Grinding and Screening Systems
Alloy is primarily a rendering equipment manufacturer and we do projects from raw material receiving through finish product grinding. While most renderers are familiar with grinding and screening, there are some options and things to consider and many smaller renderers still use bagging instead of bulk storage. Instead of a typical blog this is more of a bagging, screening, and storage primer. I will attempt to cover topics that are valuable to the smallest renderers along with big plants. The products that Alloy is the most familiar with are the typical rendering meals such as MBM (ruminant meat and bone meal), Poultry Meal, Porcine Meal, Fish Meal, and Feather Meal. There are other specialty meals but these are the primary meals in rendering plant applications.
Here is a table of contents if you want to skip ahead to a section that is pertinent to your current topic:
- Crax cooling bins (sometimes called day bins)
- Screen first or grind then screen? Vibratory screens round and square
- Conveyance methods to finished product storage
- Crax cooling
- Finished product storage methods
1. Crax Cooling Bins:
The term crax or sometimes called cake are both very old terms that go back to the early days of pressing in our industry. Although one could use the term pressed unground meal our industry has kept using the old terminology. Basically, it is the pressed solids that come out of the screw press. Essential Rendering is a book put out by the National Renderers Association (now NARA) in 2006 and they describe the product as follows. “Following cooking and fat separation the “cracklings” or “crax” which includes proteins, minerals, and some residual fat, are then further processed.”
Crax becomes brittle as it cools and a hammermill works better and wears less when it is grinding cool, brittle material. Some smaller batch plants can install a cooling bin that can contain a full days worth of product. This is where the word “day bin” originated from. It was called a “day bin” because it could handle a full days worth of product. Many renderers still call them “day bins” even though they only hold a few hours worth of product. Alloy prefers the term “cooling bin” as that is the function of the bin. The shape and design of the bin is important to reduce product bridging or non-movement. As the product begins to cool, it tends to want to bridge inside of the bin. Alloy utilizes multiple screws in the bottom of the bin that are independently driven so some screws can move forward while others move backwards breaking the bridge. Furthermore, these screw need to have AC variable speed drives so the discharge of the bin matches the throughput of the rest of the system. Crax cooling bins should also incorporate a leveling screw at the top of the bin. There is significant engineering involved in sizing the bin, the screws, the horsepower, speed of the screws, etc.
2. Screen first or grind then screen?
There are two different methods regarding grinding and screening and some rendering plant managers or plant engineers have a strong preference for one approach versus the other. Alloy has installed both approaches with success and there are advantages and disadvantages to both. I personally do not have a strong opinion on which is better as they both work fine. The question comes down to if the screen should come first or if the hammermill should come first. In terms of screens, smaller plants can get by with a circular vibratory screen, while bigger plants utilize larger shaker screens like a Rotex. If you decide to screen first, then a double- deck machine is preferred with a large mesh screen on the top and the finish mesh on the bottom. The top screen removes the big particles and extends the life of the bottom screen. Buyers of meal specify the particle size they want and virtually all applications dictate a 7-mesh or 10-mesh screen for final particulate size.
Approach 1 – Screen First then Hammermill.
The advantage of this approach is that roughly one half of the product never needs to go to the hammermill at all. So a smaller hammermill can be utilized and the wear on the hammermill is reduced, extending hammer life and reducing maintenance expenses. If a smaller hammermill is chosen, then there is an ongoing energy savings as the total connected horsepower is reduced. The disadvantage is that an additional screw and vertical lift are typically required and it takes extra floor space. There are also more pieces to maintain with the extra screw and vertical lift. In this approach, the product exiting the cooling bin goes over the screen first. Some of the product collected at the press is already smaller than what is required but the crax gets broken up in the screw conveyor system feeding the cooling bin. The screws in the bottom of the bin along with the screws going to the screen also break up the crax. So a portion of your product is already below the mesh size of the screen without ever going to the hammermill. The amount that passes through the screen prior to the hammermill depends on a number of factors (grind size of raw material, type of product cooked, type of cooker, amount of bones). Typically about 50% of the meal will go through the screen and not need to go to the hammermill. So in this approach 100% of the crax goes over the screen and the overs are collected and go to hammermill for grinding. “Overs” is the material that is not properly sized and that material goes over the screen and does not pass through it as the material is not small enough to go through the screen. Those overs are sent to the hammermill for grinding and then need to go back to the screen a second time to make sure that 100% of the material passes through the screen and is properly sized.
