## comminution - an overview | sciencedirect topics

This family of models is the oldest of the comminution models and they continue to find widespread use (Morrell, 2014a). Energy-based models assume a relationship between energy input of the comminution device and the resultant effective particle size of the product. Many rely on the feed and product size distributions being self-similar; that is, parallel when cumulative finer is plotted in log-log space (Chapter 4). The energy input is for net power, that is, after correcting for motor efficiency and drive train mechanical losses. Typically, energy is measured as kWh t1 or Joules, depending on the model.

The oldest theory, Von Rittinger (1867), stated that the energy consumed in size reduction is proportional to the area of new surface produced. The surface area of a known weight of particles of uniform diameter is inversely proportional to the diameter, hence Von Rittingers law equates to:

As Lynch and Rowland (2005) note, the means to make measurements of energy and size necessary to validate the Von Rittinger and Kick models did not exist until the middle of the twentieth century when electrical motors and precision laboratory instruments became available. The literature from this period includes work by a group at the Allis Chalmers Company who were trying to calibrate Von Rittingers equation to industrial rod mills (Bond and Maxson, 1938; Myers et al., 1947).

Often referred to as the third theory, Bond (1952) stated that the energy input is proportional to the new crack tip length produced in particle breakage. Bond redefined his theory to rather be an empirical relationship in a near-final treatise (Bond, 1985). The equation is commonly written as:

where W is the energy input (work) in kilowatt hours per metric ton (or per short ton in Bonds original publications), Wi is the work index (or Bond work index) in kilowatt hours per metric ton, and P80 and F80 are the 80% product and feed passing sizes, in micrometers.

Solving Eq. (5.1b) for n=3/2 gives the same form as Eq. (5.4) with the constant 2 K ahead of the bracket. In effect the 2 K is replaced by (10Wi), which is convenient because Wi becomes equal to W in the case of grinding from a theoretical infinite feed size to 80% passing 100m. The Bond model remains the most widely used, at least for the conventional comminution equipment in use at the time Bond developed the model and calibrated it against industrial data. It is one reason that the 80% passing size became the common single point metric (mean) of a particle size distribution.

A modification of Eq. (5.1a,b) was proposed by Hukki (1962), namely substituting n by a function of particle size, f(x). This provoked debate over the size range that the three established models applied to. What can be agreed is that all the models predict that energy consumption will increase as product particle size (i.e., P) decreases. Typical specific energy values (in kWh t1) are (Morrell, 2014b): primary crushing (i.e., 1000-100mm), 0.1-0.15; secondary crushing (100-10mm), 1-1.2; coarse grinding (10-1mm), 3-3.5; and fine grinding (1-0.1mm), 10.

Fine grinding tests are sometimes expressed as a signature plot (He et al., 2010), which is an experimentally fitted version of Eq. (5.1a,b) with n=f(x). A laboratory test using a fine grinding mill is conducted where the energy consumption is carefully measured and a slurry sample is extracted periodically to determine the 80% passing size. The energy-time relationship versus size is then plotted and fitted to give (in terms of Eq. (5.1a,b)) a coefficient K and a value for the exponent f (x).

The problem that occurs when trying to solve Eq. (5.5) is the variable nature of the function g(x). A pragmatic approach was to assume M is a constant over the normal range of particle sizes treated in the comminution device and leave the variation in size-by-size hardness to be taken up by f(x). Morrell (2009) gives the following:

where Mi is the work index parameter related to the breakage property of an ore and the type of comminution machine, W is the specific comminution energy (kWh t1), P and F are the product and feed 80% passing size (m), and f(x) is given by (Morrell, 2006):

The parameter Mi takes on different values depending on the comminution machine: Mia for primary tumbling mills (AG/SAG mills) that applies above 750m; Mib for secondary tumbling mills (e.g., ball mills) that applies below 750m; Mic for conventional crushers; and Mih for HPGRs. The values for Mia, Mic, and Mih were developed using the SMC Test combined with a database of operating comminution circuits. A variation of the Bond laboratory ball work index test was used to determine values of Mib. This is similar to the approach Bond used in relating laboratory results to full scale machines. The methodology continues to be refined as the database expands (Morrell, 2010).

