Process Optimization in Cement Plants

Introduction

In the highly competitive and dynamic cement industry, where market demands constantly evolve, optimizing processes becomes crucial for manufacturers to stay ahead. This optimization process often involves a multidisciplinary approach, incorporating engineering, technology, and data analysis to identify and address inefficiencies, reduce environmental impact, and enhance overall operational performance.

Background

The story usually starts with a cement producer grappling with challenges in meeting market demands for cement. They sought ASEC's expertise to enhance productivity and overcome limitations in cement plants. Our focus in this article is on one of the cement mills, an end-discharge ball mill equipped with a third-generation separator.

The initial design specified a feed rate of 110 tons/h, but during Performance Guarantee Testing (PGT), the achieved feed rate was 98 tons/h with a Blaine target of 3000 cm²/gm, falling short of the target. Market requirements prompted an increase in Blaine to 3,550 cm²/gm which required calculating the target feed rate to 84 tons/h. However, the actual average feed rate for this mill stood at 60 tons/h, revealing a significant variance.

The investigation highlighted a 29% feed rate shortage, leading to significant relative losses, as the SPC increased by +21%, and higher grinding media consumption by +21%.

Objectives and strategy

The investigation goals involve comprehending and resolving the underlying factors responsible for the highlighted losses through the attainment of sustainable and efficient operations for cement plant equipment. This is to be accomplished prior to considering any upgrades, with a focus on minimizing costs.

Achieving this goal requires Building an efficient investigation strategy aims to specify the elements that may cause the mill limitations, which have been defined in this case as grinding efficiency and separator efficiency, and so the investigation covers multiple dimensions that affect grinding and/or separator efficiencies:

• Ventilation assessment.

• Assessing separator efficiency.

• Ball charge sampling.

• Axial test.

• Performing internal inspection.
  

Investigation analysis

The investigation journey started with mill system Ventilation Assessment, where the analysis showed a specific airflow in the second compartment (0.16 Nm³/kg Vs 0.5 Nm³/kg reference value), while air speed in the above charge (0.38 m/sec Vs 1.5 m/sec reference value) and the Suspension percentage as per the mentioned air speed above charge in second compartment (0.94 %) which is significantly lower than recommended values (5% – 20%), and the measured false air at the mill outlet was (54% vs 25% reference value), revealing one of the reasons that caused the very specific airflow and airspeed above charge indicates very low ventilation in the mill system.

Furthermore, Fig. (1) shows the airflow at the separator outlet was (1763 m³/min Vs 2500 m³/min reference value), and the Feed/air ratio was (2.94 kg/m³ Vs 1.5–2 kg/m³ reference range) that means a high specific material load, and measured false air at the mill filter that was (19% VS 5% reference) playing its role as one of the reasons impacting the low ventilation in the separator.

Fig. (1)

Inspecting internal parts as evidential elements to understand the causes and outcomes of low ventilation, as the Mill intermediate &Outlet diaphragms permeability is 70% and 90% due to clogging by nips in 1st  compartment and cement coat in 2nd compartment which affects the level of material, fan damper malfunction louver plate which connects with damper actuator reaches to 100% before other louver plates preventing damper from full opening, mill outlet duct connected to separator coated decreasing cross-section of the duct, beside a welded steel plate in mill outlet duct to separator make obstruction for mill air stream, the existence of steel plates blinding a section and half from turbine separation area {Pic. (1)}, and caught false air, and the mill outlet and mill filter caused restrictions for draft and ventilation from the mill system; those barriers were the main reasons for the low ventilation in the mill system.

Pic. (1)      Pic. (2)

On the other hand, the consequences of this low ventilation are significant on grinding media and liners coating {Pic. (3)} affecting grinding efficiency, and material accumulations all over the system {Pic. (2) & (4)} that also causes more ventilation.

Pic. (3) Pic. (4)

Proceeding in our investigation with Separator Efficiency, were as shown in Fig. (2), the efficiency of sieve analysis resulted in a separator efficiency of 29.69 % Compared typical value of the 3rd gen. separator of 80%, while the circulating factor was (5.25 Vs 1.9 – 3.1 as per working Blaine 3550 cm2/gm), and the bypass percentage was (78.5% Vs Reference < 10 %). 

A third-generation separator exhibited an efficiency of only 30%, impacting feed rate and energy consumption.

Fig. (2)

Explaining the low separator efficiency and besides the low ventilation, the separator top seal allowable clearance shall be 14mm and not exceed 19mm as per design shown in Fig. (3), while the measured clearance was 31mm, almost double the design clearance, this variance affects the labyrinth effect allowing the material to escape

Fig. (3)

Also, material inlet distribution to the separator is mandatory to maintain separator efficiency, the material is fed to the separator through 4 branches as shown in Fig. (4), In fact it was found three out of these 4 branches blocked with material, and this means very poor material distribution which impacts separator efficiency.

Fig. (4)

The next aspect in this investigation explores grinding media analysis with ball charge sampling test.

Fig. (5)

  Fig. (6)

The filling degree in the 1st compartment is 24.6%, which is 6% less than the plant figure as per Fig. (5) with a total deficiency of charge media around 12 tons, while the filling degree in the second compartment is 24.8%, which is 7.4% lower than the plant figure as per Fig. (6) with a total deficiency of charge media around 37 tons; Also, the actual composition of the ball does not match the recommended composition.

Going deeper in the grinding media analysis involves the axial test, that results balls Classification graph and granulometry profile.

Fig. (6) Fig. (7)

The absence of classifying liners further compounded the grinding efficiency issues as Fig. (6) showed the big-sized balls exist along the 2nd compartment length and do not follow the descending profile with higher intensity by the end of the compartment, especially from meter 4.5 you can find in the classification curve that big-sized balls (Ø60, Ø50) have a high concentration in the 2nd half of compartment length, clearly impacting granulometry profile as showed in Fig. (7) the curve profile follows a normal descending manner till meter 8.5 then the horizontal profile exists till meter 11.5, then decreases at the compartment end. The horizontal profile at the curve can be explained by the non-classification of balls explained before.

Conclusion

Low ventilation was attributed to various factors, from partial blockages to malfunctioning mechanisms.

Solutions involved regular cleaning, false air arrest, adjusting separator top seal gap, damper repair, and the removal of obstructions to restore proper ventilation; those actions were applicable during regular PM stoppages, saving time and money.

While addressing granulometry issues required installing classifying liners, compensating for ball charge shortages, and sorting balls to match the design.

The low-cost solutions aimed at optimizing operation effectiveness included adjusting separator top-seal clearance and maintaining proper material distribution to separator.

The comprehensive approach restored the mill's efficiency, minimizing losses in capacity, energy consumption, and grinding media without sacrificing market requirement of keeping Blaine target at 3,550 cm²/gm, what can clearly be shown in Fig. (8).

Fig. (8)