Spotlight
Digging deep with phosphorus
A short news update from Ma’ aden about a phosphate fertilizer project in Saudi Arabia recently caught VW’ s attention, prompting desk research into how such fertilizers are produced. The large number of process steps, variation in conditions, plus the level of contaminants, means that a broad valve palette is required, often featuring corrosion resistant alloys.
By KCI Editorial
Production chain
The production chain for most commercial phosphate fertilizers starts with phosphate rock. Significant reserves of high-quality phosphate rock can be found in Morocco, China and US. The extracted rock is first ground before being passed through recovery units to mechanically separate out sand, clay and other impurities. This process is often referred to as phosphate beneficiation. The crushed, concentrated phosphate rock is mixed in a reactor with sulphuric acid to form a weak phosphoric acid. This particular step is widely known as the wet phosphoric acid process – WPA for short. The weak phosphoric acid( 40-55 %) can then be used to produce solid and liquid phosphate fertilizers( see box). SSP is the easiest to produce: the semi-solid resulting from the phosphate rock / sulphuric acid reaction is simply allowed to cool and cure, before being milled and screened to the appropriate size. Today, MAP and DAP are the most popular phosphate fertilizers as they are economical and have a high nutrient content. These fertilizers are produced by a subsequent reaction between ammonia and the weak phosphoric acid derived from the WPA process.
A growing segment
For a sense of perspective, data available from Market Research Future( 1) valued the 2024 global phosphate fertilizer market at around USD 70 billion and forecasts growth to USD 125 billion by 2034. Drivers include population growth, food insecurity, a decline in the quality and quantity of agricultural land, rising demand for alternative
foodstuffs, and government initiatives promoting the use of artificial fertilizers.
Corrosion
A recent technical article by Simbarashe Fashu and Vera Trabadelo( Mohammed VI Polytechnic University, Benguerir, Morocco) gives an excellent review on the development, performance and selection of stainless steels and nickel alloys for the WPA process( 2). These are attractive construction materials due to their superior mechanical and chemical integrity, toughness, ductility, versatility and easiness to fabricate. Discussing their findings, the authors state that austenitic stainless steels 904, 316 and 317 quickly corrode in reactor vessel environments and that high nickel contents are required for alloys to withstand stress corrosion cracking. A high chromium content is ideal for tube heat exchangers and circulation pumps in the concentration evaporators. High chloride concentrations, erosion, the presence of deposits, stagnant solutions and high mechanical loading are challenges in the filtration area, hence grades such as 316L, 317L and duplex stainless steels are frequent selections.
Valve offerings
Several manufacturers have dedicated information available concerning valves for
A quick guide to phosphate fertilizers: MAP = mono-ammonium phosphate DAP = di-ammonium phosphate SSP = single superphosphate TSP = triple superphosphate
52 Valve World May 2025 www. valve-world. net