About Phosphates

Introduction

“Phosphate” fertilizers contain elemental phosphorus (P) in its oxide form. There are a number of different phosphate fertilizers, all containing different levels of phosphate. The level of phosphate in fertilizers is expressed in terms of their P2O5 (diphosphorus pentoxide) content in order that different products can be compared. The most common phosphate fertilizers produced are:

  • Single superphosphate or SSP (18 – 20% P2O5)
  • Triple superphosphate or TSP (46% P2O5)
  • Monoammonium phosphate or MAP (generally ~51-52% P2O5)
  • Diammonium phosphate or DAP (46% P2O5)

Some history

It is generally accepted that the modern fertilizer industry stems from the work of the German scientist and philosopher Justus Von Liebig (1803-73). His Law of the Minimum states that if an element essential to a plant is lacking, growth will be poor even if all other elements are freely available to the plant. If the deficient element is supplied, then growth will increase proportionate to the amount that is supplied, until such a point that it is no longer the limiting factor. It was Von Liebig that recommended treating bones (a source of calcium phosphate) with sulphuric acid to make the phosphate more accessible to plants. He also suggested that nutrients such as phosphate, potash and lime could be produced in chemical factories.

It was in 1840 that it was discovered that phosphate-containing rock could, similar to bones, be treated with sulphuric acid to produce an effective phosphate fertilizer. The first commercial production of superphosphate (SSP) was by an English company, Lawes, in 1842. Expansion of the industry was then rapid – 10 years later there were 14 producers in the UK and some others elsewhere, and by 1870 there were over 80 production plants in the UK alone.

The essential processes to concentrate superphosphate to produce a double (~33% P2O5) or triple superphosphate were developed in Germany in the 1870s, but TSP did not become an important product until after the Second World War. Similarly the benefits of ammoniated phosphates were known through most of the 20th Century, but the products (MAP and DAP) did not become popular fertilizers until the 1960s.

 

Phosphate Rock

Phosphate production begins with phosphate rock. “Phosphate rock” is a generic term which covers both unprocessed rock in the ground, as well as product which has been mined and processed to produce a product which has a consistent quantity of contained P2O5 (also sometimes referred to as ‘concentrated phosphate rock’).

There are two basic types of phosphate rock – sedimentary rock and igneous rock. Igneous (volcanic) deposits are comparatively rare. They are characterised by having generally low levels of contained phosphate in the ground (typically between 3% and 10% P2O5), but they beneficiate (concentrate) very well, producing concentrated rock of typically 35% to 38% P2O5, and can go above 40%. Commercial production of igneous rock takes place in Russia, South Africa, Brazil, and Finland. There are also resources running north from South Africa in the Rift Valley, for example in Zimbabwe and Uganda which have been exploited from time to time.

Sedimentary deposits are more widespread, and are characterised by having a wide range of chemical and physical forms, but those which are commercially exploited tend to occur in seams of more than 2m in depth, and to be between 15% and 25% contained P2O5 in the ground (although they have been commercial as low as 10%). Concentrating the rock can be more complicated than with igneous rock, and sedimentary rock tend to be concentrated to between 28% and 34% P2O5. Sedimentary rock tend also to contain more deleterious elements, for example heavy metals, than igneous rock. Sedimentary rock is mined commercially in many countries around the world, of which Morocco, the USA, China, Tunisia, Algeria, Egypt, Jordan, Syria, Senegal, Saudi Arabia, Togo, Australia and Peru are probably the most important.

The amount of resources and reserves of phosphate worldwide is a subject of some conjecture – new resources are discovered from time to time. The International Fertilizer Development Centre (IFDC) produced a review in September 2010 which is probably the most comprehensive currently available (http://www.ifdc.org/Publications/Technical-Bulletins/T-75-World-Phosphate-Rock-Reserves-and-Resources/) which gave world reserves estimate of 60,000 million tonnes, and global resource estimate of 290,000 million tonnes. Morocco is the most important long-term source, with 51,000 million tonnes of the 60,000 million tonnes reserve estimate, and 170,000 million tonnes of the 290,000 million tonne resource estimate.

Production Processes

Phosphate rock is mined, generally in open pit mines, but underground mining does occur. The rock is then beneficiated (concentrated) using processes such as crushing, grinding and flotation. Where practical impurities are also removed. Impurities that need to be regulated to produce a usable product include:

  • Iron – Less than 1% measured as Fe2O3 ideal
  • Aluminium – Less than 1% measured as Al2O3 ideal
  • Magnesium – Less than 1% measured as MgO ideal
  • MER Ratio – the ratio of the combined Fe2O3 + Al2O3 + MgO to the contained P2O5 should be in a range between 0.05 and 0.12.
  • Calcium Oxide – the ratio of calcium oxide (CaO) to P2O5 should ideally be in the range of 1.3 – 1.6% CaO: 1 P2O5
  • Fluorine – Less than 4%
  • Silica – Less than 5%
  • Chlorine – Less than 500 ppm
  • Organics – less than 1%
  • Heavy Metals (eg. Cadmium) – less than 5 ppm.

