Before we get into the detail of making water-soluble MAP, what are the general considerations when making a water-soluble fertilizer? A water-soluble fertilizer means a fertilizer that is fully water soluble and highly pure as well as free from the water insoluble impurities (usually less than 0.1% wt) that cause clogging of spray systems and dosing devices. The first thing we look at is the quality of the feed or raw material and the required quality of the water-soluble fertilizer. These two parameters drive the choice of the process solution we will implement to achieve that goal. Usually, clients also have other constraints to consider such as energy consumption, footprint, etc that we will consider. This is applicable to MAP but also other water soluble fertilizers such as DAP, MKP, DKP or MKDP. In basic terms, MAP is produced by adding ammonia to phosphoric acid. But the quality of the acid is important. Before we get into the crystallization process, can you talk us through why acid grade is important? The quality of the acid is very important because most of the impurities that are in the feed acid will end up in the fertilizer product.
If the acid has a very good quality, it is easy to make high purity fertilizer product. If it does not, the quality of the final fertilizer product will be impacted unless some purification technique is used in the production process. A very classic way of getting the required fertilizer purity is to purify the feed acid prior to crystallization. However, this solution is not cost effective. The alternative is to make the reaction with the low feed quality but use the crystallization technology that will be play a dual role: production and purification. Pipe-reactor technology - is this the most common way to synthesize the MAP? Could you go a different route if you were making water-solubles? In the pipe reactor technology, there is no purification. What enters the reactor leaves with the fertilizer product. Which means that it requires a very good acid quality.
Crystallization is the other alternative technology. One of its greatest advantages is its ability to adapt the process to different feed-acid grades and even acid waste streams. This allows for not only controlling manufacturing cost through higher recoveries but also enhancing sustainability through a better management of waste. Crystallization technologies enable the production of high-value MAP even from low-grade phosphoric acid which is typically a waste compound with little to no value unless it is subject to costly purification steps.
Given the growing scarcity of most fertilizer raw materials and the energy applied to recover them, it seems vital that none of them go to waste. Crystallization makes this possible by converting waste streams and recycling low-grade feedstock.
After neutralization, there is insoluble content. Do you get this even with higher grade phosphoric acid? Typically, if the feed acid is a purified grade, the crystals produced will have a very low insoluble rate because the feed acid is free or low in impurities. What are your options to remove insoluble material? Is there typically a stage prior to the crystallization process via a filter or is it part of the crystallization process? The answer is both. After neutralization, we have the possibility to remove impurities and during the crystallization we will remove/separate the insolubles and impurities in general and/or operate at certain conditions to make sure they don’t co-precipitate and end up with the final product. By the way, the definition of insolubles is the amount of impurities that stays suspended when the MAP crystals are dissolved in a 10% solution at ambient temperature. This means that MAP crystals can contain impurities detected with analysis but they are not called insolubles because if you dissolve the MAP crystals they will dissolve too and will not be suspended. Can you describe crystallization of water-soluble MAP in general, and any features particular to the Veolia process? High-quality MAP can be made via reactive crystallization using technical-grade phosphoric acid and pure ammonia followed by solid-liquid separation and drying. Another alternative is to produce MAP from merchant grade phosphoric acid resulting in lower production cost because an acid purification plant is not required. MAP producers rely on crystallization experts, such as Veolia, to provide the know-how to selectively crystallize high-quality water-soluble MAP despite the impurities coming with the feed acid. Test works are very often essential in tailoring the best process solution to the feed quality and required crystal purity. In terms of manufacturing costs and energy consumption, how does crystallization compare with other processes to produce granules? Crystallization is one of the most energy efficient processes in the production of MAP. Firstly, there are huge cost and energy savings when a whole acid purification plant can be skipped! Conversely, with granulation a very pure feed is required to make a pure MAP product because what goes into the granulator will end up in the outlet product. This means a purification plant will be required upstream. In addition, an acid purification plant usually requires many steps including very expensive extraction columns in high-cost material of construction (because it is a highly acidic environment). Second, the granulator itself is less energy efficient than a crystallizer. In a granulator, a significant part of the energy is used to heat the air (which goes out of the granulator at a high temperature). Roughly, a granulator will consume twice as much heat as a single stage crystallizer. While the granulator is directly driven by steam, the crystallizer can use different techniques for energy savings (thermal vapor recompression, mechanical vapor recompression, multiple effect configuration). Further, a crystallizer allows for recovery of high quality process condensates that can be reused in other parts of the fertilizer complex, while the granulator will send air and vapor to the atmosphere with no water recovery.
Can you list some of the water-soluble MAP plants around the world using Veolia technology? What was your latest project? (Tunisia?) One of our last projects is Alkimia, which is located in Tunisia and produces 25,000 mtpa water soluble MAP crystals from a merchant grade phosphoric acid. The product is water-soluble MAP with purity > 99%wt and water insoluble < 0.1%wt with no heavy metals, fluoride, sodium and chlorides. The plant releases very low ammonia and dust emission. There is no expensive acid purification plant upstream. Purification and production occurs through crystallization and the purgeis recycled within the fertilizer complex. Veolia supplied the crystallizers, centrifuges, dryer and screening system. ●
Sana Boulabiar, process engineer, Veolia Water Technologies
The MAP crystals are the Veolia Alkimia Plant in Tunisia (produced from low grade acid)
Veolia MAP production plant in Israel
