Stage grinding with hammer mill and crushing roller mill

[Translate to Englisch:] Image alttext

Optimization of the particle size structure of a pig feed mixture with a high barley content by means of stage grinding with hammer mill and downstream crushing roller mill


Animal nutrition research findings have shown that not only the formula and the ingredients contribute to the feeding success, but that also the feed structure (particle size spectrum) and feed form (e.g. pellets, crumbles) exert a certain influence. The assessment of pellets should take into account the fact that each pelleting process also implies additional crushing of coarser particles. For assessing mealy feed such as pig feed dry screen analysis is sufficient, whereas for pellets wet screen analysis is required in order to determine also the crushing effect in the pelleting press.

In the past decades, feeding tests with mealy pig feed have shown that a coarser structure and above all a lower fines content influence the fattening result and the pigs' health favourably. Previous scientific studies have revealed an improved digestibility of finer product. Yet, the results of long-term tests do not take into account the animals' health. Various studies carried out as early as in the 90s as well as more recent results show that with the fines content increasing the risk of formation of gastric ulcers increases, too, and that also the gastric volume changes. This causes a decreasing feed intake or, in extreme cases, even the animals' death. It was also demonstrated that the improved digestibility of ingredients such as starch or protein of finely ground feed was overrated, in particular since the slightly worse digestibility in the small intestine can be compensated to a large extent by subsequent fermentative digestion in the colon. Coarser grinding also supports the formation of the pH gradient in the stomach and thus the resistance of the gastrointestinal tract to pathogens.

Apart from nutritional aspects, the reduction of fines also causes a more uniform grain structure, i.e. the fraction of medium-sized particles increases. Thus, a more stable mixture is obtained with a higher resistance to segregation on its way from the mixer to the feeding trough. In addition to that, the flow properties in the silo cells and automatic feeders are improved due to the fines reduction.

Almost all feed mills are traditionally equipped with hammer mills which are mainly used in mixed grinding. Despite the use of stage grinding with pre-/post-mill and intermediate screening it is not possible to keep the fines content at an acceptable level. In flour milling, crushing of wheat and rye with a low fines content in the first grinding stage is known. If, however, only roller mills are used for grinding of pig feed, problems are to be expected with regard to crushing the husks of barley or oats. Very early it was realised that stage grinding with a roller mill in the second grinding stage is more suitable for grinding barley.

Therefore, it is necessary to find a compromise that combines grinding with a low fines content by means of rollers and crushing by means of a hammer mill suitable for crushing husks. For this purpose, a well-known compound feed factory and the machine factory Amandus Kahl GmbH & Co. KG initiated a project that was conducted by students of the German Milling School Braunschweig with the aim of producing pig feed rich in barley with a low fines content (max. 25% <0.5 mm) and at the same time sufficient crushing of the husks.

Material and methods

In order to determine the influence of different crushing machines and grinding systems on the feed structure, a typical pig feed mixture with the following composition was chosen:

  • Barley, approx. 30 %
  • Rye
  • Wheat
  • Soybean meal
  • Rape meal
  • Mill by-products
  • Premixes

The "natural" content of fines of <0.5 mm in the mixture prior to grinding was approximately 5%, due to additives and fines of the individual raw materials. All measurements of the fines content which will be referred to later were made with the final mixture from the batch mixer.

Hammer mills and crushing roller mills were used for crushing. The operating mode of hammer mills is known. Crushing roller mills are roller mills which are meant to break the grain into smaller particles with an as low as possible fines content. Thus, granular crumbles with a low fines content is to be produced instead of meal. For this purpose, the crushing rollers (Fig. 1) are provided with a "sharp to sharp" (S:S) corrugation. The rollers have different circumferential speeds so a cutting effect is obtained instead of a crushing effect. The speed of the rollers and the lead are usually firmly adjusted, however, speed modification and a change of the lead during operation would be ideal.

Fig. 1: fast running / slowly running roller (from left to right)

The particle size obtained with the crushing roller mill (fig. 2) is determined among others by the corrugation/circumference, the lead and the grinding gap. If the formulae are frequently changed, an automatic grinding gap measurement and remote adjustment of the roller distance are advantageous. It is important that the rollers are fed over the entire width by a suitable feeding device to achieve a uniform load of the rollers and the highest possible throughput.

