Advances in granulation

FERTILIZER TECHNOLOGY

Advances in granulation

Recent innovations in fertilizer granulation include thyssenkrupp’s new process for premium ammonium sulphate production. Eirich has launched new equipment for NPK granulation, while Rhewum is also offering new screening technology.

Eirich – mixing, granulation & coating in a single machine

Headquartered in Hardheim, Germany, mixing technology company Eirich offers both stand-alone granulation equipment and turn-key fertilizer systems. The company’s equipment allows:

• Mixing, granulation and coating within one single machine
• Optimised distribution of micronutrients and binding agents
• The incorporation of secondary raw materials, such as filter cakes, sludges and nutrient solutions
• Environment-friendly granulation without fine dust generation or aerosol emissions.

Fertilizers granules need to be able to withstand the mechanical stresses and strains associated with modern farming equipment. The latest centrifugal spreaders, for example, have working widths of 24, 36 or 48 metres and accelerate granules to 150 kilometres per hour in just a fraction of a second. This means that granules need to be round and smooth and as dense and compact as possible. In addition, the risk of granules clumping together, or dust generation, needs to be eliminated. Finally – last but not least – the size distribution of granules needs to be as uniform as possible.

Eirich’s SmartMixer machine

The SmartMixer machine from Eirich enables the complete production of fertilizer granules in a single process step. Optimised mixing is ensured by combining a variable speed primary mixing tool (rotor) with an independently-moving rotating mixing pan. Uniquely, this design allows the machine to perform different process functions simply by altering the rotor speed. Mixing is performed first dry, then wet, followed by granulation – with coating at the end, if required.

The mixer can also be combined with a disk pelletiser for when customers have more rigorous grain roundness and size requirements. In these circumstances, the mixer is used to generate a feed of micro-granules for the disk pelletiser. Eirich mixers, with a usable volume of up to 12 cubic metres, can provide these at a throughput of up to 1,500 tonnes per hour.

The mixing process can be easily scaled-up. The layout and design of the smallest EL1 mixer (1 litre capacity), for example, is identical to the RV24 unit (3,000 litre capacity), the largest mixer in the series. The mixing times and speed parameters are also directly transferrable between different size mixers, making the development of new granulated fertilizers particularly easy, according to Eirich.

The mixing-dosing-granulationcoating process

Fertilizer raw materials generally need to be ground prior to granulation as a first step. For NPK fertilizers, this ensures that all three major nutrients are evenly distributed in every single granule. Eirich recommends its TurboGrinder for this preparatory step – as this machine is well-suited to the grinding and drying of fertilizer raw materials.

The ground raw materials are then weighed out in the correct proportions before being transferred to the SmartMixer machine. This granulates the materials in a batch process that take around six minutes in total.

The components are initially mixed together dry – this normally takes no longer than 30-60 seconds – before being dosed with water or an aqueous solution. The latter can be used to add binding agents and/or additional micronutrients. Adding nutrients in dissolved form at this stage ensures their even distribution, even at very low concentrations.

The machine is able to mix the liquid dose with the solids and so thoroughly wet the raw materials in a short space of time. The speed of the primary tool (rotor) is then reduced, with the aid of the frequency inverter, to initiate the granulation of the completely wetted particles.

The granulation stage is also rapid with the mixer becoming full of uniform granules within just a few minutes. These granules can then be coated, if required, with coating typically taking around 30 seconds. The mixer is then emptied in readiness for the next production batch.

Eirich case studies

Calcium dihydrogen phosphate (CDP): This is produced by reacting calcium carbonate with phosphoric acid in the mixer. The reaction product is then granulated without any need to transfer it to a different vessel. Eirich CDP systems are operating in:

• Slovenia: RV15 size mixer, 750 litres
• Bulgaria: RV24 size mixer, 3,000 litres
• Peru: RV24 size mixer, 3,000 litres. Projects with a RV24 mixer are also at the planning stage in Brazil and South Africa.

Superphosphate or double superphosphate: phosphate ore is reacted with either sulphuric acid (superphosphate) or phosphoric acid (double superphosphate). Similar to CDP, reaction and granulation are carried out in a single machine, without the need to transfer materials, resulting in significantly shorter processing times.

NPK fertilizers: Eirich built a NPK fertilizer production plant – incorporating eight mixers – for a German fertilizer manufacturer in 2014. The company has subsequently supplied mixers to a large number of other NPK plants, including those located in Australia, the UK, Poland, Russia, Sweden, Switzerland and Turkey. Hauert HBG Dünger AG has been successfully operating its Eirich NPK fertilizer plant at Grossaffoltern, Switzerland, since 2017. This produces granulated NPK fertilizers marketed under the brand name GRANUTEC Swiss Granules.

Rhewum – high performance screening

Rhewum has been developing, designing and manufacturing screening machines for newly commissioned and operational urea granulation plants for decades. The company has supplied more than 1,800 machines to fertilizer plants located in more than 60 different countries since 1956.

