NO348857B1 - Method for improving particle size distribution and reducing energy consumption of a hammer mill milling seeds - Google Patents

Description

METHOD FOR IMPROVING PARTICLE SIZE DISTRIBUTION AND REDUCING ENERGY CON-SUMPTION OF A HAMMER MILL MILLING SEEDS

FIELD OF THE INVENTION

The invention relates to a hammer mill, and more particularly to a hammer mill for processing seeds. More particularly the invention relates to use of a screen provided with elongated apertures in a hammermill. The invention also relates to a use of the hammermill provided with the screen to obtain a more optimal particle size distribution of the grinded material compared to using screens with circular apertures, i.e., with less very fine particles and very coarse particles within the milled material. The invention also relates to a reduced energy consumption and/or an increasing grinding capacity compared to the use of screens with circular apertures.

BACKGROUND OF THE INVENTION

Seed for animal feed is typically ground in a hammer mill. A typical hammer mill comprises a cylindrical chamber and a rotor coaxially mounted within the chamber. Several hammers are pivotally mounted to the rotor. The hammer mill further comprises a screen through which processed seed is pushed. The apertures in the screen are typically circular and between three to eight mm in diameter.

To grind the seed, the seed is fed to the chamber and the rotor is spun at a high speed, resulting in the hammers crushing the whole seed particles. When the seed particles have been reduced to an appropriate size, the processed particles pass through the apertures in the screen perpendicular to the direction of the flow of material inside the hammer mill. Different coarsenesses of grinding are mainly achieved through varying the screen hole dimensions, with larger holes resulting in a coarser grinding. In addition, some hammer mills are fitted with a frequency converter, allowing for adjustment of the speed of rotation of the hammers. In such hammer mills speed of rotation can be reduced to increase coarseness. However, a too coarse particle size is not optimal, since poorly ground feed may result in a too poor digestibility.

A fundamental problem with this grinding method is that the ground material will not easily pass through the holes in the screen. This is because the particles are moving at high speed along the inner surface of the screen in a direction perpendicular to the direction needed to pass out of the holes in the screen. The result is that the ground material is circulating at a high energy cost within the hammer mill much longer than necessary, being eroded down to very fine particles due to repeated contact with the hammers, other particles as well as the edge on the far side of the holes as the particles pass over the holes.

In the feed industry, it is preferable to avoid large amounts of very fine particles in the feed product and to have particles as much as possible within a rather narrow range within the optimal particle size range for each specific animal species. Diets which contain sufficiently coarse particles without too many too large particles have been demonstrated to be associated with a high nutrient digestibility while at the same time maintaining a good intestinal health. In poultry, such diets will stimulate gizzard development, which will result in increased nutrient digestibility and a better health, while in ruminants they will reduce the incidence of harmful acidification of the rumen. Moreover, diets with less very fine particles have been associated with healthier stomachs in pigs due to a lower incidence of gastric ulcers. It is therefore desirable to have a defined coarse structure without too much fine particles in feed mixtures for most animal feeds.

Grinding seeds by hammer mills is an energy consuming process. A hammer mill may typically comprise an electrical motor of 100-400 kW, which results in an energy consumption of 5 to 20 kWh per ton. A more energy efficient process or efficient operation of the hammer mills may reduce the energy consumption per ton of milled seed. Due to the quantity of seeds that are milled in a plant, such a reduced energy consumption will be a significant saving in operational costs.

Document US2617600 A describes a screen for a hammer mill. The screen comprises a combination of circular holes and elongated apertures. The elongated apertures are in one embodiment disclosed as several vertically aligned holes connected with vertical cuts or slots between the adjacent vertically aligned holes. The slots are narrower than the diameter of the holes. In an alternative embodiment the elongated apertures are described as circumferentially elongated arcuated slots. In both embodiments the elongated apertures are positioned at the end portion of the screen where air and grinded material passes through the screen tangentially relative to the screen when the screen is installed in the hammer mill. This will maintain the velocity of the air and the material that pass through the screen at this portion and thereby there is a little tendency for the slots to be become clogged.

