AbstractBased on analysing classical drum and tubular ball mills with a fixed structure, biconical and stage-type mills with a rigidly-variable structure, and mills with a variable-speed electric drive, an advanced grinding concept is suggested. It offers a new framework of views on the mechanism and technology of grinding. These views served as a basis for developing a general-purpose tubular-conical mill with a continuously-variable (integral) structure and a variable-speed electric drive.

Sergey V. KoValyuKh, Meng and

VSeVolod r. KoValyuKh, Phd, SPe KVar,

uKraine, outline the concePt for

their tubular-conical Mill with a

Variable-SPeed electric driVe.

Q=kD 2.5 LB=kD 2.4 n 0.8 0.6 L

B=kD 2.4 n 0.8 0.6 L

New____________

___________________

______________________

a generation

grinding unit

[Reprinted from Oct 09] worldcement.com

IntroductionThe ball mill (BM) is a conservative grinding unit that has resisted radical change (the first BM patent was granted in Germany in 1881). This is indicative of its merits and shows that it is indispensable, but, at the same time, this means that it has critical drawbacks. The grinding problem is addressed based on partial solutions, namely:

Mill sectioning into chambers.

Ball separation.

Choosing the fill factor and ball grades.

Choosing the liner plate profile in each chamber.

Choosing the cycle, industrial process scheme, grinding stages, etc.

The origin of all drawbacks is the fixed structure of the BM, which is an obstacle affecting the grinding parameters. A transition from a rigid BM structure to a continuously-variable (integral) one would solve the grinding problem radically.

Many factors affect grinding; however, generally, BM performance is evaluated by its milling productivity per power consumption. Let us consider several well-known empirical productivity formulas for three types of mills:

Cement mill:

(1)

Coal mill:

(2)

Ore mill:

(3)

Where B and Q is mill productivity (tph); k is constant factor; V is mill grinding space (m3); D is inner dia. (m); L is mill length (m); G is mass of balls (t); ϕ is grinding bodies fill factor; and n is mill rotational speed (rpm).

The same laws govern BM grinding; however, each

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industry has its specific features. In the productivity formulas, they have to be reflected by different factors rather than by the formula structure. Comparing (1) through (3) results in the following conclusions:

Generally, the mill productivity is a function of the grinding space because all other parameters are constant and their values have little significance.

(2) and (3) show that not only is the mill grinding space significant, but also the L/D ratio within this volume.

Only (2) indicates a minor impact of the mill rotational speed on grinding, whereas in (1) and (3), this parameter is completely absent. Here, parameter n0.8 at a constant rotational speed of classical mills is nothing else than constant factor k.

(1) and (2) indicate a minor impact of the ball charge, whereas in (3) this parameter is not accounted for at all.

Hence, all industries fail to account for one phenomenon, i.e. the impact of the mill rotational speed and the ball charge. They are the critical parameters of the grinding process with the given L/D ratio, whereas the formulas themselves do not reflect the grinding process in full.

The effect of the L/D ratio on grinding efficiencyFormulas (2) and (3) imply that productivity depends on the mill inner dia. D far more than on its length L. Here, the critical condition – the limits of validity and applicability of these parameters – is not specified. Thus, on the contrary to the technical sense, it is maintained that maximum productivity for the given grinding space can be provided at D → ∞ and L → 0.

Since the grinding process is very specific, it is best to consider known cases in different industries. Let us review the operation of coal tube-ball mills (TBM)1, 2 and cement tubular mills5 and compare the results (Table 1).

Analysis of tube-ball mill ShBM 400/800 (Sh-50) has shown its inefficiency, and only contrary to formulas (2 and 3) after redistribution of L/D dependency to increase the length and decrease the diameter3, mill ShBM-370/850 (Sh-50A) was commissioned in the power industry as a base one in the CIS countries. This reduced electric power consumption by 9.2%. The L/D ratio in Sh-50A could have been chosen bigger were

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Table 1. Operation of coal tube-ball mills (TBM) and cement tubular mills.

