Grinding In Closed Circuit: Complete Technical Guide

The finish mill of the cement plant does not work alone: around it circulates the classifier loop: the elevator carries the mill discharge to the separator, the separator divides the powder into the fines and the coarse, the fines leave to the filter as the product and the coarse returns to the mill inlet to be ground again: this loop is the closed circuit, the arrangement that most cement grinding lines of the world use, and its balance decides the fineness, the tonnage and the energy of the entire finish department.

The Complete Cement Technical Package (931 files including the books, the courses, the Excel tools and the presentations: $249.99 one-time: instant download through the PayPal payment) includes this closed-circuit file with its flow diagrams, the calculation methods and the operating tables of the separators: this article walks the file: the concept, the circulating load, the separator generations, the efficiency curves and the daily control: the reader leaves with the complete map of the loop that surrounds his own mill.

The closed circuit is the standard answer to the question of the fineness: the mill alone cannot produce the narrow distribution of the cement market: the separator adds the classification: this page follows the document and the engineer of the package can walk the PDF chapter by chapter with this article in hand.

1. The Closed Circuit: The Definition of the Loop

The file opens with the definition that the whole discipline rests on: the grinding in the closed circuit is the arrangement in which the mill discharge passes through the classifier and the oversize returns to the mill:

• The elements of the loop: the mill, the elevator or the air conveying, the separator, the filter, the return chute: the five organs of the closed circuit;
• The product stream: the fines of the separator: the finished cement of the typical circuit: the stream that leaves the system to the silos;
• The return stream: the coarse reject of the separator: the stream that re-enters the mill: the circulation: the name of the game;
• The open contrast: in the open circuit the material passes the mill once and leaves: no return: the simpler arrangement: the broader distribution and the greater sensitivity to the feed;
• The purpose: the closed loop removes the finished particles immediately and returns only the coarse: the mill grinds what still needs the grinding: the efficiency of the energy;

The definition is the base of the file: the closed circuit is not a machine but a system: the loop: the engineer who understands the loop understands why the separator of his plant is worth more than the reputation of its brand: the system, named and mapped.

2. The Circulating Load: The Heartbeat of the Circuit

The most important number of the closed circuit is the circulating load: the file teaches its definition and its arithmetic:

• The definition: the ratio of the mass of the separator return to the mass of the fresh feed: expressed as the percentage: the 100 to 350 percent typical of the cement finish circuits;
• The measurement: the plant does not weigh the return directly: the residues of the feed, the mill discharge and the product give the classic calculation: the formula of the residues: the file derives it;
• The balance: the return of the 200 percent means that two tons of the coarse re-enter the mill for every ton of the fresh clinker: the mill discharge is three tons: the internal traffic;
• The optimum: the load of the 150 to 250 percent for the most finish lines: the load too low starves the mill of the intermediate sizes: the load too high floods the elevator and wastes the power;
• The dynamics: the circulating load changes with the feed hardness, the separator settings and the mill charge: the trend of the load is the pulse of the circuit: the operator reads it;

The circulating load is the heartbeat of the closed circuit: too slow and the heart stalls, too fast and it races: the file gives the formulas, the target bands and the interpretation: the number that the process engineers of the plants quote in every optimization meeting.

3. The Elements of the Circuit: The Equipment of the Loop

The file walks the hardware of the closed circuit and describes each organ of the loop:

• The mill: the ball mill of the finish: the discharge at the outlet: the pulp of the fresh feed and the separator returns: the grinding chamber of the loop;
• The elevator: the bucket elevator of the mill discharge: the capacity of the 150 to 300 tons per hour: the conveyor that lifts the powder to the separator head: the traffic of the loop;
• The separator: the classifier: the rotor and the air: the fines through, the coarse returned: the brain of the circuit: the subject of its own chapters;
• The filter: the bag filter of the separator fines: the collection of the product: the fan of the classification air: the fines settle and the filter cleans;
• The return chute: the gravity and the air slides to the mill inlet: the return and the fresh feed mix at the feed chute: the entrance of the loop;

The equipment of the loop is the theatre of the classification: each element has its capacity limits and its failure modes: the file describes the hardware so the engineer can read the flowsheets and recognize the bottlenecks: the loop is the machine and the machine is the loop.

