Superphosphate Plant
The production of superphosphate involves the reaction of sulphuric acid and phosphate rock to convert insoluble phosphate in the rock to soluble phosphate, which is available for plants to use.
The ground phosphate rock, sulphuric acid, recycled scrubber liquor and/or water are all metered into a continuous Broadfield mixer where they are mixed. The major proportion of the reaction takes place in this mixer. It is also possible to add sulphur into the mixer to produce a sulphur-enhanced superphosphate.
The slurry-mix discharges onto a slow moving slat conveyor (the Broadfield Den) where it solidifies as the reaction continues. After a period of approximately 25 minutes the mix is cut out by a rotary cutter.
Fresh superphosphate is granulated by tumbling in a rotating drum to form spherical particles of various sizes. The superphosphate discharging from the granulation drum is screened to remove the desired size range while the larger particles are broken up and returned to the granulation drum. The desired size range is between 1 - 4 mm.
Belt conveyors convey the granulated superphosphate to storage where it remains in heaps for several days while the chemical reaction proceeds to completion. This period is referred to as ‘curing’.
Throughout the reaction a number of volatile compounds can evolve. These include carbon dioxide, sulphur dioxide, water vapour, silicon tetrafluoride, hydrogen sulphide and organic sulphur compounds. To minimise contaminants discharged to the atmosphere these gases are collected and scrubbed in the Den Scrubber with fluorosilicic acid, freshwater, seawater and an odour-reducing unit. The remaining gas is discharged to the atmosphere through the 45 metre high Manufacture Stack.
The scrubber system consists of four stages. In the first two stages the gas is scrubbed with fluorosilicic acid to absorb the silicon tetrafluoride. To control the concentration of the fluorosilicic acid, fresh water and spent odour scrubber and hygiene scrubber liquor is added continuously to the scrubber. This displaces an approximately equal volume of fluorosilicic acid, which is then used to dilute the sulphuric acid in the superphosphate plant to the desired strength for production. In the third stage, the gas is further scrubbed with seawater, which is then discharged to the Otago Harbour. Sodium hydroxide and ozone are utilised in a fourth stage to remove the odourous organic sulphur compounds. Make up of sodium hydroxide is controlled automatically through continuous monitoring of pH in the scrubber liquor. Ozone is injected after the fourth stage and oxidises any remaining compounds in the scrubber gas before exiting the stack.
The seawater from the third stage of the scrubber is pumped through a multi-port diffuser (Outfall 6a) into the Otago Harbour. At regular intervals, the contents of the fourth stage scrubber sump are pumped over to the second stage scrubber.
The same compounds evolved from the Broadfield mixer can also evolve from within the granulation plant, but to a lesser degree. For reasons largely of occupational health, these gases are collected, scrubbed to remove fluoride compounds and then discharged to the atmosphere through the Manufacture Stack.
The scrubber system is monitored and controlled by a computer system. The computer continually monitors a vast array of plant process parameters and essential equipment operating data. Motion sensors continually monitor plant components to ensure they are operating. The computer monitoring system assesses if the equipment and process is within a pre-set optimum range and will automatically shut down the plant in sequence if important process parameters deviate from the desired range.