Located in Kwekwe, Sable Chemical Industries Limited is Zimbabwe's sole manufacturer of nitrogen-based fertilizer, ammonium nitrate (AN). The company was incorporated in 1965 and started operations in 1969 using ammonia, a key raw material in the fertilizer making process, which at that time was imported through Sasol, South Africa. In 1972, Sable commissioned its own ammonia production facility via electrolysis of water. This saw the import quota reduce to 30% of total raw materials required to make AN. The nameplate capacity of the plant is 240,000 tonnes (240,000 long tons; 260,000 short tons) per annum of AN.
An air separation process is used to separate nitrogen from air for downstream use in the ammonia making process. By-products in the form of gaseous and liquid oxygen as well as small quantities of liquid nitrogen are formed. Five basic unit operations make up the air separation process: air compression, air purification, heat exchange, liquefaction and distillation. Nitrogen is compressed to 30 bar (3,000 kPa) prior to export to the ammonia making plant.
The purpose of the water electrolysis process is to produce hydrogen for use in ammonia making. The process consists of 14 electrolytic units. Electrolysis of water at Sable is a process whereby 6.5 kA direct current is applied across each electrolytic unit to separate water into hydrogen and oxygen both at 35 °C (95 °F) and 30 bar (3,000 kPa). An electrolyte consisting of 25% wt/wt potassium hydroxide, the other part being demineralised water, is used as feed to the electrolytic units. 90 MW (120,000 hp) are required to run all electrolytic units at capacity.
The ammonia making process is used for the manufacture of nitric acid and ammonium nitrate fertiliser. Using the Haber process, nitrogen from the air separation process and hydrogen water electrolysis are reacted over an iron catalyst at 320 bar (32,000 kPa) and 520 °C (968 °F) to produce ammonia gas. Ammonia is then liquefied and pumped to one of the two 1,000[clarification needed] storage spheres.
The nitric acid process produces nitric acid for use in making ammonium nitrate fertiliser. Using the Ostwald process, ammonia is vaporised and then oxidised over a 95% platinum and 5% rhodium catalyst at 930 °C (1,710 °F) and 6.5 bar (650 kPa) to form nitric oxide and superheated steam. The reaction gases are cooled to 38 °C (100 °F) before absorption by various heat recovery mechanisms. The cooled reaction gases are then passed through the bottom of an absorption column where a stream of air is added to oxidise nitric oxide to nitrogen dioxide. As the nitrogen dioxide passes up the column, it is absorbed by a stream of water flowing from the top of the absorption column to yield the desired product (57% nitric acid) which is then stored for downstream use.
The ammonium nitrate making process is a neutralisation process consisting of a reaction between ammonia and nitric acid. Ammonia is vaporised and then sparged through a proportionate amount of nitric acid to make an 83% wt/wt ammonium nitrate solution. The solution is concentrated to a 99% melt by vaporising water using two evaporators in series. 0.6% magnesium oxide is added to the melt. The melt is then pumped to the top of a prill tower into a prill pot from which it is sprayed against an approaching stream of air from the bottom of the tower, in the process making solid ammonium nitrate prills. These prills are then coated with a 0.15% dolomite-based coating agent prior to bagging and dispatch.