In the following, the characteristics of the applied grinding machines are indicated:

BWS - Crushing roller mill, twice (with 2 pairs of rollers)

  • Make: Amandus Kahl
  • Roller diameter: 300 mm
  • Roller length: 1500 mm
  • Corrugation: top 2.9 C/cm SS, bottom 3.7 C/cm SS
  • Lead: 1: 1.5
  • Drive: 45 kW

Fig. 2:      Crushing roller mill (BWS), single-stage, twice / two-stages

Crushing roller mill, once (with 1 pair of rollers)

  • Roller diameter: 250 mm
  • Roller length: 1000 mm
  • Corrugation: 5 C/cm
  • Lead: 1: 1.3
  • Drive: 15 kW

LMW - Laboratory mill 100 (fig. 3)

  • Make: Neuhaus Neotec (Amandus Kahl Group)
  • Roller diameter: 250 mm
  • Roller length: 100 mm
  • Corrugation: 4 C/cm, and other options
  • Speed: continuously adjustable for each roller
  • Lead: 1:2.3 variable
  • Drive: 2 x 2.2 kW

Fig. 3: Laboratory grinder LWM 100

HM - Hammer mill

  • Grinding chamber diameter: 1200 mm
  • Grinding chamber width: 640 mm
  • Screen perforation: Pre-mill 10/6 mm, post-mill 2 x 3.5 mm
  • Speed: 1000/1500 rpm
  • Circumferential speed: 60/90 m/s
  • Drive: 155/210 kW

Screen analyses:

  • Sample division into approximately 100 g by means of sample divider
  • Retsch sieve shaker, amplitude 1.6, duration of sieving 10 min

Four variants were chosen for the study:

  1. BWS = Crushing roller mill, (two stages/twice) without intermediate screening
  2. HM + HM  = Stage grinding: hammer mill, with pre-mill / post-mill and intermediate screening
  3. HM + BWS = Stage grinding: hammer mill + crushing roller mill (one stage) with intermediate screening
  4. HM + LMW = Stage grinding: hammer mill + laboratory grinder with intermediate screening

The variant crushing roller mill + hammer mill with intermediate screening was not part of the comparison. Preliminary tests had similar results for crushing of husks using the arrangement "first the hammer mill, then the crushing roller mill" which provides better results than the reverse order "first the crushing roller mill than the hammer mill". A hammer mill in the second grinding stage produces more fines than a hammer mill in the first grinding stage.

All tests were conducted under practice conditions and with high throughputs, except for the test with the combination of hammer mill and laboratory grinder. In this case, the product was crushed at a high capacity in the first stage, and subsequently a part of the material was post-crushed on a laboratory grinder.


To assess the crushing results, a division into the following particle size ranges was made:

  • Fine; designates the range <0.5 mm
  • Medium; designates the range of 0.5 to 1.6 mm
  • Coarse; designates the range of 1.6 to 2.0 mm
  • Very coarse; designates the range > 2.0 mm

The aim was to obtain a maximum accumulation in the medium range with a medium grain size of 1.0 - 1.1 mm. The percentage within the fine particle size range <0.5 mm should be as low as possible and not exceed 25 %. In this context it has to be pointed out that the indication "medium particle size" does not imply any information on the percentage of fines <0.5 mm in a mixture. Therefore the definition and determination of the medium range, for example 0.5 to 1.6 mm, are to be preferred.

The arrangements of the machines and the results obtained with the different crushing variants are shown in the following:

Variant 1:

Crushing using a crushing roller mill, BWS two stages (fig. 4)

Fig. 4: Variant 1, Plant design + bar chart

The result shows that the fines content is lower than 25%. Yet, there is a percentage of about 20 % in the "very coarse range" although this does not have to be considered as negative. The structure is relatively broad. As there is no intermediate screening and no post-crushing, the coarse fraction mainly consists of husks.

Variant 2:

Crushing with hammer mill stage grinding, HM + HM (fig. 5)

Fig. 5: Variant 2, Plant design + bar chart

Though a large screen perforation in the pre-mill and a low circumferential speed have been selected, a very high fines content is produced.

Variant 3:

Stage grinding with hammer mill and crushing roller mill, HM + BWS (fig. 6)

Fig. 6: Variant 3, Plant design + bar chart

With this variant a very good result is achieved which meets the objective in every respect. The percentage of fines <0.5 mm is lower than 25% and the "very coarse" fraction is below 5%. Thus the largest fraction lies in the medium particle size range. Hence, the product has a relatively narrow structure, which is uniform by appearance.

Variant 4:

Stage grinding with hammer mill and laboratory grinder, HM + LMW (fig. 7)

Fig. 7: Variant 4, Plant design + bar chart

Variant 4 is a further improvement of variant 3 due to the modification of the lead and the adjustment of the circumferential speed of the rollers. That adjustment was not possible with the existing crushing roller mills which were used. Of course, a corresponding system can be retrofitted. The variant shows that by means of adjusting the circumferential speed of the rollers and the lead, the grain structure can easily be optimised.

Reduction of the specific electrical energy in kWh/t

In all variants, the electric energy consumption in kWh/t was determined by measuring the effective power and by meter reading. The results are not presented in detail in this article. Generally, it can be stated that the electrical energy consumption of the variant with two crushing roller mills is by about 50 % lower than that of the variant with two hammer mills; and that of the variant hammer mill + crushing roller mill is by about 30 % lower than in the variant with two hammer mills.