Iran’s Pardis Petrochemical Company (PPC) has installed Rhewum’s direct excitation screening equipment at one of the world’s largest urea granulation plants. PPC selected Rhewum to design, manufacture and commission the plant’s extractor lines and main screens. The company’s technology was able to guarantee the desired high level of process efficiency and deliver high quality urea granules – combining this with high on-stream availability and reduced energy consumption (Nitrogen+Syngas 358, p46).

Rhewum’s extractor lines, diverters, special chutes, distribution feeders, safety and process screens all work together in concert to ensure a consistent screening process. The combination of vibrating extractors with linear motion screens maintains the level of fluidised granules in the granulator, as well as protecting the process by screening out any lumps.

High performance screening is essential for maintaining PPC’s product requirements (95 percent purity) and minimising product loss to the coarse and fine fractions.

The four main screens (4 x 65 t/h capacity) are located downstream in the granulation unit where they ensure the quality of the final product. The screening process is designed to separate out lumps (>15 mm) in one outlet, the coarse particles (4 to 10 mm) in the second outlet, while the product (2 to 4 mm) and the fines (<2 mm) flow into separate inlets of the product diverter.

Efficient screening steps

Step 1: Distribution. Urea particles needs to be uniformly distributed at the inlet of the screening machine to make full use of the screening area. This is achieved using a pre-positioned vibrating distribution feeder.

Rhewum’s SV feeder (Figure 1) consists of a moving inner distribution plate vibrated by two outer out-of-balance motors. These are linked to a fixed housing via a stable and dust-tight flange connection. The use of fixed flange ensures safe and dust-tight operation.

Step 2: Screen cloth. This is the most important contact part between the screening machine and urea particles. The design of the main screen cloths needs to incorporate a number of factors:

• Choosing the right mesh opening is critical when it comes to creating a high quality product.
• Keeping the screening area constantly open also increases plant capacity and reduces the costly recirculation of material.
• While easy and fast maintenance of the screen cloths leads to high on-stream availability.

An automatic cleaning cycle keeps the directly-excited screen cloths free from clogging. Each screen cloth can be changed individually, without the need to remove any of the others. This is a useful as maintenance intervals are often uneven due to unequal wear of the screen cloths. In total, there are four screen cloths installed at the PPC plant, each of which can be replaced within just ten minutes. The mesh openings for the installation were optimised during test trials at Rhewum’s pilot plant in Remscheid.

Step 3: Screening technology. The WAU screening machine – with direct excitation of the screen cloth – was selected for the PPC granulation plant (Figure 2).

PPC’s urea production requirements – the economic generation of granular products with reproducible qualities – were quite challenging. The overall objective was to reliably achieve high levels of granulation efficiency at the lowest operational cost.

WAU type screening machines are directly excited. Vibration is imparted into the sieve cloth by the high speed movement of a transversally mounted rocker shaft located beneath the sieve cloth. Outside the screen housing, robust out-of-balance motors rock the knocker shafts, conveying the high frequent oscillation directly into the screen mesh. This is highly effective as it means the screen mesh is vibrated directly into the urea granules. Machine energy consumption is also low – only 0.036 kW/tonne of screened urea.

The on-stream availability of each component in the urea granulation process is critical and needs to be kept as high as possible. Weak points need to be avoided as these will create bottlenecks and potentially – in the worst case – lead to the shutdown of the whole granulation plant. 100 percent availability of WAU screening machines is guaranteed by equipping these with 15 small out-of-balance motors. This allows performance stability to be maintained until the next scheduled maintenance period – even if one motor fails. The motors are coupled by flanges to vibrating axes which transmit vibrations to the urea particles via the knocker shafts and screen cloths.

The high number of motors fitted to the WAU machine also allows the screen decks to operate at different amplitudes. The ability to keep amplitude uniform over the complete width of the screen provides consistent screening quality over its entire area. Additionally, because screen housings are static and do not vibrate, WAU machines do not require a heavy steel structure, even when located at the top of the building.

Step 4: Trials and quality control. Test trials at Rhewum’s laboratory help to optimise the design and ensure the screening machine will function successfully later on during production. These trials are used to check if the customer’s product quality guarantees can be achieved.

Fig. 1: Rhewum SV feeder for constant and uniform distribution of screening materials.

IMAGE: RHEWUM

Fig.2: Rhewum WAU screening machine withdirect excitation of the screen cloth.

Rhewum’s SizeChecker optical measurement system (0.2-20 mm) offers a cost effective alternative to standard screen analysis. Using an automatic sampler device, the system can be used in production and integrated into process monitoring and quality control. This avoids time consuming manual sampling and analysis. The system, by automatically recording measurements, allows alarms to be set and triggered when variations in particle-size distribution occur.