Morrell (2009) gave a worked example comparing the energy requirements for three candidate circuits to illustrate the calculations. Taking just the example for the fine particle tumbling mill serves that purpose here (Example 5.1).

From the Mi data the relevant value is Mib=18.8kWh t1. Noting it is fine grinding then the feed F80 is taken as 750m. Combining Eqs. (5.6) and (5.7) and substituting the values:W=18.84(106(0.295+750/1,000,000)750(0.295+750/1,000,000))=8.4(kWht1)

Comminution is a physical pretreatment method involving milling, grinding, and chipping [42]. Such mechanical methods are aimed at increasing the accessible surface area, as well as decreasing the cellulose crystallinity, by decreasing the particle size. Comminution methods should be utilized before the biomass is subjected to any other pretreatment, as it has been shown that fine particle size can greatly improve biohydrogen production. Chen etal. [43] demonstrated particle size effect (<0.297mm to >10mm) on thermophilic dark fermentative biohydrogen production from rice straw, without using any other pretreatment, and found that the highest cumulative biohydrogen produced was from particle sizes<0.297mm (191mL H2/L), while it was 57mL H2/L for particle sizes >10mm. Themain advantage of mechanical comminution is that it does not lead to inhibitor formation; however, the process is not able to remove lignin, and is not cost-effective when considering the energy consumption. It has been reported that energy consumption of these processes increases with the decreasing particle size and with a higher moisture content of the biomass [44].

Comminution is the reduction of solid material particle size by fracture via grinding, milling, or similar processes. Comminution techniques are usually employed to produce particulate nanomaterials (e.g., powder) from larger-sized or bulk materials. Due to simplicity and low cost, attrition or grinding with the assistance of milling media such as milling balls has been used for producing nanoparticles or nanopowder since the late 1990s [31]. However, grinding techniques suffer several problems including particle agglomeration, large residual internal stress, surface defects, and contaminations in the final products.

To overcome these problems, another comminution techniqueatomizationwas developed. Atomization produces solid or aerosol particles with reduced sizes by spraying molten material or material solution or suspension under conditions such that it breaks down and then solidifies as fine powder [32]. In a typical atomization process, a molten material passing through a nozzle scatters into fine droplets by a high-speed medium (e.g., gas or water) and then the droplets solidify to powder. Obviously, the atomization technique is highly efficient for preparing micron and submicron powders at industrial scales and recent development has enabled atomization to produce nanoparticles of sizes down to 20nm [33].

Comminution is needed for the liberation of low-grade ores so that the iron content can be upgraded by gangue removal. This necessitates grinding to such a size that the iron minerals and gangue are present as separate grains. But comminution is an expensive process and economics dictates that a compromise must be made between the cost of grinding and the ideal particle size.

Traditionally, grinding has been carried out using rod, ball, autogenous, or semiautogenous mills usually in closed circuit, that is, after grinding, the material is classified according to size with the undersized portion proceeding to the flotation circuit and the oversized portion being returned to the mill. The major benefit of fully autogenous grinding (AG) is the cost saving associated with the elimination of steel grinding media. In the last 20 years, more efficient grinding technologies, including high-pressure grinding rolls (HPGRs) for fine crushing and stirred milling for fine grinding, have provided opportunities to reduce operating costs associated with particle size reduction. A HPGR has been installed at the Empire Mine in the United States for processing crushed pebbles and its introduction has resulted in a 20% increase in primary AG mill throughput (Dowling et al., 2001). Northland Resources operates the Kaunisvaara plant in Sweden, treating magnetite ore with sulfur impurities in the form of sulfide minerals. The required P80 of the ore, in order to achieve adequate liberation, is 40m. This plant uses a vertical stirred mill after AG rather than a ball mill to achieve this fine grind size with an energy cost saving of 35% or better (Arvidson, 2013).

