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2 PURIFICATION OF NON-FERTILISER GRADE WET
PHOSPHORIC ACID (PARTIAL INFORMATION)
2.1 Inorganic Phosphates – Introduction
In order to bridge <strong>the</strong> <strong>information</strong> to Section 2.2 “Purification of non-fertiliser-grade wet
phosphoric acid – <strong>the</strong> options” below, it was considered reasonable to present here first key
<strong>information</strong> included in <strong>the</strong> introduction to Inorganic Phosphates, Chapter 6 of <strong>the</strong> BREF on
LVIC-S, as follows.
The application of inorganic phosphates as fertiliser is addressed in <strong>the</strong> BREF on Large Volume
Inorganic Chemicals – Ammonia, Acids and Fertilisers (LVIC – AAF), whereas <strong>the</strong> BREF on
Large Volume Inorganic Chemicals – Solid and o<strong>the</strong>rs (LVIC-S) covers <strong>the</strong> production of
inorganic phosphates (refer to Chapter 6 of <strong>the</strong> BREF on LVIC-S).
In general terms, all inorganic phosphates can be seen as indirectly derived from phosphate
rock, Ca5(PO4)3F. The process from phosphate rock to final product may schematically be seen
to involve four major steps:
dissolution of phosphate from <strong>the</strong> rock to yield phosphoric acid
purification of phosphoric acid to a varying degree of purity
neutralisation of phosphoric acid by reaction with sodium, calcium, ammonium and/or
o<strong>the</strong>r ions to produce <strong>the</strong> required inorganic phosphate
dehydration, drying or calcination plus optional finishing to give a product in <strong>the</strong> required
form (eg. dry powder).
These steps may be carried out in one location, but quite commonly intermediate products are
used as <strong>the</strong> starting material for downstream steps. Therefore, when comparing several
production routes to manufacture a given inorganic phosphate product it is important to consider
<strong>the</strong> different strategies, process boundaries and starting points of <strong>the</strong> production.
Although strong mineral acids, such as sulphuric, hydrochloric and nitric acid are used for <strong>the</strong>
dissolution of <strong>the</strong> phosphate from <strong>the</strong> rock, by far <strong>the</strong> most commonly used is sulphuric acid.
Unpurified (merchant grade), usually called “green”, phosphoric acid is a market commodity
used by many producers as <strong>the</strong> starting point for fur<strong>the</strong>r processing.
Invariably, <strong>the</strong> resulting phosphoric acid stream contains impurities originating from <strong>the</strong>
phosphate rock, including a number of metals and fluoride. For most applications, <strong>the</strong>se
impurities need to be removed from <strong>the</strong> acid to obtain a certain level of purity of <strong>the</strong> product.
The required level of purity is largely determined by <strong>the</strong> final use of <strong>the</strong> phosphoric acid
product.
In some cases, <strong>the</strong> purification takes place in a dedicated plant by employing solvent extraction,
this leading to <strong>the</strong> production of a high quality phosphoric acid. Optionally, additional
techniques (for removal of arsenic, sulphate or fluoride) may be applied.
Depending on <strong>the</strong> required degree of purity of <strong>the</strong> final product, this can provide a feedstock for
<strong>the</strong> production of detergent, animal feed or human food phosphates.
Consequently, <strong>the</strong> purification of ‘green’ phosphoric acid may be quite shallow (e.g. ‘green’
acid pretreatment, virtually by desulphation only) or deep (concentration, desulphation, fluoride
and arsenic removal, and <strong>the</strong> purification of <strong>the</strong> ‘green’ acid by solvent extraction in a number
of steps – not necessarily in this order).
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