Document US1185620 A describes a removable screen for a disintegrating apparatus such as a hammer mill. The removable screen may comprise slots.

Document GB2171331 A describes a hammer mill comprising a screen. The screen may be provided with openings in form of slots. The slots are advantageously of rectangular shape.

Malczewski discloses that round hole screens are most commonly used in milling machines. Slotted screens, i.e., screens comprising elongated apertures, are good for sticky, high moisture and fibrous products due to the larger hole area (Malczewski, D.2023. Screen selection criteria for milling machines, Processing Magazine).

SUMMARY OF THE INVENTION

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

According to the invention using elongated holes or apertures instead of round or circular holes will reduce the problem of particles not being able to pass through the holes. Without being bound by theory, sufficiently elongated holes will result in that dry particles passing over the elongated hole will due to the curvature of the elongated hole be able to pass through the screen and out of the milling chamber without hitting the far edge of the hole. Thereby the problem of fines of dry particles due to the aforementioned erosion is reduced, while at the same time energy consumption in the grinding process is reduced. The small width of the elongated hole will hinder too large particles to pass through the screen.

The optimal particle size in feeds may vary for different animal species and depends on diet composition. While a particle size between 1.6 and 2.8 mm will be optimal for poultry, other farm animals such as swine and ruminants may have other optimal particle size ranges, for example between 0.5 and 3.5 mm.

Dry seeds have a moisture content of less than 20%wt, more preferably such as 15%wt and less, even more preferably such as 14wt% and less.

In a first aspect the invention relates to a method for milling of dry seeds. The method comprising the steps of:

a) providing a hammer mill comprising a grinding chamber and at least one screen provided with elongated apertures;

b) delivering dry seeds into the grinding chamber;

c) operating the hammer mill to grind the dry seeds to fragments;

d) passing the fragments through at least one of the screens from a first side; and e) collecting the fragments on the other side of the at least one screen.

The hammer mill may be operated at full speed. The hammer mill may be operated at a speed between 35% of full speed and 100% of full speed.

The elongated apertures may be 3mm wide and 40mm long. The elongated apertures may be 3mm wide and 60mm long. The elongated apertures may be 4mm wide and 40mm long. The elongated apertures may be 4mm wide and 60mm long. The elongated apertures may have rounded ends. The ratio of the width and the length of the apertures may be between 1:2 and 1:300.

In one embodiment the method may have reduced energy consumption compared to using a screen with circular apertures.

In one embodiment the method may have increased capacity compared to using a screen with circular apertures.

It is also disclosed a method for reducing energy consumption when obtaining a ground material of dry seeds with fragments in the range of from and including 0.5mm to and including 3.5mm compared to using a screen with circular apertures. The method may comprise the steps of:

a) providing a hammer mill comprising a grinding chamber and at least one screen provided with elongated apertures;

b) delivering dry seeds into the grinding chamber;

c) operating the hammer mill to grind the dry seeds to fragments; and

d) collecting the fragments on the other side of the at least one screen.

The fragments of the obtained ground material may be larger than 0.75mm, such as 1.0 mm, such as 1.25mm, such as 1.5mm such as 1.6mm. The fragments of the obtained ground material may be less than 3.25mm, such as 3.0mm, such as 2.8mm. The fragments of the obtained ground material may be in a range obtained by any combination of from 0.5mm, 0.75mm, 1.0mm, 1.25mm, and 1.6mm and to 2.8mm, 3.0mm, 3.25mm, and 3.5mm.

The hammer mill may be operated at full speed. The hammer mill may be operated at a speed between 35% of full speed and 100% of full speed.

The elongated apertures may be 3mm wide and 40mm long. The elongated apertures may be 3mm wide and 60mm long. The elongated apertures may have rounded ends. The width and the length of the apertures may be between 1:2 and 1:300.