To provide a better overview, the grinding results of the variants 1 to 3 are represented in an undersize cumulative distribution function (fig. 8). Especially the comparison with the distribution density shows which variants ensure a low fines content.

Fig. 8: Undersize cumulative distribution function for stage grinding BWS versus HM + HM and versus HM + BWS

The following conclusions can be drawn:

  • Grinding using a crushing roller mill (BWS) or a hammer mill + crushing roller mill causes a significantly lower fines content compared to crushing using hammer mill + hammer mill. The intended set point value of max. 25% <0.5 mm could be achieved in all variants.
  • The highest percentage in the medium-size particle spectrum is achieved by stage grinding with hammer mill + crushing roller mill.
  • By increasing the lead of the crushing roller mill, the results can be further optimized. Therefore it is recommended to provide for speed adjustment during operation.
  • When a hammer mill and crushing roller mill are used, the grain structure is more uniform and therefore less prone to segregation, while the flow properties are improved. The outward appearance also suggests a homogeneous product, in particular on account of the high concentration of particles in the medium particle size range. The barley husks are sufficiently crushed.
  • The tests have shown that the specific energy consumption can be reduced significantly by using the crushing roller mill.
  • Contrary to hammer mills, crushing roller mills do not require aspiration air at all or only a small quantity if a destoner is installed, for example. The expenditures for fulfilling the ATEX safety requirements will be reduced accordingly.
  • When a crushing roller mill is used, the savings in terms of energy costs are significant compared to that of the hammer mill.

Future prospects

The present article shows that by installing crushing roller mills in new or existing compound feed plants, the feed structure of pig feed rich in barley can be improved considerably, while the electrical energy consumption is reduced significantly.

Coarse barley husks can be reduced also by means of upstream pelleting of the grain using a flat die pelleting press with subsequent mixing before the treated barley is fed into the mixed grinding section.

Tests in the pilot plant have shown that in pan grinder mills (modified flat die pelleting press) the husks are defibered transversely and not longitudinally as in the case of hammer mills and above all crushing roller mills.

With Schule whitening machines, type VPC, it is possible to grind the husks off the whole grain to a certain degree. An alternative use of the ground off and separated husk fraction should be considered.


Animal nutrition research findings have shown that an increased content of fines in the pig feed meal can have a negative influence on the health and performance of the animals. This is caused by the formation of gastric ulcers in the animals, by a non-optimal pH-regime in the stomach and by health problems caused by pathogens in the gastrointestinal tract. From the technical point of view, a non-uniform particle size spectrum leads to segregation and a high fines percentage to poor flow properties in the silo cells. To avoid the above negative influences, a fines content of max. 25 % < 5 mm is aimed at when feeding coarse feed meal, with an average particle size distribution in the range of 0.5 to 1.6 mm. The barley husks should be ground to such an extent that adverse effects on feed intake and feed conversion are excluded and the flow properties are not affected.

The advantages of crushing mixtures having a high maize/wheat content for poultry feed production by means of the crushing roller mill are generally known. It still remains to be clarified whether it is also possible to use the crushing roller mill for the production of pig feed rich in barley. As part of a project work of students of the German Milling School Braunschweig (DMSB), realised with the support of a compound feed factory and the machinery factory Amandus Kahl GmbH & Co. KG, various crushing variants were compared in practical tests with a high plant capacity and a commercial formula with a high barley content. From a variety of tests, the following variants are compared:

  • Crushing roller mill twice
  • Stage grinding hammer mill with pre-and post-mill
  • Stage grinding with hammer mill and crushing roller mill

The tests show that - contrary to the hammer mill - by means of roller crushing the fines content in the finished product < 0.5 mm can be reduced considerably to below the target value of 25 %. However, to both reduce the fines content and achieve a high percentage within the medium range of 0.5 to 1.6 mm, the combined stage grinding with hammer mill and crushing roller mill is more suitable, since in this case the coarse fraction, i.e. mainly husks, will also be reduced sufficiently.

Results of preliminary tests show that the arrangement "first the hammer mill, then the crushing roller mill" is more appropriate than the reverse order, since post-grinding with the hammer mill produces more fines than post-grinding with the crushing roller mill. Since the requirements on the feed structure can vary very much depending on the region and competitive conditions, each application has to be considered individually. An adjustment of the speed and the lead as well as the automatic grinding gap measurement with remote adjustment is recommended if the formulae are often changed.

The use of a crushing roller mill, whether in combination with a hammer mill or not, produces in all cases a significant reduction of the specific energy consumption (kWh/t) in the range of 30 to 50%. Contrary to hammer mills, crushing roller mills do not require aspiration air at all or only a small quantity if a destoner is installed, for example. The expenditures for fulfilling the ATEX safety requirements will be reduced accordingly.