4. The Separators: The Three Generations

The heart of the closed circuit is the separator and the file presents the historical generations of the machine:

• The first generation: the air separator without the rotor: the inverted cone and the adjustable vanes: the blades at the outer casing: the coarse falls by the gravity: the fines carried by the air: the simple machine of the 1950s and the 60s;
• The second generation: the addition of the central feeding and the improved vanes: the higher fineness capability: the cyclone collectors and the re-circulated air: the intermediate step of the history;
• The third generation: the dynamic separator: the rotor cage with the radial blades: the fineness controlled by the rotor speed: the sharp classification curves: the standard of the modern plants: the machines of the world market;
• The rotor: the cage of the radial plates rotating at the 80 to 150 rpm of the large machines: the air and the powder pass the blades: the drag against the centrifugal: the cut of the size;
• The air flow: the classification air of the 50,000 to 200,000 cubic meters per hour: the fan power of the 200 to 700 kilowatts: the circulation through the machine and the filters;

The generations of the separator tell the history of the cement grinding: each generation sharpened the classification and reduced the energy: the file documents the three with their cut sizes and their limits so the plant with the old machine knows its position and the modern plant knows its advantage.

5. The Classification: The Tromp Curve and the Cut Size

The separator performance is not a single number: it is a curve: the file teaches the reading of the efficiency curve:

• The Tromp curve: the curve of the separation: the probability of the particle of each size going to the fines: plotted against the particle size: the fingerprint of the classifier;
• The cut size d50: the particle size with the 50 percent probability of the separation: the midpoint of the curve: the fineness that the separator targets: the number of the adjustment;
• The sharpness: the slope of the curve at the cut: the steep curve means the precise separation: the flat curve means the contamination: the sharpness index of the industry: the 0.6 to 0.8 typical of the dynamic machines;
• The bypass: the fraction of the feed that passes straight to the fines without the classification: the short-circuit of the powder: the bypass of the 10 to 30 percent: the enemy of the efficiency;
• The measurements: the sampling of the feed, the fines and the rejects at the same instant: the sieving of the three: the construction of the curve: the audit of the separator;

The Tromp curve is the X-ray of the separator: the file teaches the plant to build the curve, read the bypass and judge the sharpness: the optimization of the closed circuit begins with the honest measurement of the classification: the curve, the truth of the machine.

6. The Mill Side of the Loop: The Effects of the Return

The separator decides what returns, but the mill must absorb the return: the file examines the effect of the circulation on the mill itself:

• The fresh feed and the return: the mill inlet receives the mixture: the fresh clinker and the recycled coarse: the composition of the feed to the mill is no longer the pure clinker: the dynamics of the mix;
• The holdup: the material inside the mill: the product of the feed rate and the residence: the holdup stabilizes the mill: the closed circuit dampens the variations of the fresh feed;
• The first chamber: the coarse return gives the first compartment the material to break: the grinding of the rejects: the ball size distribution of the chamber must fit the returning sizes;
• The specific energy: the mill power is drawn for the total load including the return: the energy per ton of the product improves with the right circulation: the paradox of the closed grinding;
• The discharge: the mill discharge and its fineness: the residue of the discharge vs the product: the difference is the work done by the classification: the delta of the system;

The mill and the separator are the two lungs of the circuit: the file teaches the balance: the return sizes the balls of the first chamber, the holdup stabilizes the feed, the discharge feeds the classifier: the loop is the single machine and the file treats it as such.

7. The Closed Circuit of the Raw Grinding: The Drying Loop

The closed circuit is not the monopoly of the finish: the raw grinding lines run their own loops and the file compares:

• The air-swept mill: the raw ball mill swept by the hot gases: the mill discharge carried pneumatically to the classifier: the drying and the grinding in the one stream: the raw meal circuit of the dry plants;
• The raw separator: the dynamic separator of the meal: the fines to the cyclone and the meal silos: the coarse back to the mill inlet: the R90 residue of the 10 to 15 percent of the design;
• The hot gas balance: the kiln gases at the 300 to 400 degrees enter the mill and dry the 5 to 8 percent moisture of the feed: the drying inside the loop: the mill and the separator share the gas stream;
• The vertical alternative: the vertical roller mill with the integrated classifier: the classification inside the machine: the closed circuit without the external loop: the modern competitor of the arrangement;
• The meal storage: the fines of the separator to the blending silos: the homogenisation of the closed circuit product: the feed of the kiln: the meal loop ends at the kiln feed;

The raw grinding shows the closed circuit in its drying role: the loop that carries the gas, the meal and the heat: the file teaches the raw version of the arrangement so the engineer of the plant reads both of his grinding loops with the same eyes: the finish and the raw, the two closed circuits of the line.

