Metrohm has revealed how its 859 Titrotherm is ideal for sulphate control in the production of wet process phosphoric acid (WPA) in the fertiliser industry.
WPA is produced by treating phosphate rock ore with sulphuric acid, according to the simplified reaction: Ca3(PO4)2 + 3H2SO4 -> 3CaSO4.2H2O + 2H3PO4.
The key to the high yield production of phosphoric acid can be said to be the production of a highly filterable gypsum (calcium sulphate) crystal that washes well and contains a minimum of insoluble phosphorus (P).
In turn, the key to obtaining highly filterable, low P gypsum is tight control of the level of sulphate in the solution.
If the sulphate level is too high, the phosphate rock particle becomes coated with gypsum and the reaction becomes uncontrollable.
If the sulphate level is too low, the precipitated gypsum filters poorly and contains excessive quantities of P, leading to high P losses.
Generally, the H2SO4 level in the acid is maintained in the range 2-3 per cent.
It is therefore essential that the sulphate level of the WPA be controlled within very tight limits.
Traditional methods of sulphate analysis have failed in this task.
The gravimetric determination of sulphate by precipitation with barium ions is slow, and demands skill and care to obtain accurate results.
Various turbidimetric procedures have been developed also utilising the precipitation of barium sulphate, but these with methods accurate, precise results are notoriously hard to obtain.
Neither of these approaches meets the multiple criteria of being fast, accurate, precise, and easy to perform.
In contrast, the thermometric titration of sulphate by 859 Titrotherm with standard barium chloride solution fulfils all the required criteria.
Titrations generally take less than 30s and can be performed by process operators in plant field laboratories.
Titrations can be automated (with an 814 sample processor).
