World-class pumps for phosphates production

PROCESS EQUIPMENT

World-class pumps for phosphates production

Erosion and corrosion are two of the biggest challenges faced by equipment used in phosphate fertilizer production. This harsh operating environment requires exceptionally well-designed pumps made from sophisticated and robust construction materials. Hani Tello of ITT Rheinhütte and Harvinder Bhabra of ITT Goulds Pumps outline the range of options.

Fig. 2: The heated version of the RMKN magnetic drive pump is ideal for conveying molten sulphur.

PHOTO: ITT RHEINHÜTTE PUMPEN

Fig. 1: The phosphate fertilizer production process

Pumps and materials

ITT’s two global brands, Rheinhütte and Goulds Pumps, offer a full range of metallic and non-metallic pumps. These are specifically designed to operate throughout every stage of the fertilizer production process, including the handling of:

• Molten sulphur
• Sulphuric acid
• Mined and beneficiated ore
• Phosphoric acid
• Phosphate fertilizers.

The company’s robust and proven range of pumps, by incorporating world-class materials, delivers extended operational intervals between maintenance.

ITT offers many decades of applications experience and provides a single source of product expertise covering:

• Pump types: ANSI/API, DIN/ISO, slurry, axial flow
• Advanced materials: both metallic and non-metallic
• Seals: magnetic drive, hydrodynamic seals, mechanical seals.

Phosphate fertilizer production incorporates several discreet yet interlinked processes to obtain the finished phosphate product (Figure 1). In this article, we outline how pumps can be matched with different process requirements to deliver performance advantages for each of these production steps.

Molten sulphur

Because sulphur used in the fertilizer industry is classed as ‘dirty sulphur’, the design of pumps used in molten sulphur handling requires special attention, particularly their gland sealing. Horizontal pumps with a hydrodynamic seal are generally ideal. Horizontal seals need very little or no maintenance in this environment and – unlike the sealing rings in mechanical seals – are not prone to ‘sticking’ with solidified sulphur.

As well as RCEV cantilever type pumps, ITT Rheinhütte provides full pump coverage for the clean molten suplhur process. The heated version of the RMKN magnetic drive pump (Figure 2) is very suitable for conveying molten sulphur and meets DIN EN ISO 15783 technical requirements. The GVSO (VS4) pump is similary well-suited.

Sulphuric acid

Vertical and horizontal centrifugal pumps are employed in sulphuric acid plants. These convey both liquid sulphur and dilute and concentrated sulphuric acid. These acids are corrosive and can contain sulphur dioxide and sulphur trioxide. Corrosion-resistant materials with high chemical stability are therefore required. Construction materials must be able to convey these fluid media safely, even at high temperatures, and withstand concentrated sulphuric acid (99%) up to 240°C.

ITT Rheinhütte has enjoyed several decades of good experience in handling sulphuric acid using horizontal pumps fitted with hydrodynamic seals. These pumps utilise a specialised material called Siguss. This is adapted to the highly demanding and stringent operating conditions at sulphuric acid plants and provides maximum ‘erosion-corrosion’ resistance.

Siguss contains around 15 percent silicon, up to 5 percent chromium, but no nickel, and is suitable for all concentrations of sulphuric acid up even at boiling temperature. Siguss is thus the only material that can pump sulphuric acid (and mixtures with minor amounts of chlorine gas or nitric acid) without forfeiting its outstanding durability. Siguss is also highly wear resistant.

Non-metallic materials are often used in pumps conveying sulphuric acid in the low concentration areas of the plant. This includes the pumping of washing acid – a mixture containing 15-75 percent sulphuric acid and sulphur dioxide. Non-metallic materials are generally superior to most metallic materials in this part of the process.

Mining and beneficiation

Phosphate ore is extracted using various methods, including underground, open pit, and strip mining, before being transported to a beneficiation plant for further processing. Ore transport depends on the mining method with belt conveyors used for crushed dry ore and centrifugal pumps used when the ore is in a slurry form.

Processing may not be required if the P2 O5 grade of the run-of-mine ore is sufficiently high. Beneficiation is, however, usually necessary to upgrade the mined ore and generate phosphate rock concentrates. Processing typically takes place over several stages.

The ore firstly goes through a scrubbing process before being crushed and screened to reduce the particle size. It is then fed to a grinding mill for further size reduction. This liberates the target phosphate from unwanted gangue constituents. The resulting slurry then undergoes initial separation to remove slimes, usually with hydrocyclones, followed by several stages of froth flotation. The phosphate concentrate obtained is filtered and thickened in preparation for the phosphoric acid plant, the next step in the production process, while the waste materials are pumped away as tailings.

