
used are at least stoichiometric, but the desirable range is from about 
1.5 to about 4.0 times the equivalent amount of the superphosphoric acid 
present in the filter media. Amounts in excess of about 4.0 times would be 
uneconomical and would probably require neutralization of the alkali metal 
or ammonium phosphate solution to render it suitable for further use. 
Sufficient amounts of the alkali metal or ammonium salt is required to 
react with the superphosphoric acid adhering to the cake crystal surface 
so as to lower the pH sufficiently to suppress the solubility of the 
magnesium acid pyrophosphate crystals. The alkali metal or ammonium salt 
reacts with the phosphoric acid to form phosphate salts, water and/or 
carbon dioxide when carbonate or bicarbonate-containing solutions are 
used. 
The following Examples comprise embodiments of the invention and are not 
intended to limit the scope of the invention claimed.

EXAMPLE 1 
10 grams of wet filter cake (superphosphoric acid mother liquor/magnesium 
acid pyrophosphate mixture) was slurried with 100 ml of 20% NH.sub.4 OH 
solution for 10 minutes and then filtered. On a stoichiometric basis, 
about 2.4 grams of NH.sub.3 would be required to react with and neutralize 
the superphosphoric acid fraction to diammonium phosphate. A four-fold 
excess was used to determine its effect on the solubility of the magnesium 
acid pyrophosphate crYstal in a warm, alkaline solution. 
The filtered crystal and filter paper were then slurried again in a second 
100 ml of 20% NH.sub.4 OH solution and filtered a second time to remove 
the remainder of the acid phosphate and determine if there was noticeable 
hydrolysis of the filter cake solids. The resultant crystalline cake Was 
dried to remove surface water and to determine the magnesium acid 
pyrophosphate cake portion on a weight basis. 
The components involved in this experiment were: 
______________________________________ 
Superphosphoric Acid 
Dry, 
Wet Filter Cake 
(Clear Filtrate Acid) 
Washed Cake 
______________________________________ 
71.41% P.sub.2 O.sub.5 
73.31% P.sub.2 O.sub.5 
69.06% P.sub.2 O.sub.5 
5.93% MgO 0.31% MgO 20.21% MgO 
______________________________________ 
10 gm Wet Filter Cake+95.3 gm NH.sub.4 OH.fwdarw. Wash A+Filter Cake A 
Filter Cake A+95.3 gm NH.sub.4 OH.fwdarw. Wash B+3.15 gm Cake B (dried) 
The resultant wash solutions had an analysis of: 
______________________________________ 
Wash A 4.70% P.sub.2 O.sub.5 
46 ppm MgO 
Wash B 0.78% P.sub.2 O.sub.5 
8.9 ppm MgO 
______________________________________ 
A material balance based upon these analyses checks out fairly closely. The 
adhering superphosphoric acid taken into the wash solution should have 
contributed 182 ppm MgO to Wash A and 31 ppm MgO to Wash B. However, the 
analytical results shown above indicate that a magnesium oxide or 
hydroxide had to be formed and filtered as a precipitate with the filter 
cake. During the washing periods, it was noted that a cloudy, fine 
material existed in the solution but it was removed by the solids to give 
a clear filtrate each time. It should be noted that the theoretical 
analysis of magnesium acid pyrophosphate (MgH.sub.2 P.sub.2 O.sub.7) is 
70.19% P.sub.2 O.sub.5 and 20.10% MgO. The cake obtained in this 
experiment was a very pure form of this material. 
EXAMPLE 2 
A second experiment was performed using the same wet filter cake as in 
Example 1, with the exception that soda ash (sodium carbonate. Na.sub.2 
CO.sub.3.10H.sub.2 O) was substituted for ammonium hydroxide. In this 
experiment, 10 gm of wet filter cake was slurried with 100 ml of solution 
containing 22.5 gm Na.sub.2 CO.sub.3.10H.sub.2 O for about 10 minutes and 
then filtered. On a stoichiometric basis, about 15 gm of Na.sub.2 
CO.sub.3.10H.sub.2 O would be required to react with and neutralize the 
superphosphoric acid fraction to a disodium phosphate. Therefore, a 1.5 
fold excess was used to determine its effect on the solubility of the 
magnesium acid pyrophosphate crystal. The decrease in the stoichiometric 
quantity would also indicate a change in the chemistry if the resultant 
MgO level in the wash solutions increased significantly. 
10 gm Wet Filter Cake+120 gm Na.sub.2 CO.sub.3 Soln..fwdarw. Wash Cake 
A+Filter Cake A 
Filter Cake A+152 gm Na.sub.2 CO.sub.3 Soln..fwdarw. Wash B+Cake B (dried) 
Wash A was noticeably cloudy but the filtrate was clear. Similarly, Wash B 
had a slight haze which was separated from the solids and found to have 
MgO and silica. 
______________________________________ 
##STR1## 2.87% P.sub.2 O.sub.5 
97.1 ppm MgO 
______________________________________ 
EXAMPLE 3 
Additional testing was performed on filter cake obtained from treating 
superphosphoric acid derived from other phosphate rock sources to 
determine that this process would be equally effective. In this 
experimental run, the concentration of soda ash in the wash solution was 
varied to determine the stoichiometric effect on the resultant MgO level 
obtained in the wash solutions. 
______________________________________ 
Superphosphoric Acid 
Dry, 
Raw Cake (Clear Filtrate) 
Washed Cake 
______________________________________ 
68.09% P.sub.2 O.sub.5 
70.91% P.sub.2 O.sub.5 
69.06% P.sub.2 O.sub.5 
6.63% MgO 0.26% MgO 20.21% MgO 
______________________________________ 
10 grams of cake were slurried in 5.1 grams Na.sub.2 CO.sub.3.10H.sub.2 
O/100 ml H.sub.2 O (Wash A) and repeated with the same for second wash 
(Wash B). This was at 0.35 times the stoichiometric level required. 
______________________________________ 
Wash A 2,300 ppm MgO 
Wash B 618 ppm MgO 
Dry Cake 3.27 gm 
______________________________________ 
EXAMPLE 4 
The experiment in Example 3 was repeated using 0.8 times stoichiometric 
amounts of Na.sub.2 CO.sub.3, and the wash was essentially neutral. The 
wash solution showed a fivefold reduction in MgO level indicating a much 
lower hydrolysis of the MgH.sub.2 P.sub.2 O.sub.7 than in Example 3. 
The adhering, purified superphosphoric acid retained in the wet filter cake 
represents 48 to 75% of the P.sub.2 O.sub.5 loss encountered in the 
magnesium removal process. By treating this wet filter cake in a 
countercurrent, ammonium or alkali metal solution of equivaIent excess, 
the purified P.sub.2 O.sub.5 in acid form can be separated and recovered 
from the MgH.sub.2 P.sub.2 O.sub.7 crystal. This can be done using 
ammonium hydroxide, sodium carbonate, sodium bicarbonate, or equivalent 
potassium salt solutions. 
The results obtained with this process will reduce the total P.sub.2 
O.sub.5 losses encountered in the magnesium removal process to a 7 to 15% 
P.sub.2 O.sub.5 level, the composition level of the pure magnesium acid 
pyrophosphate initially crystallized. The resultant solutions may be used 
as a raw material feed into processes manufacturing fertilizer, detergent 
base sodium salts, etc., to make full use of the separation.