It is also disclosed a method for increasing grinding capacity when obtaining a ground material of dry seeds with fragments in the range of from and including 0.5mm to and including 3.5mm with an increased capacity of a hammer mill compared to using a screen with circular apertures. The method may comprise the steps of:

a) providing a hammer mill comprising a grinding chamber and at least one screen provided with elongated apertures;

b) delivering dry seeds into the grinding chamber;

c) operating the hammer mill to grind the dry seeds to fragments; and

d) collecting the fragments on the other side of the at least one screen.

The fragments of the obtained ground material may be larger than 0.75mm, such as 1.0 mm, such as 1.25mm, such as 1.5mm such as 1.6mm. The fragments of the obtained ground material may be less than 3.25mm, such as 3.0mm, such as 2.8mm. The fragments of the obtained ground material may be in a range obtained by any combination of from 0.5mm, 0.75mm, 1.0mm, 1.25mm, and 1.6mm and to 2.8mm, 3.0mm, 3.25mm, and 3.5mm.

The hammer mill may be operated at full speed. The hammer mill may be operated at a speed between 35% of full speed and 100% of full speed.

The elongated apertures may be 3mm wide and 40mm long. The elongated apertures may be 3mm wide and 60mm long. The elongated apertures may have rounded ends. The width and the length of the apertures may be between 1:2 and 1:300.

Use of the screen according to the invention result in superior particle size distributions of processed seeds compared to use of a standard 3mm screen, and a reduced energy consumption.

FIGURES AND SPECIFIC DESCRIPTION

Figs. 1A-B show schematically a working principle of a hammer mill; Fig.1A: with a new, unused screen; Fig.1B: with a worn screen;

Figs.2A-B show a photograph of a screen for a hammer mill; 2A: a whole screen, the whole screen is bent with the short sides closest to a camera; 2B: sections of two screens side by side at a larger scale compared to Fig.2A;

Figs.2C-E show different embodiments for distributing elongated apertures on a screen;

Figs. 3A-C show results from a first experiment comparing maize, wheat and oats particle size distribution from grinding processes using a horizontal hammer mill comprising a standard screen with either 3mm, 6mm, or 8mm circular apertures and the same hammer mill comprising a screen with elongated apertures 3mm x 40mm; Fig.3A: effect of screen pattern on proportion of too fine particles (<1mm); Fig.3B: effect of screen pattern on proportion of too coarse particles (>2.8mm); Fig.3C: effect of screen pattern on proportion of optimal sized particles for poultry (1.6-2.8mm);

Fig.4 shows effect of screen patterns on grinding capacity at 18 ampere mill load, from the same experiment as described for figures 3A-C;

Figs. 5A-B show results from a second experiment comparing maize particle size distribution from grinding processes using a commercial horizontal hammer mill comprising a standard screen with 3mm circular apertures, at different hammer speeds and the same hammer mill comprising a screen with elongated apertures 3mm x 60mm; Fig.5A: effect of screen pattern on proportion of too fine maize particles (<1mm); Fig.5B: effect of screen pattern on proportion of optimal sized maize particles for poultry (1.6-2.8mm);

Figs. 6A-B show the wheat particle size distribution, from the same experiment as described for figures 5A-B; Fig.6A: effect of screen pattern on proportion of too fine wheat particles (<1mm); Fig.6B: effect of screen pattern on proportion of optimal sized wheat particles for poultry (1.6-2.8mm);

Figs. 7A-B show the oats particle size distribution, from the same experiment as described for figures 5A-B; Fig.7A: effect of screen pattern on proportion of too fine oat particles (<1mm); Fig.7B: effect of screen pattern on proportion of optimal sized oat particles for poultry (1.6-2.8mm);

Figs. 8A-C show the energy consumption in amperage for the grinding of maize, wheat and oat, from the same experiment as described for figures 5A-B; Fig.8A: maize at fixed feeder rate; Fig.8B: wheat at fixed feeder rate; Fig. 8C: oat at fixed feeder rate;

Figs. 9A-C show results from a third experiment comparing wheat particle size distribution from grinding processes using a commercial horizontal hammer mill comprising a standard screen with 6mm circular apertures, at different hammer speeds, and the same hammer mill comprising a screen with elongated apertures 3mm x 60mm; Fig.9A: effect of screen pattern and hammer speed on proportion of too fine wheat particles (<1mm); Fig.9B: effect of screen pattern and hammer speed on proportion of too coarse wheat particles (>2.8mm); Fig.9C: effect of screen pattern and hammer speed on proportion of optimal wheat particles for poultry (1.6-2.8mm); and

Fig.10 shows the effect of screen pattern on the energy consumption in amperage for grinding wheat at a fixed feeder rate, from the same experiments as described for figures 9A-C.