An important part of the comminution circuit is size classification. This can be accomplished with screens or cyclones or a combination of the two. Since cyclones classify on the basis of both particle size and specific gravity, cyclone classification in the grinding circuit directs coarse siliceous particles to the cyclone overflow. In a reverse flotation circuit, these coarser siliceous middlings can be recovered through increased collector addition but at the expense of increased losses of fine iron minerals carried over in the froth. However, if the required grind size is not so fine, then screening can be used instead of cycloning to remove the coarser particles for regrinding and, thus, produce a more closely sized flotation feed (Nummela and Iwasaki, 1986).

Comminution is used to reduce the particle size of biomass and increase accessible surface area, reducing crystallinity and degree of polymerization (DP), thus increasing the biomass biodegradability [4]. Typical mechanical comminution includes chipping and milling as shown in Table 2.1. Chipping is usually necessary to reduce the size of raw lignocellulosic biomass for further processing, for example, to make log to wood chips. Milling process that can be used for biomass pretreatment involves various types, such as ball milling, hammer milling, knife milling, vibro milling, tow-roll milling, colloid milling, wet-disk, and attrition millings [4]. As indicated by Agbor et al., harvesting and preconditioning can reduce lignocellulosic biomass from logs to coarse sizes (about 1050mm), and chipping can reduce size to 1030mm, while grinding and milling can reduce the size to 0.22mm [11]. Grinding and milling are also effective to alter the inherent ultrastructure of biomass, such as crystallinity and DP of cellulose [7]. As shown in Table 2.1, vibratory ball milling has been found to be more efficient in reducing cellulose crystallinity of spruce and aspen chips than ordinary ball milling [12]. Disk milling has been reported to achieve higher enzymatic hydrolysis yield than hammer milling [13]. The moisture content of biomass should be also considered in the selection of milling methods [4]. For dry biomass, hammer and knife millings are more suitable, while ball and vibro milling are more widely used and suitable for both dry and wet biomass [7,14].

Size reduction by milling also has been employed independently to improve the biodegradability of biomass conversion to biogas, bioethanol, and biohydrogen. When rice straw was subjected to disk and ball milling, the yields of glucose and xylose by enzymatic hydrolysis reached up to 89% (from 78%) and 54% (from 41%), respectively [15]. Glucose and xylose yield increased to 40% and 32% by wet disk milling for pretreated sugarcane bagasse and rice straw, respectively [16]. An obvious advantage of mechanical comminution for size reduction is that no inhibitors are generated during the process. However, various inhibitors are generated in chemical pretreatments, such as hydroxymethylfurfural (HMF), furfural and acetic acid, and so on, which are toxic to yeast or other microorganisms.

Comminution indicates a series of phenomena that occur at different stages of thermo-chemical conversion (combustion and gasification) processes in a FB reactor. All of them contribute to the size reduction of the fuel and char particles, together with the shrinkage related to devolatilization and chemical reactions. Primary fragmentation occurs immediately after injection of the mother fuel particles into the bed as a consequence of thermal stresses and internal pressures caused by the release of volatiles: it may break the particles into several coarse fragments and into a multitude of fines (Chirone et al., 1991). Attrition and secondary fragmentation of char particles occur in parallel with the chemical conversion of char and are both strongly assisted by this process. It continuously generates new asperities on the external surface of the particle, which are then mechanically abraded, so enhancing attrition (Arena et al., 1983). It also increases the size of internal pores and the weakening of the carbon bonds inside the particle, so that a secondary fragment is formed when a bridge is too weak to withstand the hydrodynamic forces on the char (Arena et al., 1996). There are also situations where all internal bridges may collapse suddenly, giving rise to a special type of secondary fragmentation, known as percolative fragmentation, which leads to the production of relatively fine sub-particles. Miccio et al. (1999) showed that percolative fragmentation is the main mechanism of fines generation during biomass char circulation in the freeboard of a FB gasifier, probably as a consequence of the highly porous char generated from these types of fuels. The unfragmented and fragmented char particles produce a further generation of very fine particles (about one order of magnitude finer than those from secondary fragmentation) due to the abrasive action of bed particles. All these different comminution phenomena can have a major or minor extension, or may not even be present, depending on the nature (and the size) of the mother fuel particle (Fig. 17.4).