Parameter Coal TBM Cement ball mills with closed-cycle grinding

Sh-50 Sh-50А

Dimensions (m) 4.0 / 8.0 3.7 / 8.5 3.95 / 6.4 3.65 / 10.3

Grinding space (m3) 100.0 91.5 100.0 104.0

Ratio L/D 2.0 2.3 1.62 2.82

Ball charge (t) 100 105 123.0 141.0

Ball fill factor 0.22 0.25 0.27 0.3

Productivity (tph) 50.0 50.0 43.0 45.0

Power (kW) 2460 2000 1490.0 1862.0

Electric power consumption (kWh/t)

49.2 40.0 34.7 41.4

Table 2. Ball charging

Parameters Ball charge in three chambers of tubular mills

Working condition Crushing Grinding Re-milling

Ball dropping velocity paths

Waterfall (impact)

Waterfall-cascade (crushing)

Cascade (attrition)

Ball dia. (mm) 100 - 60 60 - 30 30 - 20

Weighted average ball (mm)

74.5 44.8 25.1

Ball fill factor (%) 30 27 24

Gain of number of balls as per chambers

Δϕ = 3% = const.

worldcement.com [Reprinted from Oct 09]

it not for the constraint on mill length. The mill performance was improved by reducing the ball lifting height, increasing the turnover of the ball charge and the time of drying wet coal. Actually, by drawing on the experience of operating cement mills, coal TBMs are approaching the dimensions of tubular mills. Note that the efficiency of mill Sh-50 with a variable-speed drive will exceed that of the upgraded mill Sh-50A.

Two cement mills (Table 1) with closed-cycle grinding5, having practically the same grinding spaces and productivity, differ significantly by their L/D ratio and insignificantly by 6.7% as to their electric power consumption.

In ore BMs, there were many cases of increasing the mill length from 600 mm to 1200 mm by inserting rings, thus increasing the productivity from 14 to 20 tph.

On the one hand, these cases confirm the impact of the L/D ratio on the grinding process, and on the other hand, they show that the grinding efficiency of coal mills fails to

agree with formulas (2 and 3), and that of cement mills with (1). Grinding practice5 shows that bigger diameter mills, in contrast to small-diameter ones, yield greater productivity, but a coarse grind, and vice versa.

Hence, the solution of the L/D ratio problem is in a trade-off between the merits and drawbacks of big and small-diameter mills. It is optimally achieved in mills with a continuously-variable structure.

The phenomenon of ball mills with a rigidly-variable structure (variable L/D ratio)An attempt was made to solve the problem of effective coal grinding by using biconical mills with a rigidly-variable structure, in which a complex-shape casing comprises two opposing cones, a short inlet one with a 120° angle and a long outlet one with a 60° angle. There is a cylindrical section between the cones. A feature of such mills was that for the first time in power engineering there was an attempt to implement one of the key grinding principles, i.e. separation of the multitude of balls along the mill casing. The mutually exclusive eclectic design of the biconical mill was an obstacle to solving this critical problem1, 2. Unfortunately, in contrast to References 1 and 2, papers 3 and 4 give no data on the character of separation of grinding bodies, this being indirect evidence of its absence. Nevertheless, the biconical mill is the first, though not the sole unit with a rigidly-variable structure.

Over their length, tube-ball cement mills are separated into three chambers with two diaphragms and an outlet grid with an airlift. The involved and ineffective separation of the mill into chambers was an attempt to at least partially separate the balls by size, ensure a different fill factor for grinding bodies and a weighted average ball, and provide different ball velocities along the mill. In the ore mining industry, multistage grinding in several short mills solves the task of mill sectioning into chambers. The fill factor recommended by world practice5 and the ball grades (Table 2) in each chamber are different and – what is extremely important – their number is continually reduced by Δϕ = 3% as per chambers.

Hence, though the key engineering tasks in tubular mills have not been solved in full yet, the key priorities of an optimal grinding technology were identified.