PRIOR ART

The feed and food industry commonly use hammer mills for reducing particle size of seeds. Such hammer mills most commonly have a horizontal rotor provided with radially oriented bar-like hammers pivotably mounted to the rotor within a grinding chamber. One or two perforated screens make up a part of an outer wall in the grinding chamber. In modern hammer mills of this kind, the perforated screens cover most of the grinding chamber. Seeds are guided into the grinding chamber from above. The tips of the rotating hammers hit the seeds and break the seeds into fragments. The fragments leave the grinding chamber through the perforations in the screen. The fragments are collected in a space outside the screen where the fragments drop down due to gravity and air flow through the hammer mill and are collected below the grinding chamber. In modern hammer mills, the screens cover more than 80% of the circumference of the grinding chamber. The screens may typically cover more than 85% of the circumference such as more than 90% of the circumference.

Figure 1A shows schematically a bar like hammer 1 and a screen 2. The screen 2 comprises several circular holes 3 that are drilled perpendicular to a surface 21 of the screen 2. The screen 2 depicted in figure 1A has holes 3 with sharp edges 31 on the internal surface 23 of screen 2. A hammer tip 11 will hit a seed 4 and break the seed 4 into fragments 41. Some fragments 41 will pass directly through a hole 3, while other fragments 41 will hit the edge 31 of the hole 3. The fragments 41 may thereby be reduced further in size to small particles 43.

Most of the fragments 41 and particles 43 will pass through the hole 3 and be collected outside the screen 2. As shown in figure 1A, some fragments 410 and particles 430 will recoil into the grinding chamber (not shown). Recoiled fragments 410 and particles 430 will in turn be hit by a hammer tip 11 until they pass through a hole 3.

The fragments 41 bouncing at high speed into the edge 31 will over time wear the edge 31 on the far side of the hole 3 relative to the rotational direction of the hammers 1 as indicated in figures 1A and 1B. The edge 31 becomes rounded or chamfered. As illustrated in figure 1B, a larger portion of the fragments 41 and particles 43 will recoil into the grinding chamber when the screen 2 becomes worn. This reduces the effectiveness of the hammer mill. The hammer mill must run for a longer time to get the seeds 4 through the screen 2 which means reduced capacity and increased energy consumption.

It is common practice to turn the screens 2180° to even the wear on the edges 31, and to run the engine in both directions to wear the edges on both sides. However, the screens 2 need to be replaced at regular intervals due to the wear on the edges 31.

Screens 20 with elongated apertures are depicted in figures 2A and 2B. A whole screen 20 is shown in figure 2A. The screen 20 comprises several elongated apertures 5 of same shape and size. The apertures 5 are formed as elongated through holes with parallel sides 51 and rounded end portions 53 as best seen in figure 2B. These apertures 5 are referred to as rectangular shaped apertures 5 due to their overall appearance. In other embodiments the apertures 5 are formed with straight end portions 53 as in rectangles.

The apertures 5 have a longitudinal direction that corresponds to the rotational direction of the hammers 1. As seen in figure 2A, the apertures 5 of same shape and size are distributed over most of the area of the screen 20. The apertures 5 may be distributed over the screen 20 in different patterns. Three possible embodiments are depicted in figures 2C-E. The skilled person will know that other patterns are possible.

By example only, the apertures 5 may be 3mm wide and 30mm long or 3mm wide and 60mm long. The width of the apertures 5 is adapted to the size of dry seeds 4 to be milled. The required length of the apertures 5 can be calculated by taking into consideration the thickness of the screen 20 and the circumference of the grinding chamber.

