Phosphate Rocks Chapter 25: Phosphoric Acid

Phosphoric acid is made by reacting phosphate rock with a strong acid^(C). Hydrogen from sulphuric acid swaps places with calcium in the rock, giving solid calcium sulphate, which is known as gypsum, and liquid phosphoric acid. A filter then separates the solids from the liquid. 

The reaction is easy. But the trick to making phosphoric acid is good filtration, and the trick to good filtration is crystallisation. Large, well-formed crystals with even shape and size allow the liquid to drain easily. Unfortunately, calcium sulphate crystals come in several shapes and sizes: a mixture of awkward needle-shaped crystals, rhomboid lozenges, x-shaped swallowtail twins and thin flat plates. Small crystals plug the gaps between large crystals, tiny crystals get in-between, blocking the filter cloth, and before long the liquid has nowhere to go. The reaction conditions determine the crystal structure of the gypsum which in turn determines the ease of filtration. 

After establishing the ideal reaction conditions to make good crystals, you still have to separate the solid from the liquid. 

There are many types of filter for continuous processes, but the tilting pan Prayon filter is a mechanical work of art. Imagine a huge clock face, the diameter of four tall men lying head to toe. Imagine that inside the fixed clock face is a giant rotating bicycle wheel, divided into twelve segments by spokes. Each of the segments is filled with tea strainers. Now imagine that the huge bicycle wheel is rotating slowly. At twelve o’clock there is a teapot, constantly pouring a mixture of freshly brewed tea and tea leaves onto the segment as it passes underneath. As the strainers move round, the strong tea drains through and only the damp tea leaves are left. At six o’clock there is a kettle, constantly pouring hot water onto the damp tea leaves to make weak tea, which is collected before nine o’clock and poured back over the leaves at three o’clock 

Underneath the turning tea strainers there are a series of collectors, cups, jugs, a vacuum cleaner, hairdryer and a dustbin. 

Under one and two o’clock there is a porcelain cup: this is where the strong tea, the product, will collect. There is a blank under three o’clock where the weak tea is added to wash the tea leaves. Under four and five o’clock is a jug, where the weak tea will collect and be recycled back into the teapot. Under six there is a blank where the hot water is added. Under seven and eight there is a jug which will collect watery tea to be added back at three o’clock to make weak tea. Under nine and ten o’clock is the vacuum cleaner to suck the tea leaves dry. Under eleven o’clock is the bin. At this point the strainer is tipped upside down and a hairdryer blows through the bottom to push the dry, spent tea leaves into the waste. The strainer tips back and the process starts again. 

If you keep adding dry tea leaves to the teapot, but fail to add more liquid, the tea leaves stay small, they don’t stretch and swell and grow. The tea is strong, but there isn’t enough of it, and lots of valuable flavour is left unextracted in the waste tea leaves. If you add too much liquid, then the tea becomes weak and tasteless. 

OK, so now replace the tea leaves with gypsum and the tea with phosphoric acid. 

But don’t drink it. 

It’s not a perfect analogy but at least you now understand counter-current filtration, and the importance of getting the process into a perfect balance.