Diammonium Phosphate ( DAP ) Production from Saudi Arabian Phosphate Deposits
The ability of the western world to feed itself depends largely on large mechanised farms using fertiliser to boost soil productivity. The principle fertiliser is mainly phosphate-based and extracted from Florida or deposits that stretch along the southern Mediterranean and into the Middle East. At the eastern end of this line of deposits, in northern Saudi Arabia, one of the world’s largest reserves of phosphates is about to be exploited. The fertiliser this produces will move the country away from dependence on oil for export earnings and at the same time boost agricultural productivity in the export markets.
Phosphates are sedimentary rocks, formed from the remains of long-dead organisms. Around 60 to 70 million years ago the Mediterranean and its margins marked the site of a large sea that separated Europe and Asia from Africa. This sea, known as the Tethys, extended across the Middle East and North Africa and was marked by a strong westerly current that flowed across an area that begins in north-west Saudi Arabia and continues through Jordan, Egypt, Tunisia, Algeria and Morocco. Together they represent the world’s largest phosphate province, for this current deposited decaying organic remains of plants and animals (mainly plankton) into a sedimentary series containing shales, dolomites, and limestones. Upon diagenesis this ocean organic ooze became phosphorus deposits.
As a result of this the Tethys sea, Saudi Arabia hosts some of the largest, but undeveloped, phosphate rock deposits in the world. Within the Kingdom, limestones, cherts and phosphate rocks, known as the Turayf Group, were laid down in flat-lying sequences extending over great distances. Subsequent earth movements associated with the opening of the Red Sea have preserved the phosphate-bearing beds in a series of grabens or downfaulted basins. Each of the three formations of the Turayf Group (Urn Wu’al, Mira and AI Jalamid formations) has a phosphate horizon at its base extending from 2m to more than 40m in thickness. These deposits were first identified in 1965 by water boreholes located beside the Trans-Arabian oil pipeline. Subsequent investigation gave scope to the extensive nature of the deposits, each with potential for commercial development. Other deposits suitable for commercial development in the surrounding area include the Thaniyat Turayf and As Sanam deposits.
‘The time for development of these reserves has now come,’ says Dr A E Dabbagh, President and CEO of Ma’aden, the Saudi Arabian Mining Company. The reasoning is two-fold:
· Firstly, the market is right following the decline of production in Florida , which has decreased by one third during the past decade
· Secondly, the necessary transport link between the phosphates deposits and an industrial Gulf port has been approved.
Furthermore, the availability of ammonia, sulphur and energy in the region facilitates the manufacturing of fertilisers. There is also the fact that as a non‑oil based industry it will diversify the foreign exchange earnings of the Kingdom and, as it is a big undertaking with a commercial life of several decades, it will have a positive affect on the Kingdom’s economy for a considerable period. ‘It is a significant initiative,’ notes Dr Dabbagh, ‘and will result in the creation of a major mining operation.’
Ma’aden is currently developing a mining plan at Al Jalamid. Here, 213 million tonnes (Mt) of reserves underlying a mining area of 18km2 have been identified, with a good stripping ratio (2: 1) and excellent beneficiation characteristics. It is part of the Thaniyat member, and the phosphorate is as much as 17m thick (dependent on the palaeotopography), with an average thickness of 6.5m. The upper phosphate horizon, as determined from 305 boreholes, is the only one of commercial interest, as the middle and lower members require stripping ratios in excess of 5:1. The original phosphate content of these reserves was around 10%, but this has been upgraded by weathering, de-dolomitisation and calcite leaching, to between 15% to 32%, averaging 21%. The phosphorite is visually similar to the adjacent horizons, with mining cutoff established by chemical analysis at 15% P2O5.
The deposits will be exploited by drilling and blasting, while draglines will remove the overburden and front-end loaders and 85 tonne trucks will transport the ore to a semi-mobile crushing station.
Milling to provide a fine-grained slurry will then take place, before conversion to diammonium phosphate (DAP) fertiliser.
Processing tests conducted on Al Jalamid ores favour the use of flotation. Using a wax‑type reagent, the calcite and dolomite is floated from the phosphorate to produce 4.5 million tonnes per year (Mt/y) of concentrate averaging 32.5% P205 from 11.2 Mt/y of mined ore. Energy for the project will be made available via turbine generators fired by fuel oil, with an installed capacity of 28MW. The mining and processing are all designed to use the minimum of capital and cash costs in this remote area.
In any desert environment water is of primary concern. Extensive drilling has determined processing quality water is available from local boreholes. Hydrological modelling of the underlying Tawil aquifer has revealed a sustainable flow rate of 13 million m3 per year, more than sufficient for processing. A reverse osmosis plant will provide potable water.
The mine-produced concentrates will require transport to Jubail via a new rail link, where the Kingdom already has a plant capable of producing one million tonnes of DAP annually from imported phosphoric acid. This will require enlarging, including the sulphuric acid, ammonia and granulation plants, to convert the concentrate to DAP. These chemical plants will be become the largest in commercial operation, converting the 4.5 Mt/y of concentrates into 2.9 million tons per annum of DAP In brief, the phosphate concentrate will be dissolved in sulphuric acid to produce phosphoric acid (H3PO4) The sulphur from the acid combines with the calcium phosphate concentrate to form a solid waste product, gypsum (CaS04) separated by filters. To the phosphoric acid, ammonia (NH3) from natural gas is added to produce diammonium phosphate or DAP This reaction produces a ‘soft solid that can be dried and granulated to about 3mm to form a easily-handled product. The typical DAP fertiliser is adjusted with a filler to arrive at the industry standard of 18-40-0 (N-P-K).
To maintain the planned production of 2.9 Mt/y of DAP, four sulphuric acid plants, three phosphoric acid and three DAP plants will be required. However, Jubail already has a large industrial infrastructure with ready supplies of sulphur and natural gas from established petrochemical plants, together with electrical power and a skilled workforce, so the development of the necessary plants should be straightforward.
Throughout the development due regard has been paid to protecting the environment. At the mine site much of the process water in milling and flotation is recycled. The overburden and processed tailings are returned to the mine, infilling the cut ore panels. Sulphuric acid plant studies have revealed that less than 2kg of S02per tonne are produced - at the phosphoric acid plant less than 10 grams of fluorine per tonne of phosphoric acid are produced, while under 30g per tonne of fluorine are discharged from the DAP plant. These are well within the stringent standards of the US Environmental Protection Agency adopted by the project.
The cost of the entire project has been calculated at around US$2,500 million with the mine costing US$400 million and the railway link US$1,200 million. Sufficient funds exist within the Kingdom to implement this project, with the feasibility plans indicating a four year investment and construction programme to bring the project into operation and establish Ma’aden as a major player in the world phosphate industry. It would appear that supplies of fertiliser have been secured for years to come.
Source: Materials World, Vol. 11, No. 5, pp. 23-24 May 2003.
For more information on this source please visit The Institute of Materials, Minerals and Mining.