8. The Finish Circuits of the Cement: The Configurations

The finish department offers several closed-circuit configurations and the file maps them:

• The simple ball circuit: the mill and the dynamic separator: the circulating load of the 150 to 250 percent: the classic arrangement of the thousands of plants: the workhorse of the industry;
• The ball with the pre-grinder: the roller press ahead of the mill: the press discharges the cakes, the deagglomerator splits them and the mill finishes the grinding: the combined circuit of the modern energy-saving lines;
• The two-stage circuit: the first mill and the second mill in series with the separator after each or the common separator: the finer cements of the special markets: the flexibility of the stages;
• The separate grinding: the grinding of the clinker and the slag in the two separate circuits blended afterwards: the flexibility of the blends: the cements of the many qualities from the two lines;
• The finish vertical: the vertical roller mill of the finish with the integrated classifier: the closed circuit in the single machine: the new lines of the energy-conscious markets;

The configurations of the finish show the adaptability of the closed circuit: the same principle serves the simplest and the most sophisticated lines: the file compares the configurations with the energy and the flexibility criteria: the engineer of the project selects the arrangement for the market of his plant.

9. The Operation of the Circuit: The Control of the Fineness

The daily operation of the closed circuit is the control of the loop: the file presents the working controls of the separator and the mill:

• The rotor speed: the primary lever: the faster rotor, the finer the cut and the finer the product: the speed of the separator is the first knob of the quality control;
• The air flow: the classification air and its velocity: the higher the air, the coarser the carry: the damper settings and the fan speed: the second lever of the fineness;
• The feed rate: the tonnage of the fresh feed: the loading of the loop: the rate vs the fineness: the balancing act of the mill operator;
• The mill charge: the ball level and the top-up schedule: the grinding power of the chamber: the charge meets the return: the condition of the mill limits the circulation;
• The product targets: the Blaine or the residue of the product: the sampling every hour and the adjustment: the control loop of the fineness: the certificate of the day;

The operation of the closed circuit is the art of the two knobs: the speed and the air: with the feed rate as the constraint: the file trains the operator in the adjustments, the sampling and the reaction times: the fineness of the cement is decided at the separator of the circuit: the control room of the loop.

10. The Troubleshooting of the Closed Circuit: The Symptoms

The loop has its failure modes and the file dedicates the diagnostic chapters to them:

• The coarse product: the Blaine falls: the rotor speed check, the air flow check, the feed rate check: the cascade of the possible causes in the file’s checklist;
• The blinding of the sieve: the electrostatic and the moist fines blind the screens of the laboratory sieves: the false residues: the lab procedure and the ultrasonic cleaning: the wrong data poisons the control;
• The wear of the rotor: the abrasive particles erode the blades and the cage: the imbalance of the rotor: the vibration of the separator: the periodic inspection and the replacement of the parts;
• The fan problems: the imbalance of the classification fan, the erosion of the blades, the falling air flow: the symptoms of the fineness and the power: the vibration analysis of the fan;
• The loop overload: the elevator trips, the mill floods: the circulating load beyond the hardware: the reduction of the feed and the reset of the loop: the recovery procedure of the plant;

The troubleshooting of the closed circuit is the systematic narrowing of the causes: the file provides the trees of the diagnosis so the plant fixes the loop in hours instead of days: the symptoms, the checks and the remedies: the file is the emergency book of the finish department.

11. The Energy of the Classification: The Savings of the Sharp Loop

The economic justification of the closed circuit is the energy: the file presents the numbers of the savings:

• The energy of the open vs the closed: the open mill grinds the fine particles repeatedly and wastes the energy: the closed circuit removes them: the savings of the 10 to 20 percent at the same fineness;
• The separator generations: the third generation versus the first: the sharper the classification, the lower the energy per ton: the gains of the 15 to 30 percent in the finish grinding with the modern separator:
• The bypass reduction: the lowering of the bypass from the 30 percent to the 10 percent recovers the wasted grinding: the energy of the short-circuit: the audit target of the plant;
• The particle distribution: the closed circuit narrows the distribution: the same strength at the lower Blaine: the cement of the narrower curve needs less grinding for the same performance:
• The audit: the kWh per ton before and after the separator upgrade: the payback of the investment: the file shows the method of the energy accounting of the loop;

The closed circuit pays its maintenance with the energy it saves: the file quantifies the savings with the typical numbers and the audit method: the engineer of the package can present the business case of the separator modernization to the management with the numbers of this chapter.

12. The Calculations and the Excel: The Tool of the Circuit

The practical heart of the file: the calculation methods and the spreadsheet that the package provides:

• The residue balance: the circulating load from the residues of the feed, the discharge and the product: the classical formula: the example worked in the file: the numbers of the plant;
• The mass balance: the full balance of the loop: the fresh feed, the return, the discharge, the product: the consistency check of the measurements: the closing of the balance to the 5 percent;
• The Tromp construction: the template of the efficiency curve: the three sieve analyses in the columns and the curve generated: the bypass and the sharpness computed by the tool;
• The scenario analysis: the speed and the air scenarios: the product fineness and the tonnage forecast: the comparison of the operating points: the optimization sheet of the plant;
• The reporting: the daily report of the circuit: the load, the fineness, the energy: the trend chart: the long-term view of the loop: the file and the tool together;

The Excel tool of the package performs in seconds the calculations that the file teaches by hand: the engineer learns the method from the pages and the speed from the spreadsheet: the 931 files of the package include this calculator: the closed circuit, computed and controlled.