In summary, Rhewum’s screening technology improves the overall efficiency of the whole urea granulation plant by:

• Reducing energy consumption
• Increasing availability
• Having a lighter steel structure, and
• Avoiding unnecessary maintenance. Following its convincing performance at Rhewum’s test facilities, the WAU type machine installed at PPC’s granulation plant will continue to prove its high performance urea screening abilities over the coming decades. Tests conducted by Rhewum back in Remscheid have confirmed that PPC’s purity and yield expectations have been exceeded at the Persian Gulf site.

Innovative low-cost ammonium sulphate granulation

Ammonium sulphate is a popular and effective dual nutrient fertilizer. It simultaneously supplies sulphur and nitrogen, boosting crop growth and yields, as well as promoting the transfer of micronutrients such as manganese, iron, and boron from soil to plants.

Among the key benefits of granular ammonium sulphate – compared to liquid or crystalline alternatives – are its improved storage, spreading, and mixing qualities. Granulated ammonium sulphate – unlike the more commonly produced crystalline product form – is also easier to incorporate into fertilizer blends.

Unfortunately, cost is a constraining factor as conventional granulation plants need to consume expensive ammonia and sulphuric acid to manufacture premium ammonium sulphate products.

However, in a recent breakthrough, German industrial giant thyssenkrupp Industrial Solutions (tkIS) has developed a low-cost granulation process for ammonium sulphate. This innovative process converts ammonium sulphate solution – an inexpensive by-product – into premium grade granules (Sulphur 372, p54).

Because it is only produced by a few suppliers in small volumes currently, there is unmet demand in the market for granulated ammonium sulphate. This provides new entrants into this niche market with attractive sales opportunities, particularly as the price for premium granules, around $100 per tonne, is significantly above that of standard ammonium sulphate products. thyssenkrupp says the new process is the outcome of “years of energy and innovative thinking” by the R&D team within its fertilizer division.

“We want to give fertilizer manufacturers the opportunity to convert an industrial by-product into high-quality nitrogen fertilizers,” comments tkIS, adding: “There is a worldwide demand for granular ammonium sulphate, which very few manufacturers currently make.”

Fig. 3: Fluidised bed granulation process for the production of ammonium sulphate

The patented tkIS ammonium sulphate process

thyssenkrupp’s patented process (Figure 3) starts with ammonium sulphate solution – an industrial by-product generated during the manufacture of caprolactam and coal oven gas. Initially, a preventative additive is mixed with the solution. This reduce dust formation during subsequent granulation by giving the end-product a high crushing strength. The resulting liquid mixture is sprayed into a fluidised bed granulator and processed into solid granules. Materials exiting the granulator are screened. Any oversize particles are crushed and returned to the granulator alongside undersize particles. The product granules generated by the process are round, very hard, and resistant to impact and abrasion.

Following successful lab and bench-scale tests, thyssenkrupp built a pilot plant in 2016 with an initial capacity of half a tonne per hour. “As all the tests have also been successful here, we are upping the process to industrial scale with capacities of 5-20 tonnes per hour,” says tkIS. “Fertilizer manufacturers worldwide will benefit from improved cost efficiency.”

The fluidised bed granulator (Figure 4) is the centre piece of the production plant, as it is where the liquid ammonium sulphate is turned into a solid product.

In the thyssenkrupp process, liquid ammonium sulphate solution is sprayed onto seed granules. These granules grow bigger in the granulator via a sequence of spraying, wetting, drying and layering (Figure 5).

Fig. 5: Conversion of liquid ammonium sulphate solution into granules

Fig. 4: The fluidised bed granulator is the centrepiece of the production plant.

PHOTO: TKIS

The particles in the granulator are fluidised by a hot air stream. This stream can require steam heating. Heat integration is also possible using hot off-gases instead, if available from adjacent plants, to reduce operating costs. The air exiting the granulator is scrubbed before being discharged to the environment as clean air.

The ability to integrate the new process into existing chemical complexes will be essential for its economic success. As well as the heat integration mentioned above, there are opportunities to install the fluidised bed granulator in parallel with an existing crystalliser unit. By reducing the load to the crystalliser, this would have the added benefit of producing larger crystals.

Process development

In general, the granulation of ammonium sulphate is prone to dust generation and the formation of granules with low hardness. The toughest challenge during the development of the new process was therefore finding a suitable liquid granulation additive. This is added to the liquid feedstock before spraying the solution onto the seed granules.

Process development started with small-scale, single nozzle laboratory batch tests. This enabled the main process parameters to be scoped and the selection of a suitable granulation additive. During the second phase of development, a technical-scale test facility was used to demonstrate continuous operation and to identify other granulation parameters such as the recycle ratio. Finally, the process was up-scaled and risks were mitigated by demonstration in a pilot plant with a capacity of 12 tonnes per day.

The quality of ammonium sulphate solutions generated by upstream processes can vary significantly. As the granulation process can be a sensitive to certain impurities, pilot plant trials offer a valuable way of testing feedstock quality during the feasibility phase, before deciding whether to invest in a full-scale fluidised bed granulation plant.