Let us consider one more unconventional tubular three-chamber stage mill manufactured by the MAAG Company. It reduces chromite and magnesite at the Zaporizhia Refractory Plant in Ukraine. The diameter of the first chamber of the mill is 2.3 m, and that of the second and third ones is 1.8 m. The stage mill is yet another striking example of a unit with a variable L/D ratio, which merits special attention.

The operation of different-design ball mills can be compared provided the initial conditions be strictly met, i.e. grinding of the same material with equal mill grinding spaces and ball charges.

For comparison, Table 3 gives the certificate data on grinding cement of one grade with industrial mills of different designs. At the refractory plant, these mills grind chromite and magnesite. This additionally and unambiguously warrants the validity of results obtained. The two compared mills have almost the same initial conditions. However, the grinding space of the stage

Table 3. Certificate data on grinding cement of one grade with industrial mills of different designs.

Parameters Series tubular mill, dimensions 2.0 х 10.5 m

MAAG stage mill 2.3/1.8 х 11 m

Equivalent dia. (m) 2.0 1.96

Grinding space (m3) 28.0 27.5

Ball charge (t) 32.0 29.0

Power consumption (kW)

540.0 365.0

Cement productivity (tph)

12.0 21.9

Chromite and magnesite productivity (tph)

5.4 10.0

Power consumption per 1 t of material (kWh/t)

45.0 16.6

Reduced efficiency 1.0 2.71

Figure 1. Mill productivity Q and power consumption N (dashed lines) vs. ball fill factor (ϕ1 = 0.28; ϕ2 = 0.32; ϕ3 = 0.36) and mill rotational speed n.

[Reprinted from Oct 09] worldcement.com

mill is smaller by 2% and the ball fill is 10% less than in the compared mill, thus creating an advantage for the tubular mill. The same productivity ratio of the compared mills when they grind cement and chromite–magnesite (21.9:12 = 10:5.4) excludes randomness of the results analysed and additionally proves their validity.

In spite of the tubular mill’s advantages with regard to initial conditions, the efficiency (the reduced ratio of electric power consumption per tonne of ground product) of the stage mill is 2.71 times higher than that of the classical design mill, and the specific electric power consumption is 16.6 kWh/t.

Hence, a fundamental result has been obtained, which proves the efficiency of mills with a variable L/D ratio. Such mills partially implement the following: separation of grinding bodies, variable ball filling per chambers, and various ball velocity conditions along the mill.

The effect of the mill rotational speed and the ball fill factor on grinding efficiencyThe basic drawback of ball mills with a constant rotational speed is that the optimal ball lifting height changes continuously (drifts) with variable internal factors, such as intensive attrition of the ball charge and the lining profile. The widely changing external factors (coarseness, moisture content, grinding ability, and components ratio) require adjustment of the balls’ lifting height depending on the character of the grinding process. This is impossible to ensure, given the lining profile and the degree of filling of grinding bodies. The classical system for automatic control of the source material charge does not compensate for internal and external factors. A transition to grinding cement of other grades requires readjustment of mill operating conditions. According to process requirements, the operating conditions can be selected effectively and instantaneously (compensate for external and internal perturbations) by adjusting the grinding process automatically with the mill rotational speed.

From equation (2) it follows that parameters n0.8 and ϕ0.6 have no critical impact on mill productivity. By the same reasoning, these parameters are absent in equations (1) and (3).

The outcome of the authors’ long-term studies in BM with a variable-speed electric drive in different industries was not only providing a 50% productivity boost and accurately stabilising (±0.5%) grinding fineness, but also proving that the grinding efficiency at specified L/D ratios depends mainly on the mill rotational speed strictly in compliance with the grinding bodies fill factor6.

The variable-speed electric drive provides the following functionality:

To perform smooth mill starting, thereby ensuring extended reducing gear life.

To choose an optimal ball dropping path onto the “ball impact spot” with account of continuously changing characteristics of the material being milled and the condition of the balls, as well as to adjust the static characteristic drift with time in the automatic mode.

To implement an optimal combined control system based on two channels, i.e. the inertia-less mill rotational speed control channel and the inertial

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Figure 4. Mill productivity and preset rotational speed vs. grinding bodies wear.

