Fertilizer Process: Composting, Screening and Dehydration
The process, can be summarized in the production in Composting, Screening, Dehydration, Mineral and Organic addition with Mixing, Pelleting, Cooling, Sifting and Packing.
The composting if the first and probably most important phase of the production that will determinate if you will have a good product or not. The composting is the period that the raw material has to stay in a closed area, continuously mixed and ventilated in order to reduce the moisture content and allow to the microorganisms to attack the organic matrix and to transform the manure in compost. The composting time can vary from 60 to 90 days. The mixing operation can be done with specific equipment as shown on the below photos or with a loader. After the composting time, the compost has to rest and mature for about another 30 days. Now there are some new technologies that, by spraying some additive to the manure, accelerate the microorganisms process thus reducing the composting time to 30 days.
The screening process it is used to separate the lumps of compost from the powder, before the dehydration phase. Usually for this operation is used a rotary drum screener and the diameter of the holes is usually determinate by the diameter of the pellet that has to be produced. If the screening is not done in the proper way, by using screens with a too large perforation, the risk is that lumps with high moisture content won’t be dehydrated properly increasing the total humidity percentage and if the aforesaid lumps arrive to the pellet mill, the die can be easily clogged.
After the composting and maturation period, the material, has a moisture content higher than 20%, it must be dried. The equipment mostly used for this operation is a drum dryer but can be used a conveyor dryer as well. In order to have a good quality product, the dehydration has to be as soft as possible, that means that the lower is the temperature inside the dryer, the better is the product that the customer will have after this operation. Off course, the lower is the temperature, the longer is the time that the material has to remain into the dryer which means that the size of the dryer will be bigger than that of a dryer used with high temperatures. Because of the aforesaid, it is always better to increase the composting period, in order to have a more dried product and, consequently, have a smaller dryer. The machine has to be produced in Stainless Steel AISI 304.
Some companies prefer to dry the pellet after the pellet mill instead of drying the raw material before pelletize. In this case the material can arrive at the pellet mill with a maximum content of moisture of 30% and, after the pelleting process, the pellets will go through a Belt Conveyor Dryer (the use of a Drum Dryer in this position is not recommended because it will damage the pellet and create a lot of fine).
Both the solutions are good; with the dryer before the pelleting the process is smoother and the pellet will be more compact while, with the dryer after the pelleting, since the material arrives to the pellet mill with a higher content of moisture, the production is higher, and the pellet, after the drying, will be more rough and some agronomists say that the performance of this kind of pellets is better. Moreover, with the dryer after the pelleting, there’s no need to install a cooler because, on the last 3 meters of the conveyor, air is injected at air temperature, cooling the pellet.