The Physical Sciences Initiative The process is continuous, with the raw materials being fed in continuously and the magnesium oxide being continuously produced. Feedstock (raw materials, preparation)
The feedstock is slaked lime (calcium hydroxide) and purified sea water. These have to be produced from the raw materials, which are limestone, fresh water and sea water. The limestone and fresh water are used to produce the calcium hydroxide, which is then reacted with the purified sea water to produce magnesium hydroxide.
Treatment of raw materials
About 6 million litres of fresh water are used each day. Sulfuric acid is used to lower the pH of the fresh water to about 4. The water is then passed downwards through a tower against a rising current of air which strips off carbon dioxide.
About 1000 tonnes of limestone are used each day. The limestone is very pure calcium carbonate (over 98%), and is crushed and washed before being heated in a lime kiln to form quicklime (calcium oxide):
CaCO3 → CaO + CO2
This reaction is carried out in a rotary kiln at 1500 oC. Fresh water is then added to the quicklime to produce slaked lime (calcium hydroxide):
CaO + H2O → Ca(OH)2
About 150 million litres of sea water is used each day. The sea water is taken from the Boyne estuary for about 8 hours each day, and is stored in a large reservoir, where sand and other solid impurities settle out. It is then pumped through underground pipelines to the Premier Periclase plant, four miles upstream. In a similar manner as with the fresh water, the sea water is acidified with sulfuric acid and passed downwards through a tower against a rising current of air which strips off carbon dioxide. The sea water then undergoes further clarification to remove excess sand and other particles by a settling process.
Catalysts, or pressures other than atmospheric pressure, are not required for any of the reactions involved in the Premier Periclase process. There are no reversible reactions involved, so no compromises in relation to striking a balance between reaction rates and product yields are needed. In the reaction of sea water with calcium hydroxide
Co-products (separation, disposal or sale)
The main co-product is calcium chloride, which is in solution in the spent sea water. This water also contains excess calcium hydroxide, and therefore has to be neutralised before it is discharged to the sea. There are at present no commercially profitable co-products from the Premier Periclase process.
Waste disposal and effluent control (waste water treatment, emission control)
Combustion gases are subjected to dust removal using electrostatic precipitators, and sulfur oxides are removed using scrubber systems. The electrostatic precipitators work by electrostatically charging dust particles, which are then attracted to and removed by oppositely charged plates. Following this the gases are scrubbed with water. This process removes harmful gases and some grit.
Used seawater contains some calcium hydroxide which is neutralised using fresh degassed sea water. The water is then filtered and pumped out to sea. Extensive environmental monitoring is carried out in the plant, for example monitoring of waste gases for dust and sulfur oxides, and monitoring of effluent for pH and suspended solids.
http://chemistry.slss.ie/resources/downloads/ch_cw_magnesiumoxide.pdf
