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2.2 Purification of non-fertiliser grade wet phosphoric acid –
<strong>the</strong> options
Description
Since, in most cases, <strong>the</strong> purified non-fertiliser-grade wet phosphoric acid is selected for <strong>the</strong>
production of inorganic phosphates, <strong>the</strong> techniques available for <strong>the</strong> purification of “green”
phosphoric acid (to remove sulphate, arsenic, fluoride and o<strong>the</strong>r impurities) need to be analysed
and compared in detail from <strong>the</strong> technical, economical, and environmental point of view in
order to reduce <strong>the</strong> impact of <strong>the</strong> production of feed-phosphates on <strong>the</strong> environment in <strong>the</strong>
whole chain of operations, starting from <strong>the</strong> intermediate phosphoric acid product and ending at
<strong>the</strong> final food-grade phosphate product.
Such a comparison needs to be carried out for two typical cases A and B pertaining to <strong>the</strong> plant
location, and for two alternative process routes X and Y available to achieve <strong>the</strong> different degree
(depth) of purification of “green” phosphoric acid:
and
A. Phosphoric acid purification at <strong>the</strong> wet phosphoric acid production site, typically (but not
always) outside of <strong>the</strong> inorganic phosphates production site
B. Phosphoric acid purification at <strong>the</strong> inorganic phosphates production site (in some cases it
may also be <strong>the</strong> site of <strong>the</strong> phosphoric acid plant),
X. Desulphation (adding lime), neutralisation (adding NaOH/Na2CO3) and purification by
precipitation, involving various techniques available for chemical purification of <strong>the</strong> wet
phosphoric acid, dependent on <strong>the</strong> required degree of acid quality
Y. Concentration, desulphation, and very deep purification of <strong>the</strong> wet phosphoric acid by
solvent extraction, involving more elaborate physico-chemical techniques.
Wet process phosphoric acid is purified by numerous methods and to a wide variety of
standards depending on <strong>the</strong> fur<strong>the</strong>r application of <strong>the</strong> acid.
The most basic method, and <strong>the</strong> one which all suppliers of merchant-grade acid carry out before
shipment, is clarification, by settling or o<strong>the</strong>r mechanical means, to remove suspended solids. In
case <strong>the</strong> acid is used for <strong>the</strong> production of fertilisers no fur<strong>the</strong>r treatment is usually applied.
Chemical purification methods can be employed if <strong>the</strong> acid is to be used for specific purposes,
not requiring a high quality. Active carbon treatment is <strong>the</strong> usual means of removing organic
impurities. Fluorine is removed by adding reactive silica and distilling off silicon tetrafluoride.
Phosphate rock or lime may be added to <strong>the</strong> impure acid to remove excess sulphate. Metals ions
can be selectively precipitated by various chemicals. By adding a Na2S solution to <strong>the</strong> acid,
arsenic can be precipitated as arsenic sulphide. Removal of o<strong>the</strong>r cationic impurities, especially
Fe, Al, Mg and Ca, can be achieved by neutralising <strong>the</strong> acid with sodium carbonate or caustic
soda. The phosphoric acid in this process is converted to a phosphate salt solution.
More elaborate techniques involving (organic) solvent treatment are used to obtain purer acid
such as that required for animal feed supplements (mainly cadmium removal) and especially <strong>the</strong>
food industry. Liquid/liquid extraction processes are most commonly used. Processes are
operated for <strong>the</strong> separation of single components (e.g. uranium and cadmium) as well as of
practically all impurities in wet phosphoric acid. The quality of such purified acid nearly equals
that of <strong>the</strong>rmally produced acid. Besides liquid/liquid extraction processes, precipitation
processes are also being employed.
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