Figure 2. Electric power consumption γ vs. ball fill factor (ϕ1 = 0.28; ϕ2 = 0.32; ϕ3 = 0.36) and mill rotational speed n.

Figure 3. Grinding bodies wear vs. operating time.

worldcement.com [Reprinted from Oct 09]

(adjustment) channel for controlling the source material charge.

In-depth studies in BM with a variable-speed electric drive (within 0 to 1.4 ncr) in the field of parameters “rotational speed – grinding bodies fill factor” were conducted on the 2 x 10.5 m mill at the Belgorod Cement plant, Russia7, 8. The three operating conditions obtained (Figures 1 and 2), besides having a practical significance, are theoretically interesting because they explain the effectiveness of different mill rotational speeds (n > nrated; n = nrated = 0.76 ncr; n < nrated), thus having reconciled their many supporters and opponents.

The data in Figures 1 and 2 results in the following conclusions:

A bigger ball charge requires a lower mill rotational speed.

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To ensure maximum productivity at specified grinding fineness and minimal electric power consumption, it is necessary that ϕ = 0.36 and nopt = 0.65ncr.

This operating condition can be implemented only with a variable-speed electric drive, allowing one to not only find the optimal operating condition, but also to adjust the static characteristic drift in time with an automatic control system.

The speed conditions n = 0.76 ncr (ϕ = 0.32) and especially n = 0.9 ncr (ϕ = 0.28) are false optimums because the actual ball fill is at a lower level and limited by the power and rotational speed of the non-controlled electric drive.

The regularities obtained are universal for any standard size mills.

These results and conclusions are in full agreement with those of Dr. Beke Bela8 who analytically confirmed the full agreement of dependencies (Figures 1 and 2) by comparing the authors’ results6 with those of Italian researchers9 obtained independently at the Guidonia Cement plant in Rome on three 4.8 x 15.4 m mills with a variable-speed gearless 6000 kW electric drive. The control range of these mills is within 0.66 ncr to 0.79 ncr, i.e. only 13%.

The following is an example of compensation for ball charge attrition using the mill automatic speed control system.

According to the statistics of grinding bodies wear of 1 kg/t of cement being ground, the additional charge schedule (after 9 days) and the Davis volumetric-weight ball wear theory, the company has built an approximate ball charge wear vs. grinding time function ϕ =ƒ (t) (Figure 3).

Based on this theory, the ball mass in the beginning of any n-th interval is equal to its initial mass G1 multiplied by constant factor q raised to n less one:

(4)

Where q is the common ratio of the geometric progression q = G2 / G1.

Formula (4) holds not only for one ball, but also for the entire fill of grinding bodies.

Based on the grinding bodies wear vs. grinding time function, conversions yield the productivity and specified rotational speed dependence on grinding bodies wear (Figure 4), i.e. Q = f (ϕ) at n = 0.65 ncr = constant and Q = f(ϕ) at n = var. They are needed to obtain the productivity vs. time graphs for operating conditions with a continuously-controlled mill rotational speed.

The dependencies Qopt = f (ϕ) at n = var. and Q = f (ϕ) at n = 0.65 ncr = constant are given in Figure 4. They allow defining mill productivity growth due to an optimal rotational speed condition.

In this case, the variation of the rotational speed setting in an automatic control system is defined by grinding bodies wear. It can be effected by the programme set point nset = f (ϕ) shown in Figure 4 as a dashed line.

Treatment of experimental curves (Figure 5) yields an estimate of productivity Q growth when controlling the rotational speed with changing controller setting n* as the balls wear. The growth value is 6.2%. A variable-speed electric drive not only dramatically increases the grinding

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Figure 5. Mill productivity vs. grinding bodies wear.

Figure 6. Tubular-conical cement mill. VTF – vibratory tilted feeder; CU – control unit; EP – electric precipitator; МF – mill fan.

[Reprinted from Oct 09] worldcement.com

unit productivity, but additionally compensates for perturbations related to a changing grinding bodies fill factor. Optimal mill rotational speed control maintains the mill operating conditions in the suboptimal zone in the time between successive additional charges.