Phosphoric Acid (H3PO4) can be produced by either a wet process or a thermal process. In the latter elemental phosphorus is produced by reducing rock in an electric arc furnace. The phosphorus is then burned in a counter current of steam, where it reacts first with oxygen to produce pure P2O5, which then reacts with the water to produce phosphoric acid. The process has the advantage of producing very pure phosphoric acid, but is expensive due to the very high levels of electricity required. In wet processes phosphate rock is acidulated with a strong acid, typically sulphuric acid (H2SO4) but nitric acid (HNO3) or hydrochloric acid (HCl) can also be used. The basic reaction is one where the acid reacts with the calcium fluorophosphate in the rock to produce phosphoric acid, calcium sulphate (if sulphuric acid is used), and hydrogen fluoride:

Ca10F2(PO4)6 + 10 H2SO4   10CaSO4.nH2O + 6H3PO4 + 2HF

There are a number of different processes available depending upon how the calcium sulphate crystallises (dehydrate, hemihydrate etc.). Dihydrate processes produce acid of around 30% P2O5, which are then concentrated to 54% (standard commercial fertilizer-grade phosphoric acid). Hemihydrate processes produce more concentrated acids.

SSP is produced by mixing finely ground phosphate rock with a diluted sulphuric acid (68% to 75% H2SO4). The fluid mixture then goes to a den where it solidifies over a period of between 30 minutes and 4 hours. It is then removed from the den for final curing, which may take between 2 and 6 weeks, where all reactions are completed. It is then broken up and packed. The mixture can also be granulated either before or after final curing should granular SSP be required.

TSP is produced in the same way as SSP, except that phosphoric acid is used instead of sulphuric acid. The denning time is shorter – typically 10 to 30 minutes, but overall final curing time is much the same at 3 to 6 weeks.

Ammoniated Phosphates – MAP and DAP – Phosphoric acid is neutralised with ammonia to produce a slurry of ammonium phosphate. The proportions of ammonia and phosphoric acid used will largely determine whether MAP or DAP is produced. These days most production is done in a pipe reactor, and the slurry is fed into a granulator to produce granulated product.

 

Demand

Phosphates are used in 3 main markets:

  • Fertilizers account for over 87% of all demand
  • Animal feed supplements account for around 8% of demand
  • Industrial and food-grade applications account for around 5% of demand

Based on data from the International Fertilizer Industry Association (IFA – see www.fertilizer.org) demand for phosphate rock in 2011 was just over 190 million tonnes rock, containing around 58.6M tonnes P2O5.

Of the 190 million tonnes of rock used:

  • Around 150 million tonnes was used to produce phosphoric acid
  • Around 30 million tonnes was used to produce SSP and fused magnesium phosphate (FMP) for fertilizers, and defluorinated phosphate (DFP) for animal feeds.
  • Around 5 million tonnes was used as a direct application slow release fertilizer
  • Around 5 million tonnes was used to produce elemental phosphorus.

 

Supply

Around 38 countries currently mine phosphate rock, whilst slightly more – 41 – have the capacity to produce concentrated phosphate fertilizers such as MAP, DAP and TSP. The top nine rock producers control around 48% of the world’s rock capacity, whereas the same nine producers only control 34% of MAP, DAP and TSP capacity.

Animal Feed Phosphates

The main animal feed products produced are:

  • Dicalcium phosphate (DCP)
  • Monocalcium phosphates (MCP)
  • Defluorinated feed phosphate (DFP)

Total demand for feed phosphates is up to 3.5M tonnes P2O5, or up to around 8.5M tonnes of products.

Percentage of Feed Phosphate Market in Key World Areas

 

Capacity

Production %

Consumption %

China

29%

30%

26%

USA

16%

18%

10%

 

 

 

 

West Europe

16%

15%

16%

Brazil

10%

12%

13%

Others

29%

25%

35%

 

Industrial and Food Phosphates

Industrial and food-grade phosphates are generally produced with phosphoric acid of greater purity than either fertilizers or animal feed products. Wet process fertilizer acid can be purified by solvent-extraction processes. In addition, pure grades of thermal acid can be made. Industrial and food-grade acids can be used directly in applications ranging from metal treatment to the production of cola beverages, or made into a range of phosphate salts.

The main markets for industrial and food-grade phosphates are:

  • Detergents, cleaners and soaps
  • Metal treatment
  • Water treatment
  • Speciality (fully soluable) fertilizers
  • Food and Beverages
  • Toothpaste and pharmaceutical

There are a very wide range of other applications phosphates are used in such as fire retardants. Some new applications, most particularly the potential use of phosphoric acid in batteries for electric and hybrid vehicles is generating significant interest.

Total demand for industrial and food phosphates has been either static or declining over the last five years. Historically the largest single market has been the use of sodium tripolyphosphate (STPP) in detergent formulations (both laundry and dishwash). Due to environmental concerns which have been hotly debated since the 1970s these detergents are steadily being replaced with non-phosphate formulations. Although growth in other sectors has been good, especially in food applications, the net effect on demand has been at best neutral over the last five years.

China is an important global producer of STPP and other basic phosphate salts. The major Western producers (e.g. ICL Performance Products, Innophos, Chemische Fabrik Budenheim, Prayon) have increasingly concentrated on providing added-value formulated solutions to their customers.

 

Data Sources:

  • IFA
  • 'World Phosphate Rock Reserves and Resources', IFDC, September 2010
  • Phosphate Rock Market Outlook, P.Burnside, CRU Phosphates 2012 Conference
  • Global Market for Inorganic Feed Phosphates, A.Jung, CRU Phosphates 2012 Conference
  • Walking on a Knife Edge: Phosphate Fertilizer Market Outlook, J.von Gurnet, CRU Phosphates 2013 Conference