Rod or bar mills suffer from a lot of wear if the product is very abrasive. This limitation gave rise to the autogenous mill where the grinding bodies are themselves pieces of ore. These pieces are a lot more efficient when they fall from a great height. This is why the diameter of these mills could achieve 11m and their length only 4.5m. They could require an installed power of up to 9,000kW.

by erosion, meaning detaching fine particles from average-sized grain surfaces or from boulder surfaces. As such, the production of fines could be significant, which could be an inconvenience (but not always).

Generally, ore density is much less than the density of steel. To activate fragmentation, we expect to have on the inside of the equipment a low proportion (5% by volume, for example) of steel balls. Wear on this low mass of balls has very little effect on the actual operation cost. Mills that operate in this way are called semi-autogenous mills.

The feed must have a suitable granulometry. In particular, there needs to be an adequate proportion of boulder sizes greater than 250mm. The shortage of large boulders must be compensated by the addition of balls and operation in a semi-autogenous mill.

Elements Sj (xj) of the fragmentation speed matrix give two maxima as a function of xj. The peak corresponding to the lowest values of xj deals with crushing grains, and the other peak with boulder abrasion. The analytical expression for Sj is very empirical and its value is between 5 and 100h1.

This zone corresponds to a variation from approximately 1 to 6 in grain size. To locate the average in the range of grain sizes, Stanley [STA 74] distinguishes abrasion from crushing by visually examining fragments. Crushing produces conchoidal fractures while abrasion produces rounded fragments.

The mass M of solid retained in the mill at the granulometry shows two peaks if we consider the frequencies mi (by mass) as a function of grain size. The peak that corresponds to boulders is 10 times more significant than the one for grains (see Stanley [STA 74, pp. 89 and 90]).

As such, Stanley [STA 74 pp. 81] is an example of the breaking matrix that is a combination of three matrices: erosion, crushing and intermediary. But he goes much further and replaces the erosion matrix with a combination of two matrices where, respectively:

Menacho [MEN 86] provides the distribution frequency for the product exiting the mill (his equation (21)). It is limited to adding the erosion and breaking effects. He calls the average residence time of the powder or of the pulp in the mill.

For an equal production and having a large equipment diameter, energy consumed is greater than that required in ball mills. On the other hand, the consumption of steel from wear on the few balls present is less but the wear on bearings is greater.

The autogenous mill could be fed with boulders reaching up to 800mm and give, in one single operation, a product size that does not exceed a few tens of millimeters (the reduction ratio could exceed 1,000).

The possible flow increases with the diameter D of the equipment as D2.82 and energy consumption as D2.62 only. This explains the tendency for these machines to be gigantic as they can treat up to 300 tonne.h1.

The autogenous mill is equipped with a classification device. In wet mode, the pulp mixture is evacuated through a grid and then treated by a hydro-cyclone or a sieve. Granules are recycled back into the equipment. In dry mode, an air current goes through the equipment and drives the fines fraction, which economizes the entire classification circuit.

Comminution in ball mills and vertical mills differs fundamentally. In a ball mill, size reduction takes place by impact and attrition. In a vertical mill the bed of material is subject to such a high pressure that individual particles within the bed are fractured, even though the particles are very much smaller than the bed thickness.

Early issues with vertical mills, such as narrower PSD and modified cement hydration characteristics compared with ball mills, have been resolved. One modification has been to install a hot gas generator so the gas temperature is high enough to partially dehydrate the gypsum.

For many decades the two-compartment ball mill in closed circuit with a high-efficiency separator has been the mill of choice. In the last decade vertical mills have taken an increasing share of the cement milling market, not least because the specific power consumption of vertical mills is about 30% less than that of ball mills and for finely ground cement less still. The vertical mill has a proven track record in grinding blastfurnace slag, where it has the additional advantage of being a much more effective drier of wet feedstock than a ball mill.