Hence, optimal TBM operating conditions at a preset L/D ratio uniquely and essentially depend on the combined interaction of the mill rotational speed and the ball fill factor. As a control action, the mill rotational speed allows the creation of an optimal combined control system for the TBM capable of adjusting the static characteristic drift by compensating for external and internal perturbations, stabilising cement grinding fineness with high accuracy and increasing the productivity 1.5 times.

A general-purpose tubular-conical mill with a continuously-variable structureA study of the theory and practice of grinding, especially for mills with a controlled rotational speed and a variable L/D ratio, yielded the key concept of grinding and applied it for developing an advanced milling unit, i.e. a general-purpose tubular-conical mill (TCM) with a variable-speed electric drive. The TCM design (Figure 6 for cement, and Figure 7 for coal) differs in that the conical casing size is reduced towards the discharge side. In contrast to other mills, TCM can be mounted on a common frame. This makes it possible to incline the conical casing with respect to the longitudinal axis within the angle of inclination between the cone generatrix and the horizontal line. Casing tilting helps to control (redistribute) ball filling over the cone length both manually and automatically, and it is a fundamentally new and effective control action.

For the first time, the TCM implements the key concept of grinding. It features the only possible and optimal location for each working energy carrier – the grinding ball – depending on both its diameter and the entire ensemble of balls in strictly appropriate cross-sections over the entire length of the conical casing, simultaneously providing for optimal ball dropping velocity paths and their turnover in each of these sections. This is the core of the grinding problem.

Implementing the key grinding concept immediately yields the gamut of TCM features. They systemically merge into one unit and in full scope the entire set of advanced technological and design solutions that provide the following:

An automatic, ideally size-descending separation of the ensemble of balls of different diameter in each cross-section over the entire length of the conical casing.

A smooth transition of optimal ball dropping velocity paths and ball turnover as a function of cone radius, viz. from impact in the beginning to crushing in the middle, and attrition at the mill end.

A ball charge fill factor decreasing optimally over the conical casing length.

Optimal self-regulation of the ball grades (of the average weighted ball).

An optimal trade-off between the merits and drawbacks of big and small-diameter mills (the L/D ratio).

Adjustment of optimal operating conditions for one charge by changing casing inclination with respect to the longitudinal axis to redistribute the ball charge in the mill beginning and end.

The small cone inclination lowers the pressure of the balls on the mill end heads, thus reducing their mutual wear and the harmful “near-wall” effect.

The ideal separation of the grinding bodies eliminates diaphragms, thus increasing the grinding space and reducing the number of hatches in the TCM casing by three-fold.

A closed-cycle grinding cycle in a one-chamber TCM with an open-circuit separator as in coal TBMs, and elimination of an outlet grid with an airlift, thus dramatically simplifying the entire grinding process.

Improved aerodynamics since a one-chamber TCM acts like a cone-nozzle (a convergent tube).

Automatic additional charging of balls one by one with a maximum diameter over the entire length of the one-chamber TCM during continuous mill operation.

The TCM can operate without reducing its productivity with worn and smooth liner plates because the velocity conditions are rigidly tied to the variable mill radius, i.e. the liner plate profiles have no critical effect on TCM performance.

Level off the irregular wear of grinding bodies over the entire mill length and reduce ball wear due to effective ball filling and optimal mill operation.

Increased unit duty factor.

Reduced power consumption, dimensions, metal capacity, capital outlay and scope of maintenance.

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Figure 7. Tubular-conical coal mill. RCB – raw coal bunker; VTF – vibratory tilted feeder; CU – control unit; MF – mill fan; DB – dust bin.

worldcement.com [Reprinted from Oct 09]

The above TCM advantages are in line with modern grinding concepts. One can contend that the stage mill design with a rigidly-variable L/D ratio is a partial solution of the generalised integral-multistage TCM design with a continuously-variable structure. By increasing the number of stages in the MAAG mill, its efficiency will grow to exceed 271%. Integration of an infinitely big number of stages into the mill will yield the TCM design.