Approach 2 – Hammermill then Screen
Another way to handle this product is to send 100% of the crax to the hammermill where everything is ground then go to screening. The advantage of this approach is that it is simpler there are fewer pieces to maintain and less floor space is required. In addition, a two deck screen is no longer required as everything will be properly sized before screening. In many plants, they do not incorporate an overs system at all. The only overs are fibers, hair or other particulate that you do not want in your finished product anyway. The only time you see good product left on the screen that is not properly sized gives immediate visual evidence that there is a hole in the hammermill screen. The hole in the hammermill is not indicated if you go over the screen first as there are always overs in the first approach. The disadvantage is a bigger hammermill is required and the hammers and screens wear faster.
Hammermill and Screen Options:
A number of options need to be considered on a hammermill and as this is just a primer I will not get into the details. But, things to consider are: the size of the throat in rendering applications, magnets and their construction, fire suppression system requirements, air handling equipment for proper hammermill performance, dust collection, vibration isolation, safety switches, support structure and more. In terms of the screening systems, we can design the size of the vibratory screen and recommend the round type or the Rotex version. With the Rotex, a properly designed cable suspension system is required.
3. Conveyance to finished product storage:
There are four conveyance methods I will discuss here. The most common method is a series of screw conveyors, vertical lifts, and/or bucket elevators to finished product storage. When I started in the industry, this was about all I would ever see in rendering plants.
The next method I have seen is drag conveyors. The advantage of this approach is that the conveyor can go from horizontal to vertical and or some other angle without many different conveyors or motors. Alloy has not used this technique, but I have seen them utilized with success in a number of plants.
Pneumatic conveyance - Alloy has utilized Kice pneumatic systems in recent years with great success. I was working with one of the biggest US packer-renderers and we were designing a plant for one of their sister companies in Central America. The rendering expert I was working with told me about the Kice system and how they had been using it for a long time in multiple plants. The advantage of this approach is that it is incredibly clean with just a small pipe running through rendering on the way to storage. The system uses either a blower or a fan depending on if it is a positive system or a negative system and it utilizes rotary air locks to maintain the pressure in the system. A pre-packaged bagging system is required to deal with the dust on these pneumatic systems. Alloy installed the Kice system in Central America based on the recommendation and we have now utilized it the US as well.
The last system we have seen in a modern rendering plant but we have not used before in one of our projects. Chain-vey is a newer product that is similar to a drag conveyor but it runs is a small tube. The tube we witnessed was clear so you could see the product and the paddles inside the tube as the product was being moved. This has a similar advantage as the Kice system in that it is really clean and the pipe can make a number rises and turns that would be difficult for other devices to accomplish. This has an advantage over a pneumatic system in that less air is required so filtering is not as critical.
Whatever system is chosen, it is important to note its advantages and disadvantages and allow for proper design.
4. Crax Cooling-
Most plants do not need anything special for crax cooling as the crax has plenty of time to cool before loadout. Plants that utilize bags and plants that are short on storage, require crax cooling before storage. Alloy utilizes a crax cooling system that is designed for this purpose.
If you have any needs relating to grinding, screening and storage talk to one of our applications engineers. We design in 3D models, so it is easy to conceptualize what our thoughts are for your process. We can handle the big projects from concept through installation and start-up, in addition we also work on smaller systems or individual pieces of equipment.