The various pump types used in mining and beneficiation include: mine dewatering pumps, rubber or metal lined slurry pumps, and process pumps for clear fluids. ITT’s Goulds Pumps range (Figure 3) of horizontal and vertical pumps adequately covers all of these applications.

Phosphoric acid

The wet phosphoric acid process is commonly used in fertilizer production. In this process, phosphoric acid (H3 PO4 ) is generated by attacking the beneficiated phosphate rock concentrate with sulphuric acid (H2 SO4 ) inside a reactor. In a highly exothermic reaction, the calcium present in the rock reacts with the sulphuric acid to form a calcium sulphate (CaSO4 ) slurry. Flash cooler pumps are used to circulate the slurry to reduce the temperature. This mixture contains up to 40 percent solids of around 250 microns and has a pH of <1.0 at temperatures greater than 10O°C.

The slurry is transferred to filters to separate gypsum crystals from the phosphoric acid which is then fed to evaporators for concentration. The main evaporation circulation pump is generally an axial flow type handling high flow rates at a relatively low head. The phosphoric acid product undergoes further concentration before finally being stored as a 54 percent concentrate.

Fluosilicic Acid (FSA) is also present in the phosphoric acid production process (Fertilizer International 504, p44). Overall, the process presents some very challenging applications for pumps in terms of corr– osion and erosion.

ITT offers non-metallic pumps that are a perfect fit for phosphoric acid applications. These offer a longer service life and are an economic option compared to high alloy materials. A range of end-suction pumps is also used during phosphoric acid production. These provide service water to the acid plant and the water treatment plant (for waste disposal and water recycling).

ITT has vast experience in all these applications and its Rheinhütte and Goulds Pumps brands (Figure 4) offer a complete package of suitable metallic and non-metallic pumps. Types rang from horizontal slurry and process pumps, to vertical pumps, and axial flow designs.

Fig. 3: ITT’s Goulds XHD pump (left) and Goulds 5500 pump (right) are widely used in phosphate ore mining and beneficiation processes.

PHOTOS: ITT GOULDS PUMPS

Fig.4: ITT’s Goulds AF pump (left) and Goulds IC pump (right) are well suited to phosphoric acid plant applications.

PHOTOS: ITT GOULDS PUMPS

Fig. 5: ITT’s abrasion-resistant, heavy duty Rheinhütte RCE pump (left) and Goulds RCNKu pump (right) are suitable for chemically aggressive fertilizer production processes.

PHOTOS: ITT RHEINHÜTTE PUMPEN/GOULDS PUMPS

Phosphate fertilizers

Phosphoric acid is used as a feedstock in the production of various types of phosphate and compound NPK fertilizers. These include:

• Diammonium phosphate (DAP)
• Monoammonium phosphate (MAP)
• Triple superphosphate (TSP).

Production of these multi-nutrient fertilizers is very challenging for pumps, as it usually combines highly corrosive fluids at high operating temperatures in the presence of solids, with some very low net positive suction head (NPSH) requirements.

ITT has developed suitable products for the fertilizer production process and possesses long-standing experience in this application. Horizontal and vertical pump designs in various materials are generally used. Heavy-duty pumps with hydrodynamic seals are typically necessary to cope with the presence of solids and extend the life of both the pump and its construction material.

The Rheinhütte RCE pump (Figure 5) is designed for heavy-duty applications in chemically aggressive and abrasive media. It is the first choice in the basic chemicals industry – especially in fertilizer production – as well in the environmental technology sector and many other industries.

Summary

ITT offers a single source of pump products globally through its renowned Rheinhütte and Goulds Pumps brands. This wide-ranging portfolio is ideal for fertilizer industry customers looking for reliability, safety and innovative pumping options, and is supported by comprehensive process and applications knowledge.

ITT also has an active pump and materials research and development programme. This is designed to meet the changing needs of the phosphate fertilizer industry and evolving production processes. ITT’s R&D programme is supported by state-ofthe-art global manufacturing facilities and a sales and service network that is strategically located close to customers.

About the authors

Hani Tello is head of global product sales at ITT Rheinhütte. Harvinder Bhabra is the global product manager for slurry and axial flow pumps at ITT Goulds Pumps.