The required length is the length needed for the fragments 41 to pass through the aperture 5 without hitting the end portion 53. The actual length of the apertures 5 may be longer or shorter than the required length. If the length is longer than the required length, the fragments 41 will pass through the apertures 5 without hitting the end portion 53 at all. If the length of the apertures 5 is shorter than what is required for the fragments 41 to pass through the apertures 5 unhindered, the considerably longer distance the fragments 41 can pass over the apertures 5 will still result in a significant reduction in wear on the end portion 53.

EXPERIMENTAL SET UP AND RESULTS

Experiment 1

The first experiment was run on a small horizontal hammer mill provided with an electrical motor of 18.5 kW. In this particular hammer mill, rotation speed cannot be varied, and the feeder rate is automatically regulated to reach an amperage set by the operator. The standard screens used were commercially available grinding screens comprising circular apertures of either 3, 6 and 8mm diameter and a maximum light opening. These were compared with a tailor-made screen comprising a maximum number of elongated apertures of 3mm width and 40mm length. The lengths were oriented parallel to the rotational direction of the hammers. Whole seed batches of the common feed seeds maize, wheat and oats were ground on the same day using all the screens. Aliquots of seeds from the same seed batch were ground on the different screens. Feeder rate was recorded in order to measure energy consumption, and particle size distribution of the resulting ground material collected after grinding was measured by passing 100 g of ground material through a series of sieves placed on standard laboratory shaking machine at a fixed amplitude. Results from Experiment 1 are presented in figures 1 and 2.

Referring to Fig.3A, grinding processes wherein a hammer mill provided with a standard screen of either 3mm, 6mm or 8mm circular apertures result generally in more too fine cereal particle sizes (<1mm) compared to grinding processes wherein a screen with elongated apertures (3mm x 40mm has been used).

Now referring to Fig.3B, grinding processes wherein a hammer mill provided with a standard screen of either 6mm or 8mm circular apertures result generally in more too coarse cereal particle sizes compared to grinding processes (>2.8mm) wherein a screen with elongated apertures (3mm x 40mm) has been used. As expected, using a standard screen provided with 3mm circular apertures results in very few too coarse particles.

Fig.3C shows the percentage of particles of a size which can be considered to be neither too small nor too large, in other words within an optimal particle size range (1.6mm to 2.8mm). This shows that compared to any of the standard screens the screen with elongated apertures yielded the highest percentage of such optimal particles for the cereals maize, wheat and oat.

To summarize, the results presented in figures 3A-C show that the screen with elongated apertures results in less too fine and too coarse particles and consequently a higher proportion of optimal particles, compared to any of the current commercial screens.

Figure 4 further shows that with the exception of the screen with 8mm circular apertures when wheat and oats were ground, the screen with elongated apertures has a higher grinding capacity (kg/h). Due to the fixed energy consumption (18 ampere) in this particular hammer mill, this means a lower energy consumption in the grinding process per ton grinded cereals.

Experiment 2

The second experiment was run on a commercial horizontal hammer mill provided with an electrical motor of 200 kW. In this hammer mill, rotation speed can be varied while the feeder rate is kept constant. The standard screen used was a commercially available grinding screen comprising circular apertures of 3mm diameter and a maximum light opening. This screen was compared with a tailor-made screen containing a maximum number of elongated apertures of 3mm width and 60mm length. The lengths were oriented parallel to the rotational direction of hammers. Whole seed batches of the common feed seeds maize, wheat and oats were ground on the same day using both the screens. Aliquots of seeds from the same seed batch were ground on the different screens. Rotation speed was varied. Amperage was recorded in order to measure energy consumption. Particle size distribution was measured as described in Experiment 1. Results from Experiment 2 are presented in figures 5, 6 and 7.