The new mechanism of separation of the ball charge in the TCM is initiated at the instance of ball separation and its free dropping along a path created by the conical casing, which differs from the conventional one for classical mills. The ball dropping vector in the TCM is perpendicular to the conical casing generatrix. Rather than dropping vertically, the balls drop at a small angle to the side opposite to the material flow along the mill, i.e. to the charging side. Under gravitational force, a bigger diameter ball (with a bigger mass) drops farther, whereas a small ball falls closer. This separates the grinding bodies by size. Besides, during their joint dropping, the bigger diameter ball drives the smaller ball to the charging side. The free ball dropping process and their ideal separation is best seen in translucent TCM models.

Different ball dropping paths over the length of the conical casing, combined with ball separation, have a crucial bearing on the grinding process. Conventionally, this priority task is solved by an involved and limited-value selection of the liner plate profile. The liner plates, however, undergo rapid wear and the expected effect degrades drastically. Besides, required ball dropping paths of the conventional mill do not vary along the entire chamber length. In TCM, the ball dropping paths are rigidly tied to the conical casing radius, and the liner plate profile has no essential impact on the paths. The liner plates in the TCM serve to protect the casing and increase the ball charge friction coefficient during lifting to the point of separation for free dropping.

The authors have designed liner plates that are installed in each ring of the conical casing by alternating only three standard sizes, whereas their total amount has been reduced by 20%. A rolled stock lining has also been designed for the conical casing. In the TCM, instead of having six process hatches in the casing for maintenance and additional charging (recharging) of balls, there are only one or two hatches. This streamlines the design, increases casing strength, and reduces dust formation and the scope of maintenance. The conical casing dramatically reduces and simplifies scheduled charging and recharging of the ball fill.

Figure 8. Process diagram of a closed-cycle grinding plant with a 4Ч13.5 m mill. 1 – bag collector; 2 – separator with external cyclones (N rotor = 180 kW, fan Q = 150 000 m3/hr and N = 250 kW); 3 – elevator (Q = 470 tph, N = 75 kW); 4 – air chutes; 5 – 4 x 13.5 m mills, N = 3200 kW; 6 – aspiration column; 7 – 6 x 800 mm dia. cyclones; 8 – bag filter; 9 – fan (N = 125 kW, Q = 60 000 m3/hr); 10 – expander-tube pumps (Q = 150 tph); 11 – belt conveyor ; 12 – clinker and additives batchers; 13 – fan (N = 55 kW, Q = 25 000 m3/hr).

[Reprinted from Oct 09] worldcement.com

World practice5 in charging balls (Table 2) advises a stagewise decrease of ball size per chambers with a constant value of ΔΦ= 3%. In a one-chamber TCM, this concept is implemented automatically. In doing so, the upper layer of balls is arranged horizontally over the entire casing length. This simplifies the involved and time-consuming process of additional charging of the stopped mill by setting up continuous additional automatic charging by simply dropping a maximum-diameter ball into the chute of an operating mill at intervals depending on ball wear per tonne of ground material. As the ball wears, it will move along the cone and be replaced with a next new one. In the CIS countries, additional ball charging is scheduled for every 9 days (this schedule, as a rule, is not observed because of time-consuming operations and mill stopping). However, in only 9 days, the wear and tear of the entire ensemble of balls reduces the productivity by 13.7% (Figure 5).

A critical and not yet solved problem is choosing a ball grade generally related to the diameter of the average weighted ball daw.

(5)

Where q is the weight of balls of respective diameters included in the grinding charge (t); d is ball dia. (mm).

The grade of grinding bodies should ensure an effective grinding charge, which would create a big material contact surface, prevent free passage of the material through the spaces between the balls without grinding, and at the same time, ensure passage of ground size fractions along the mill. In the TCM, this task is solved by automatic self-regulation of the average weighted ball irrespective of the initially chosen ball grades.

