Art of Grinding-Particle size reduction

Particle size reduction is the first engineering step in Feed manufacturing process. The objective of particle size reduction is to improve feed efficiency by increasing surface area of grains and other ingredients. This increased surface area of nutrients is exposed to animal digestive system. This leads to complete digestion of ingredients and results in better feed efficiency.  Researchers are continuously studying the Effect of particle size on animal performance – especially in broiler performance. It is also recognized that size of particle and uniformity of particle size are important for pellet quality and animal performance. The research data clearly suggests that particle size is more critical in mash feed than pellet or crumbled feed. For a pellet feed manufacturing plant, the grinding is the second largest energy consumption process. Hence, the grinding process is lot more critical in terms of both animal performance and feed mill productivity. There are two types of machines used for particle size reduction-Hammer mill and Roller mill. Most of the feed mills uses hammer mills. Particularly, aqua feed manufacturing plants uses roller mill after hammer mill to achieve fine grinding.
In Hammer mill, set of moving high speed hammers breaks the grains which comes in contact by hammering effect. Whereas in roller mill, particle size is reduced by pair of rotating rolls by constant compression force. The roller mills produce more uniform particle size than hammer mill.
Challenges in Grinding
The efficiency of hammer mill depends on several factors (Martin, 1985). Some of the key factors are:

  1. Tip speed
  2. Hammer design
  3. Number of hammers
  4. Open area in screen
  5. Hammer mill motor HP or kW
  6. Moisture content of raw material
  7. Gap between hammer tip and screen (sieve)
  8. Raw material flow rate to hammer mill
  9. Air flow through hammer mill
  10. Raw material screening system

The first five points (Tip speed, Hammer design, number of hammers, Open area in screen and Hammer mill motor HP/ kW) are arrived by the machine manufacturer while designing the hammer mill. But the user has a main role in providing the data of raw material, tentative recipe and the local conditions etc. to choose the right machine by the supplier. These parameters should be maintained constant in hammer mill.
Maintaining minimum variations in rest of the points is a big challenge for the operation team.
Moisture content – When moisture content of grain is low, the hammer mill produces more fine particles and increases the throughput of hammer mill (TPH) and reduces the Specific energy(kW/Ton) of hammer mill. When the moisture content of raw material is high, grinding produces coarse particles. The performance of hammer mill is consuming more Specific energy and reduces the throughput.
Gap between hammer tip and screen (sieve) – The gap will increase along with wear and tear of hammers. Increased gap affects the performance of hammer mill both in productivity and quality of grinding.
Material flow rate –High flow rate of materials gives coarse grinding as well overloads hammer mill motor. Low flow rate affects the throughput of hammer mill.
Air flow through hammer mill – Air will be trapped in the grinding chamber because of high speed rotation of rotor and hammers. Air trap inside the grinding chamber leads to grinding storm and results in poor performance of hammer mill. The air trap will be avoided through proper functioning air assist system.
Raw material screening system –It is very important to screen the materials from impurities like stone, maize cob, iron and non-ferrous components, dust etc. Especially when metal components enter the grinding chamber, it brakes the hammer and damages the screen. Stone removers, jute removers and magnets are used to screen the materials.
Critical check point in Grinding

  1. Particle Size and Distribution

Both particle size and particle size distribution are important for animal performance and feed mill productivity. The research data recommend to have particle size of 600 to 900for corn-based broiler diet to achieve the optimum animal performance. Geometric Mean Diameter (GMD) and Geometric Standard Deviation (GSD) are the key factors of interest. Another important factor in particle size analysis is the distribution of particles. A histogram is plotted with the data of particle size analysis. It is important to calculated, 68% of particles lies between what particle sizes.
The particle size and its distribution should be continuously monitored and recorded. Particle size is critical to quality factors (CTQ) to a pellet press. Because it influences both feed quality (PDI) and productivity.

  1. Specific Energy of Hammer mill

It is a measure of the actual productivity of grinding equipment, (Hammer mill or roller mill). The Specific energy is the amount of electrical energy consumed by the equipment (hammer mill) to produce 1 ton of ground material. It is expressed in kW/Ton. To measure specific energy of hammer mill, it recommended to have a separate energy meter and a timer in the hammer mill’s electrical circuit. The timer helps to measure the running time of hammer mill. Some feed millers install the energy meter in the mash section. Modern hammer mills are having provisions to measure these metrics. Periodically, the energy consumption in kW and output of hammer mill in tons are measured. Specific energy is calculated as
Specific Energy = Hammer mill energy consumption in kW  Hammer mill output in tons
The metrics of Specific energy should be continuously monitored and recorded. By analysing this metrics, the operation team gets lot of insights about type of formulation, raw material conditions like moisture content, hammer mill conditions, wear and tear of hammers (beaters) and screens (sieves). The data isalso useful to implement Condition Based Maintenance (CBM) program to hammer mill.

  1. Maintenance cost

The spare parts used in feed mill can be classified in two types:

  1. Production consumable spares and
  2. Machine spares

Hammers and screens can be accounted as production consumable spares. All other mechanical and electrical spares associated to hammer mill can be accounted as machine spares. The purpose of this classification is to analyse the cost of grinding more specifically.
Material of construction (MOC) and hardness of the hammer is critical for its wear resistance property. For sieves, hole pitch and hole pattern are important factors which influences the throughput (TPH) and specific energy of hammer mill and the particle size distribution.
On daily routine, the following check points should be checked and take appropriate action for healthy hammer mill operation
Magnets condition
Hammer condition
Sieve conditions
Leakages in hammer mill
The particle size of ground material is a critical to quality factor in a feed mill. Because, it influences the performance of every sub process of feed mill – mixing, conditioning, pelleting and cooling. In addition to that particle size influences feed quality. It has the direct connotation with digestibility of feed. Hence periodic evaluation of particle size is a critical analysis for a feed mill QA program. Periodic monitoring of hammer mill performance is an important activity as a in-process quality assurance (IPQA) program.
This is the part-II of the column contributed by M Kanagaraj who is an expert on feed milling, for any further details he can be reached at
References are available upon request.
by M Kanagaraj, Catalyst Techvisor Private Limited