Figures 5 to 7 show that the screen with elongated apertures (rectangular 3mm x 60mm) responds in a similar way as a standard commercial screen provided with 3mm circular apertures to a reduced hammer speed. The amount of too fine particles (<1mm) decreases, while the amount of optimal sized particles for poultry (1.6mm to 2.8mm) increases. Thus, the potential for producing a feed with a high content of particles within the optimal size range for poultry is very high when combining the screen with elongated apertures and a reduced hammer speed. In addition, it is not possible to produce similar high levels of optimal sized particles for poultry using a standard commercial screen provided with 3mm circular apertures. This is particularly clear for grinding of high-fibre ingredients such as oat as seen in figure 7B.

Another surprising advantage of using the screen with elongated apertures is shown in figures 8A to 8C. While energy consumption measured as amperage increases considerably when hammer tip speed is reduced for a normal commercial screen, the screen with elongated apertures does not exhibit any considerable increase in energy consumption when hammer tip speed is reduced. Thus, a coarser and more optimal grinding by reducing hammer tip speed is a more viable option with the screen with elongated apertures, see figures 5B (maize; 45% of full speed), 6B (wheat; 50% of full speed) and 7B (oat; 70% of full speed).

Experiment 3

The third experiment was run on the same commercial horizontal hammer mill as described in Experiment 2. The standard screen used was a commercially available grinding screen comprising circular apertures of 6mm diameter and a maximum light opening. This screen was compared with the tailor-made screen as described in Experiment 2. Aliquots of wheat from the same batch of wheat were ground on the different screens while rotation speed was varied, i.e., full speed; 70% of full speed; 60% of full speed; and 50% of full speed. Amperage was recorded in order to measure energy consumption, and particle size distribution of the resulting ground material collected after grinding was measured by passing 100 g of material through a series of sieves placed on a standard laboratory shaking machine at a fixed amplitude. Results from this experiment are presented in figures 8 and 9, where they have been compared with similar values recorded for the same type of cereal ground at the same conditions in the second experiment.

The results from the third experiment are shown in figures 9 and 10. The purpose of this experiment was to compare two screens with a more similar coarse grinding potential with regard to the effect of hammer speed. As shown in figure 9A, the screen with elongated apertures (3mm x 60mm) produced less fines than the standard commercial screen provided with 6mm circular apertures at all speeds and had a higher potential for maximizing particle sizes in the optimal range. However, even more important, with the screen with elongated apertures, the proportion of too coarse particles were much lower compared to the screen with 6mm circular apertures, and with a smaller increase as hammer speed decrease, as illustrated in figure 9B. At the same time, as illustrated in figure 9C, the amount of optimal particles is generally higher with the screen with elongated apertures. This illustrates a fundamental problem with standard commercial screens with circular apertures. When circular apertures are used to control particle size, the large diameter of the circular aperture required results in that too much unground or coarsely ground material escapes the milling chamber, particularly when combined with a reduced hammer speed. The screen with elongated apertures, however, due to the constricted width, will to a large extent hinder too large particles in passing through the screen independent of hammer tip speed. As shown in figure 10, although both screens exhibit a low increase in energy consumption when the speed decreases, the energy consumption is considerably lower with the screen with elongated apertures compared to the standard commercial screen.

To summarize, the screen with elongated apertures to a large extent overcomes the fundamental problem of a too heterogenous grinding result and a too high energy consumption in the hammer milling process, and results in a grinding with a higher amount of optimal particle sizes for animal feed obtained with a reduced energy consumption.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims (5)

C l a i m s

1. A method for milling of dry seeds (4), the method comprising the steps of:

a) providing a hammer mill comprising a grinding chamber and at least one screen (20) provided with elongated apertures (5);

b) delivering dry seeds (4) into the grinding chamber;

c) operating the hammer mill to grind the dry seeds (4) to fragments (41); d) passing the fragments (41) through at least one of the screens (20) from a first side; and

e) collecting the fragments (41) on the other side of the at least one screen (20).

2. The method according to claim 1, wherein the hammer mill is operated at full speed.

3. The method according to claim 1, wherein the hammer mill is operated at a speed between 35% of full speed and 100% of full speed.

4. The method according to claim 1, wherein the method has reduced energy consumption compared to using a screen (2) with circular apertures (3).

5. The method according to claim 1, wherein the method has increased capacity compared to using a screen (2) with circular apertures (3).