Wet process phosphoric acid or furnace process phosphoric acid is mixed with a sodium compound like soda ash or caustic in liquid soaps to create a reaction mixture containing monosodium phosphate (NaH2 PO4) and disodium phosphate (Na4 HPO4) (STPP).
This mixture is then precipitated and the insoluble impurities are removed.
Due to the high thermal energy needs of the furnace process, STPP made using phosphoric acid from the wet process is frequently less costly than STPP made with phosphoric acid from the furnace method.
However, wet-processed STPP may include greater quantities of impurities, particularly silicon and magnesium, making it unsuitable for a number of applications.
In particular, it has been noted that silicon impurities and, to a lesser degree, magnesium impurities may contribute to the turbidity or cloudiness of aqueous solutions containing STPP.
STPP made from furnace process acid may be more expensive, however clear aqueous solutions of STPP are favored in some industries, such as the food industry.
Because of this, we need a better way to make STPP from wet process acid, ideally one that leaves as little silicon and magnesium behind as possible.
The STPP feedstock was previously thought to have insoluble impurities removed using precipitation processes.
To be found in Chemical Abstracts, Vol. There is a specific reference to 80, page 17044s.
It has been shown, however, that these solely precipitation procedures of purification are ineffective when the silicon and magnesium levels in the feedstock are between about 100 and about 800 ppm by weight.
However, their removal is not easily performed by precipitation without also losing significant amounts of phosphate compounds, even at these low concentrations, which may be responsible for the undesirable turbidity.
So, it stands to reason that a more advanced method of manufacturing STPP from wet process acid would need more than simply the precipitation of silicon and magnesium.
The method outlined in the present invention is geared in this direction.
THE INVENTION IN BRIEF
Thus, the present invention provides a wet-process technique for creating sodium tripolyphosphate from phosphoric acid.
The following steps are all a part of this process:
An aqueous solution of Na2 HPO4 and NaH2 PO4 containing silicon and magnesium impurities is formed by reacting wet process phosphoric acid with a sodium compound (a) (a)
(b) increasing the molar ratio of F:(Si+Mg) to at least about 5:1 by adding a fluoride component, such as NaF, to the reaction mixture;
- c) Magnesium precipitates out of the reaction mixture in large quantities;
(d) separating the reaction mixture from the magnesium precipitate to get a clean water-based solution;
(e) re-heating the purified solution at a temperature of approximately 400 °C. over 700 degrees Celsius in heat to the point where,
First, a significant amount of the fluoride and silicon in the filtered solution evaporates away; and second,
a solid sodium tripolyphosphate product is manufactured; and (ii)
The next step is to isolate the solid sodium tripolyphosphate byproduct (f).
CAUSE AND EFFECT EXPLANATION
The first step in the process of the present invention is to react wet process phosphoric acid with a sodium compound such as sodium carbonate or sodium hydroxide to produce a reaction mixture including an aqueous solution of monosodium phosphate (NaH2 PO4) and disodium phosphate (NaH2 PO4) (Na2 HPO4).
Using Na2 CO3 as the sodium source, the following reaction equations (A) and (B) explain the typical reactions that occur at this stage:
2H.sub.3 PO.sub.4 +Na.sub.2 CO.sub.3 2NaH.sub.2 PO.sub.4 +CO.sub.2 +H.sub.2 O. is a possible representation of this formula. (A)
From this, we get 2NaH.sub.2 PO.sub.4 and Na2CO3, as well as 2NaH.sub.2 HPO.sub.4, carbon dioxide (CO2), and water (H2O). (B)
The first stage in the process of the present invention involves forming the well-known chemical compounds NaH2 PO4 and Na2 HPO4 by reacting phosphoric acid with sodium carbonate or sodium hydroxide; any and all conventional reaction parameters may be utilized to achieve this.
n other words, other from using wet process phosphoric acid and a sodium source like Na2 CO3 or NaOH as starting reactants, the initial phase of this process is not limited by any particular reaction conditions.
As a reactant, the wet method phosphoric acid or any other standard aqueous solution of phosphoric acid may be utilized.
Waggman, W. H., Phosphoric Acid, Phosphates, and Phosphatic Fertilizers, 2nd Edition, Chapter 12, Reinhold Publishing Corporation, New York (2002), discusses in detail the manufacture of phosphoric acid in all its forms (1952).
All of Waggman’s contributions are cited in detail here. In conclusion, the wet method involves digesting natural phosphate ore with sulfuric acid in an aqueous solution to make phosphoric acid.
Gypsum (CaSO4.2H2 O) is what’s left behind when the phosphoric acid is removed.
Wet process acid, as was previously indicated, might include a broad variety of pollutants.
Silicon, magnesium, and fluoride concentrations in wet process acids are shown in Table A.
The 50-1000 ppm silica range
Magnesium, 50-1,000 mg
Varying Fluoride Content
Phosphoric acid in aqueous solutions containing between 15% and 25% P2 O5 by weight is preferred for use in the present invention.
The sodium compound utilized as a reactant in the present invention, such as Na2 CO3 or NaOH, is readily accessible for commercial application.
This reactant is often derived from very pure grades (i.e., more than around 98% by weight) of soda ash or caustic due to its low degree of chemical compatibility.
However, other salts, such as sodium bicarbonate, may still be employed with the present invention.
In a first reaction tank, wet process phosphoric acid is reacted with sufficient Na2 CO3 or NaOH to produce an aqueous solution of NaH2 PO4, as described in equation (A) above. This pair of reactants works best with a 2:1 molar ratio.
If more Na2 CO3 or NaOH has to be added to the first reaction tank, it is usual practice to transfer some of the NaH2 PO4-containing aqueous solution to a second tank first.
To illustrate, the equation depicts the formation of an aqueous solution when disodium phosphate is dissolved in water (B).
Also, for this second reaction, the optimum molar ratio is around 2 moles NaH2 PO4 to approximately 1 mole Na2 CO3.
The contents of the first two vessels are combined in the third vessel at a molar ratio of Na2 HPO4 to NaH2 PO4 of between 1.6 and 1.75, preferably between 1.65 and 1.7, and most ideally at 1.67. Using an aqueous solution containing Na2 HPO4 and NaH2 PO4 with a molar ratio of around 1.67:1 will guarantee a high STPP concentration in the final product.
Many of these operations, such as mixing and reactions, need temperatures higher than 60 degrees Celsius. The temperature is somewhere about 100 °C