The vertical mill is more complex but its installation is more compact. The relative installed capital costs tend to be site specific. Historically the installed cost has tended to be slightly higher for the vertical mill.

Comminution of LC materials through a combination of chipping, grinding, and/or milling can be applied to improve their digestibility by reducing their particle size and crystallinity. The reduction in particle size leads to an increase in available specific surface and a reduction in the DP [178]. The process also causes shearing of the biomass. Mechanical pretreatment also results in substantial lignin depolymerization via the cleavage of uncondensed-aryl ether linkages [276]. All these combined factors lead to an increase in the total hydrolysis yield of the lignocellulose in most cases by 525% (depending on kind of biomass, kind of milling, and duration of the milling), and also reduces the technical digestion time by 2359% (thus an increase in hydrolysis rate) [277]. It was shown that, without any pretreatment, corn stover with sizes of 5375m was 1.5 times more productive than larger corn stover particles of 425710m [180].

Milling is considered to be an environmentally friendly pretreatment process. In these pretreatments, no hydrolysis or fermentation inhibitors, such as furfural or hydroxymethyl furfural, are produced [278]. However, size reduction of biomass is an energy-intensive process that warrants improvement to raise the energy efficiency involved in bioethanol production. The National Renewable Energy Laboratory indicated that size reduction required one-third of total energy inputs for biomass to ethanol conversion [279].

The use of mechanical chopping [280], hammer milling [281], grind milling [282], roll milling [283], vibratory milling [284], and ball milling [276] have proved successful as a low-cost pretreatment strategy.

Previously, milling reactors have been used as a means of pretreatment. The milling process has been studied prior to and in combination with enzymatic hydrolysis, where mechanical actions, mass transport, and enzymatic hydrolysis are performed simultaneously in order to improve the hydrolysis process. The attrition mill bioreactor [285] and the intensive mass transfer reactor, including ferromagnetic particles and two ferromagnetic inductors [286, 287], are two examples of these processes. Mais et al. [288] used a ball mill reactor for the pretreatment and hydrolysis of -cellulose and reported the number of ball beads as an effective parameter to improve enzymatic hydrolysis of -cellulose.

More recently, studies have shown that energy consumption for grinding biomass depends on initial particle size, moisture content, material properties, mass feed rate, and machine variables [281]. Optimum operating conditions may lead to reduced energy expended for size reduction [289]. Bitra et al. [290] determined the direct mechanical input energy for hammer mill and knife mill [289] size reduction of switchgrass, wheat straw, and corn stover over a range of mill operating speeds, screen sizes, and mass feed rates.

Although milling pretreatments are often described as high-energy requirement techniques, wet disk milling has been recently described as a potential feasible mechanical technique to treat rice straw [291]. Glucose and xylose yields by wet disk milling, ball milling, and hot-compressed water treatment were 78.5% and 41.5%, 89.4% and 54.3%, and 70.3% and 88.6%, respectively. The energy consumption of wet disk milling was lower than that of other pretreatments.

Sugarcane bagasse and straw were treated under comparative conditions by ball milling and wet disk milling techniques and their physical properties and susceptibility toward enzymatic hydrolysis and fermentation were evaluated [278]. Ethanol yields from total fermentable sugars by using a C6-fermenting strain reached 89.8% and 91.8% for bagasse and straw hydrolysates, respectively.

Ball milling pretreatment combined with the addition of dilute acid and alkali proved to be an effective processing to enhance the enzymatic hydrolysis efficiency of corn stover [292]. The results also indicated that the treatment effect of wet milling is better than that of dry milling.

Hardness using Mohs scale (see Appendix 1) is directly linked to abrasiveness. We can without any doubt show the practical qualification of this scale by using the following qualifiers given in Table5.1.

The connection between hardness and abrasiveness is the fact that a solid A that scratches another solid B has an index where IMA>IMB. It is the method that was used to organize different solids using the Mohs scale.

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