Effects of Mill Speed and Air Classifier Speed on Performance of an Industrial Ball Mill

Research Article

Austin Chem Eng. 2020; 7(1): 1072.

Effects of Mill Speed and Air Classifier Speed on Performance of an Industrial Ball Mill

Fazeel Ahmad*

Department of Chemical Engineering, University of Wah, Wah Engineering College, Pakistan

*Corresponding author: Fazeel Ahmad, Department of Chemical Engineering, University of Wah, Wah Engineering College, Pakistan

Received: April 18, 2020; Accepted: May 11, 2020; Published: May 18, 2020


The research focuses on the mill speed and an air classifier speed effect on the two compartment Cement ball mill performance in terms of Blaine, Sulphur trioxide contents, mill power, mill residue and mill residence time. Within the content of this work, sampling campaigns were organized around a cement grinding circuit and varying cement ball mill speed as well as an air classifier speed at various dosage feed rate. The fact that such an examination has not been made previously by using industrial data, which makes this work unique. The fineness is measured in terms of Blaine number. It was deduced that depending on the speed of mill and air classifier, their effects on Blaine , SO3 , mill power and mill performance were varied, ultimately all of them improved the performance of grinding and classification operations were The Blaine quality dictates strength, setting time and overall performance of cement. Optimum performance of ball mill could potentially refine Blaine fineness, thereby improving the cement quality. This study investigates the effects of separator speed and mill speed on Blaine fineness, mill residue, consumed power. Variations in clinker feed rate, mill speed, separator speed, grinding aid could proportionally impact the grain quality of Blaine. When the separator speed is increased from 850 to 900 the Blaine is increased from 2800 to 3000 cm2/ gram and mill residue decrease from 15 to 10. Therefore, optimum parametric combination could reduce power consumption while improving the cement quality. The fact that such a study has not been carried out at an industrial scale, makes it one of its kind. Knowledge of effects of parametric variations on the quality of end product could be helpful for controlling product quality. Furthermore, proper grinding of clinker produces fine Blaine at first place and reduces the need for recycling of coarse grains.

Keywords: Clinker; Cement; Blaine; grinding process; Ball mill speed; Air Classifier speed.


Cement Ball Mill

A ball mill is a type of grinder used to grind, blend and sometimes for mixing of materials for use in mineral dressing processes, paints, pyrotechnics, ceramics and selective laser sintering. Ball mill in the cement industry is used to reduce the size of clinker into fine particles also called as cement. Mill speed and air classifier speed were the investigated parameters for the closed cycle mill. Almost six speed level are used in the closed cycle mill are 750, 800, 830,850, 900, 950 rpm. Blaine is the important characteristic of ball mill which is influenced by the mill speed and separator speed. Ball mill is grinding equipment which is used to reduce the size of clinker into cement. It uses grinding media in the form of balls. Clinker coming from the silo is sent into hopper and mill for impact action. Clinker is introduced into the ball mill. The rotating mill is filled with different steel balls of sizes 25 mm, 40mm, 50mm, 60 mm, 70mm and 80 mm with 45%, 40% and 15% weight percentage, respectively. It is crucial to increase the efficiency of comminution process to reduce the amount of energy used and greenhouse gas emissions. First chamber consists of large size media, having diameters of the order of 80-40 mm while second chamber consists of media; having size of the order of 40- 25 mm. Electric power is also affected by the media and processing parameters.

For instance, for 1000 kg cement production almost one- third power is consumed in cement mill [1]. The lab scale results could differ from the actual or industrial scale results because of different operational scale and different control of parameters. In some cases, the reduction in power consumption could be as low as half in industrial mills as compared to lab scale mills, despite using the same grinding aids. Such a change has also been reported to significantly alter the mechanical properties of cement. Using grinding aids, the decrease in power consumption in lab scale mill has been reported as 30% and 34% for glycol and amine whereas, for industrial scale, the decrease has been reported as 5% and 12% respectively. Water demand for industrial mill has been reported as 20% more as compared to the lab scale mill. In comparison of industrial and lab scale mills for 28-day compressive strength, the increase of almost 10 MPa has been reported for the industrial mill [2]. The optimization in the processing parameters could be equally useful for other mill configurations like vertical mils and high speed mills [3]. Mechanical milling method has been observed to be one of most economical and popular method for improving Blaine [4].

