Shipping black phosphoric acid

Black wet process phosphoric acid contains suspended and precipitated organic material and inorganic material. Upon standing, the precipitated material (mostly gypsum) forms a cake that is very difficult to be resuspended. By adding small amounts of nitric acid to the black acid, the nature of the precipitated material is changed and it does not cake and can be easily resuspended with slight agitation. Thus, the treated black acid can be shipped and stored and it can be clarified at the destination by adding water.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is concerned with the treatment of black wet process 
phosphoric acid and with the treated acid. 
2. Description of the Prior Art 
Insofar as is now known, the treating method of this invention has not been 
proposed. 
SUMMARY OF THE INVENTION 
This invention provides a method for improving shippability of black wet 
process phosphoric acid that comprises admixing with said black acid 
between about 0.5 weight percent and about 5 weight percent HNO.sub.3. It 
also provides the thus treated black acid. 
DESCRIPTION OF SPECIFIC EMBODIMENTS 
As is well known in the art, black wet process phosphoric acid (black 
acid), obtained by treating phosphate rock with sulfuric acid, contains 
suspended and precipitated organic material and inorganic material. Upon 
standing much of this material will settle and cake, leaving a black 
supernatant liquor. After such material has settled and caked, it is 
extremely difficult to re-suspend it. 
Because of the caking of the settled material, some grades of black acid 
cannot be shipped any great distance nor can it be stored satisfactorily 
for very long. In many cases, the caked material resting on the bottom of 
a tank car, storage tank, or other container presents a serious and 
difficult problem of removal. Vigorous agitation fails to re-suspend the 
caked material. 
In co-pending application Ser. No. 177,034, filed concurrently with this 
application by the same inventors named herein, it is disclosed that black 
acid can be clarified by admixing with it a small amount of nitric acid 
and sufficient water to effect clarification. 
It is the discovery of this invention that when nitric acid is admixed with 
black acid without adding water the settled material does not cake and it 
can be readily re-suspended with gentle agitation. Indeed, it appears that 
the usual movements of a vehicle, such as a tank car, carrying nitric acid 
treated black acid will be sufficient to keep the solid material in 
suspension. It has also been found that when the nitric acid treated black 
acid reaches its destination it can be clarified by adding the required 
amount of water thereto. 
Generally, concentrated nitric acid containing about 70-72% HNO.sub.3 can 
be used. More concentrated acid can be used, however, including fuming 
nitric acid containing 90% or more HNO.sub.3. The amount of nitric acid 
(HNO.sub.3) used is generally between about 0.25 weight percent and about 
5 weight percent, based upon the total weight of the final mixture, and 
preferably between about 0.5 percent and about 3.5 percent. The amount 
used will depend somewhat on the nature of the particular black acid to be 
treated, i.e., whether it is a "clean" acid (relatively low impurity 
content) or a "dirty" acid (relatively high impurity content). 
In the following examples, two black acids were used. Acid A was a typical 
"clean" acid and Acid B was a "dirty" acid. Typical analyses of these 
acids, in weight percent, are estimated as follows: 
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Acid A 
Acid B 
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P.sub.2 O.sub.5 53.6 50 
H.sub.3 PO.sub.4 74 70 
H.sub.2 SO.sub.4 2 5 
Other impurities 2 5 
H.sub.2 O 22 20 
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In the following examples, all parts and percents are by weight. 
EXAMPLES 1 THROUGH 3 
Blends of Acid A and 70% nitric acid were prepared as follows, expressed in 
parts: 
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Example Acid A 70% Nitric Acid 
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1 99.3 0.7 
2 98.6 1.4 
3 97.2 2.8 
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After standing for about 66 days, each blend had an off-white precipitate. 
Blends 1 and 2 had clear light green supernatant liquor and Blend 3 had a 
slightly hazy supernatant liquor. The precipitated material in all blends 
did not cake and was readily re-suspended with mild agitation. 
EXAMPLES 4 THROUGH 6 
Blends of Acid B and 70% nitric acid were prepared as follows, expressed in 
parts: 
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Example Acid B 70% Nitric Acid 
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4 99 1 
5 97 3 
6 95 5 
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After standing for about 131 days, all blends had a light colored 
precipitate and a very dark supernatant liquor. The precipitated material 
in all blends did not cake and was readily re-suspended with mild 
agitation. 
Although the present invention has been described with preferred 
embodiments, it is to be understood that modifications and variations may 
be resorted to, without departing from the spirit and scope of this 
invention, as those skilled in the art will readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the appended claims.