The diverse character of grinding (Table 2) over the length (of the chambers) in classical TBMs results in irregular wear of grinding bodies. This is especially obvious in ore rod mills. Machined rods, representing an integral fixed-length grinding body, undergo wear to a conical shape, i.e. the wear intensity in the end of the mill is higher than in its beginning. The conical casing, which ensures optimal ball operation conditions, levels off the irregular wear of grinding bodies over the mill length, and reduces overall ball wear by 20%.

During grinding in a BM, the source material size composition changes so that the amount of large particles decreases and that of small particles increases. Excessively ground particles, which are not removed in time from the bulk of material in the mill and ground further, hinder grinding of large particles and retard the entire grinding process. The smaller the particle, the less inner defects it has, the more it resists destruction, and the more energy and time is required for its grinding.

Closed-cycle grinding prevents excess material grinding and increases the productivity by 20 - 25%. The main drawbacks of the closed-cycle scheme are as follows: a host of complex and unwieldy auxiliary equipment whose total power consumption exceeds 40% of the BM power. Figure 8 is the structural diagram of the auxiliary equipment for a closed-cycle mill10.

TCM eliminates the drawbacks of the classical grinding scheme by implementing a simple closed cycle of the coal TBM type (Figures 6 and 7), using one mill fan with 20% power of the TCM rating. Electric power saving for 3200 kW 4.0 x 13.5 m mills just by simplifying the auxiliary equipment process flow is over 600 kW. This is without account of cost, capital outlay, floor area, dust formation and its operation.

Open-circuit separators separate material less effectively than do centrifugal ones, though sufficiently well for coal mills, the coal dust residue being within required limits (6 to 7% on an R90 screen). For better quality separation of the finished product, a centrifugal open-circuit separator can be used, or a centrifugal separator can be installed after the open-circuit one in the grit recycling channel.

In addition to dramatically simplifying the classical scheme of closed-cycle auxiliary equipment, this eliminates the outlet grid with the airlift together with the diaphragms in the single-chamber TCM. This increases the TCM grinding space by 11.5%. According to (1), this additionally increases the productivity.

A general-purpose TCM is intended for operating in the open or closed grinding cycles with both dry and moist materials in different industries. The TCM can operate with/without a variable-speed drive.

Thus, the authors suggest a conceptually new mechanism and technology of grinding, which served as a basis for developing an energy-intensive TCM with a variable-speed drive. The overall TCM efficiency exceeds 271% without accounting for the following efficiencies: variable-speed drive (up to 50%), closed-cycle grinding (20 to 25%), regular additional ball charging (13.7%), and increased TCM grinding space (11.5%).

A general-purpose TCM with a variable-speed drive is a comprehensive way of improving new-generation grinding plants. Its design supports easy, complete and optimal implementation of the ball operation sequence, which is in line with modern grinding theory and practice.

The originality and novelty of the technical solutions suggested have been confirmed and protected by many patents.

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LETiN, L.A. and RoDATiks k.F., Middle-speed and low-speed mills, Energia Publishers, Moscow, 1981.

kiEsELhoFF, M.L. and PoLFERoV, k.F., ‘high-productivity tubular ball mills’, Teploenergetika, 12, Moscow, 1962.

kAPEL’soN, L.M. and kARPoV, B.s., ‘investigating coal grinding conditions in a biconical ball mill’, Teploenergetika, 12, Moscow, 1962.

DUDA, V. Cement, stroiizdat Publishers, Moscow, 1981.

koVALyUkh, V.R., ‘optimisation of the grinding process in ball mills with a variable-speed drive’, Cement, 8. st. Petersburg, 1985.

koVALyUkh, M.V., ‘A new-generation tubular-conical coal mill’, Teploenergetika, 1, Moscow, 2001.

BEkE, B., ‘Phenomena in sheet regulated ball mill’, World Cement Technology, 8, 1979.

oLiVERo, L., CAiRE, F., PiNToR, G. and BiANChi, P., ‘Betrieblich Anpassung von Zementmuhle mit variabler Drehzahl’, Zement – Kalk – Gips, 12, 1977.

TkAChEV, V.V. ‘Closed-cycle grinding plant’, Cement, 8, st. Petersburg 1983.

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