Same effect has also been witnesses in case of grinding aids. Results, typically of power consumption have been over served as skewed in a lab scale ball mill, when compared to the industrial scale ball mill [5]. Likewise, the ambient conditions, climate, raw material and human factor could also impact the performance of ball mill [6].

Therefore, some type of calibration is required in the as obtained results of a lab based ball mill, before anticipating or applying those on an industrial scale. This implies that mill processing parameters could potentially affect the Blaine, emissions and residue.

This motivated the authors to investigate the effects of such processing parameters on a real-time industrial scale. The feed to the ball mill consists of clinker about 95 percent and remaining are grinding aids like gypsum, ash and ethylene glycol. Some media contains cement particles on their surface which shortens their grinding capability. Additives and powders of differential grain sizes could influence the mechanical strength of the cement [7]. The preparation of powder with typical specification and on an industrial scale could be challenging, as it demands understanding of effects of parametric variations on the powder quality. Altun et al., [8] investigated the effects of media, mill speed, feed rate and stirrer speed on the size reduction and power consumption on a customized horizontal mill.

It was observed that feed rate causes decrease in energy consumption. This is because the balls in empty mills would strike to the walls and cause increase in electric load and noise. The analysis of separator speed, residue and Blaine quality were overlooked in the study. The hold time could potentially reduce grain size as particles would be subjected to grinding for relatively longer time. Too prolong grinding times could however cause extra fineness, which leads to agglomeration. In an industrial unit, slight variation in hold could significantly affect the overall quality of cement. Therefore, wise selection of grinding time could improve Blaine and cement strength [9, 10]. Likewise, grinding time could also affect the fineness, which results in improved Blaine and cement strength. However, too fine grains could halt Blaine quality due to cause agglomeration [11]. Ghiasvand et al. [12] reported that the increase in milling time increases the Blaine for almost all types of cements. Typically, the increase in Blaine, measured in cm2/g is twice as compared to time measured in minutes. The increased separator speed could enhance filtering capabilities of the cement,, thereby allowing finer particles to pass, which could also improve Blaine [13].

Schnatz [14] varied L/D ratio and ball charge filling ratio in a discontinuous semi-industrial ball mill to study their behavior on the specific energy consumption. It was concluded that the milling time depends and L/D ratio influence the fineness quality of cement. The study however, did not determine an optimal mill time. Some studies have compared the Blaine from the grinding product of open as well as closed mill [15]. Too much passage at the critical speed may not increase Blaine further and tends to achieve maximum value of Blaine. A further increase in the separator speed may not improve Blaine as it tends to saturate the Blaine [13].

O-Sepa separator

O-Sepa separator also called as Cage separators, air classifier or High Efficiency Separators. The material enters through the top of the separator. Material falls and is dispersed by the distribution plate. The separator material feed is carried out mechanically by means of suitable continuous conveyors. Fines are conveyed by air in external cyclones or directly to a bag filter. The main separating device is a cylindrical rotor. The rotor is like a cage composed of blades closely spaced. The rotor is operated by a variable speed drive. The rotor speed determines swirl in the classifying zone and therefore the cut of the separator


The ball mill is a unit that has feeding doors at the start, and discharge door at the end and grinding mechanism in the at center. The clinker enters form the inlet door is ground in the rotating mill till the desired Blaine value is achieved. Finally, the refined Blain leaves the mill and enters the separator. Wear of grinding balls were investigated after the grinding process was completed.

Materials and Method

Cement ball mill reject sample from a industry located in Pakistan in Pezu District KPK. Study about cement was done for Ordinary Portland cement with 5% gypsum content and 95% clinker. During the sampling time, the capacity of cement mill was 120 t/h and the specific energy utilization was 30 kWh /t and the Mill balls were sphere in geometry with diameters varying from 25-80 mm. The balls were made of chrome-nickel alloy steel. The ball mill vessel was made of stainless steel to prevent corrosion. A typical mill operating conditions are mentioned in table 1.