Prilling is an extremely effective finishing technology and offers many advantages over other solidification methods. But what is prilling? And what is a prill? To define prilling, we need to examine both the main principles of the process and the end product. But before we do this, let's go back to where it all started...
From Dream to Invention
Legend has it that one night in 1782, William Watts, a plumber from the city of Bristol in the UK, was walking home after a night of drinking ale in the local pub. Along the way, he decided to take a rest near the St Mary Redcliffe Church, on which he had renovated the lead roof. As he lay on a bench, thinking about the disappointment of his loving wife, who was home alone, he drifted off into a dreamy sleep.
In his dreams, his wife's anger kept haunting him. He saw her towering over the church and, in her anger, she poured molten lead down the tower onto his face. The lead didn't form a stream but instead shaped into perfect spherical balls. He woke up to find that it was only the Bristol rain falling from the sky.
The next day he decided to experiment and, together with his wife, climbed the spiral stairs up to the church’s roof. They drilled holes in the bottom of a pan, melted some lead, and fed it through the holes. Just like in the dream, the molten lead formed spherical droplets, which solidified into perfectly spherical (definition of prill) lead balls before hitting the ground. These balls became known as patent lead shot, which was used in shotguns designated for hunting .
Previously, lead shot was produced according to a slow and laborious molding process, producing irregular and expensive lead balls. Watts, an avid hunting enthusiast himself, was so convinced of this production method that he decided to build the world’s first prilling tower, right on top of his house .
Until this day, spherical products from the chemical industry are produced in very much the same way as it was invented by William Watts; currently known as the process of prilling.
The Basics of Prilling
When only looking at the main principles of prilling or the meaning of prilling, not much has changed since William Watts first created his famous lead shots. However, the technology to achieve these principles has been modernized and significantly improved. Like the original process, the prilling process consists mainly of melt droplets falling from an elevated height. Solidification of the melt droplet occurs due to the cooling medium it is falling through (that medium usually being air).
The different melts and the different particle sizes that are prilled, require different tower heights to make sure the droplets are completely solidified before they hit the bottom of the prilling tower. Prilling towers can be everything from 20m up to more than 60m in height and are usually constructed from either cement or steel, depending on the melt and how corrosive it is.
The heart of a prilling machine, a vital part of the prilling tower, is the prilling bucket. This is where the droplets are produced, which ultimately turn into the desired prills as they fall down the tower. Prilling buckets can produce prills with a typical diameter of between 0.5mm and 3mm. The formation and breakup of a liquid jet are governed by the physical properties of the melt. Therefore, each new prilling facility has to be designed in accordance with the material that needs to be prilled and the customer’s preferred size of the produced prill.
A prilling tower is operated with a gas stream to cool the prills. This is generally a normal indoor flow of outside air. In some cases, a nitrogen gas stream is required to create an inert atmosphere inside the prilling tower. The gas stream can be operated in two different ways; a countercurrent or co-current flow. The most common is the countercurrent flow, since this operation allows the residence time of the prills in the tower to be increased and also gives a better heat distribution over the tower.
At the bottom of the tower, the prills need to be collected and sent to packaging. A scraper is placed at the bottom, which gently pushes the prills to a screw conveyor that transports the product to big bags.
There is always some dust formation inside a prilling tower. This can be due to evaporation, satellite drop formation, or due to parts of the prills breaking off upon impact with the floor. Excessive amounts of dust can have serious repercussions, both for the environment and the overall safety of the facility. Therefore, due to new environmental regulations, the prilling towers of today have filter systems to combat these kinds of emission problems.
Furthermore, quite a few materials that are commonly prilled can reach their lower explosive limit (LEL) inside the tower when excessive amounts of dust accumulate. This LEL was one of the reasons why the prilling industry stopped producing sulfur prills. Research at Kreber has shown that with our current innovations, this will possibly change in the near future. For more information about our most recent developments, please read the following article: sulfur formation and solidification technologies.
Prilling is often the most preferred method because it offers the following advantages over other solidification methods:
- Most Cost-Effective
Prilling is one of the most economical finishing technologies in the world for large outputs. Compared with other solidification methods such as granulation and pastillation, CAPEX and OPEX are multiple times lower at increased capacities.
- Highly Flexible
Prilling production is easily scalable within the range of 70 to 110% of the originally designed capacity. This makes the prilling production method an extremely flexible option for your total production process. The prilling process is also easily scalable in its throughput, making it an interesting alternative for companies looking for small batch production.
- Guaranteed Continuity
The high degree of ‘self-control’ makes prilling a robust finishing technology with a very stable processing procedure. It can therefore play a vital role in guaranteeing the continuity of your process and quality of your product.
The process requires minimal energy consumption. In fact, it is the most energy-efficient solidification technology available. This helps you minimize your operational expenses.
- Convenient Operation
Our equipment is easy to operate thanks to our well-defined process operating parameters. Furthermore, the low amount of rotating and intricate parts result in a reliable process, giving the client low overall operational and maintenance costs.
- Small Footprint
With the design of our prilling tower and additional equipment, you benefit from a relatively small footprint, compared to other solidification technologies.
Thanks to a constant stream of technical innovations, such as state-of-the-art air inlet filtration and scrubber technologies, prilling has evolved into a cleaner finishing technology than it has ever been.
Primary finishing method
Prilling and granulation are the two solidification methods most widely used in the fertilizer industry. For a breakdown and comparison of these major finishing technologies, feel free to read the following article: the difference between prilling and granulation.
Urea and ammonium nitrate are the two main fertilizers that still utilize prilling as their primary finishing method to this day. Prilling is ideal for these materials since the production rate needs to be reliable and the output high. Prilling has also been widely used for BPA production and the production of NaOH, while also gaining ground in other markets thanks to new innovations in the field.
New Technology and Innovation
Even though prilling has been around for a very long time, it is still evolving. While prilling was already a great choice for any market wanting a robust finishing method for their material, new innovations are in the works to improve and streamline the process even more.
An excellent example of a new innovation in prilling is a closed-loop system. Environmental emissions are practically nullified, significantly increasing the safety of the operational process . Upcoming developments are the mitigation of the satellite droplet formation, which will also optimize the prill size distribution. These innovations make the prilling process a better alternative, compared to the other main finishing methods.
- Harrison, David. Dream Lead to Invention. Bristol Times, 26 November 2002.
- Watts, Williams. Small Shot. 1347 United Kingdom, 10 December 1782.