13. The Special Cements of the Closed Circuit: The Fineness Cases

The closed circuit adapts to the products beyond the classical 42.5: the file closes the industrial chapters with the special fineness cases that the loop handles:

• The slag cements: the granulated slag grinds harder than the clinker: the circuits of the CEM III run the higher circulating loads and the finer media: the Blaine of the 4,000 to 4,500 with the power-hungry loops: the hard case of the classification;
• The ultra-fine cements: the special products of the Blaine above 5,000: the grinding aids and the fine rotors of the high speed: the retention in the separator and the coating of the mill: the extreme of the closed circuit;
• The limestone cements: the filler and the clinker ground together: the fine limestone sloughs the separator circuits: the residues of the product and the distribution narrow: the loop in the blended line;
• The white cements: the ferrite-minimized clinker and the white grinding media in the white plants: the closed circuit without the iron contamination: the special linings of the white lines: the color checked in the loop;
• The oil well and the specials: the coarser distributions of the oil well cements: the open-like behavior in the closed circuit: the broad curves of the special classes: the flexibility of the loop configuration;

The special products show the reach of the closed circuit: the same loop serves the hardest slag and the whitest cement: the file teaches the adaptation: the media, the speeds and the air adjusted per the product: the engineer of the package walks his plant’s product range with the classification knowledge of this chapter in hand.

The quality control of the specials closes the chapter: the fineness certificates, the residues and the distribution curves of the daily reports: the loop of the circuit and the certificates of the products: the closed grinding is the factory that produces the fineness of the whole portfolio: the specialist cements of the niche markets are made at the same separator that makes the general cement: the difference is in the settings.

14. The Modern Developments: The Intelligence of the Loop

The final chapters of the file open to the future of the closed circuit:

• The online analyzers: the particle size analyzers on the product stream: the real-time fineness: the feedback loop of the separator speed: the automation of the fineness control;
• The expert systems: the model-based control of the loop: the mill power, the elevator load and the separator conditions in the one algorithm: the stability of the operation: the 3 to 5 percent production gains of the tuned plants;
• The sensor fusion: the acoustic sensors of the mill charge and the vibration sensors of the separator: the digital twin of the loop: the predictions of the wear and the maintenance;
• The energy integration: the waste heat of the kiln in the raw loop and the electric demand management of the finish loop: the integrated energy of the plant: the loop in the grid of the plant;
• The climate accounting: the CO2 per ton of the cement ground: the energy of the loop in the carbon balance of the product: the reporting of the modern industry;

The closed circuit evolves with the technology: the same loop of the elevator and the separator gains the eyes of the sensors and the mind of the control models: the file positions the engineer at the crossing: the classical understanding of the loop and the modern instruments: the plant of the future runs its loops with the intelligence of the data.

15. Conclusion

The grinding in the closed circuit: the loop of the mill, the elevator and the separator: the arrangement that most of the cement of the world passes through: the file of the package teaches the totality: the definition, the circulating load, the separator generations, the Tromp curve, the operation and the savings: the engineer who masters the closed circuit masters the finish department of his plant: the loop, the balance, the product: the cement, classified right.

The Complete Cement Technical Package includes this closed-circuit file with the diagrams, the calculation methods and the Excel of the circuit: the one-time $249.99: the instant download: the library of the cement: the file of the loop: the engineer of the package: the separator in the head: the energy of the circuit: the fineness and the tonnage, balanced: the knowledge of the classification, the professional.

The Frequently Asked Questions

Is the closed circuit always better than the open?

For the cement of the modern market the closed circuit wins on the energy and the particle distribution: the open circuit survives in the small plants and the coarse products where the simplicity beats the savings: the file compares the two with the numbers so the engineer decides per the case: the loop for the fineness, the open for the simplicity.

What is the ideal circulating load for the finish mill?

The typical band of the 150 to 250 percent for the classical ball and separator circuits: the optimum depends on the separator sharpness, the ball charge and the product: the file teaches the measurement and the tuning: the plant finds its own optimum by the experiments guided by the formulas: the load of the loop, the number of the balance.

How often should we measure the Tromp curve?

The audit of the classifier every 3 to 6 months for the stable plants and at every performance complaint: the curve catches the wear of the rotor, the wrong air settings and the bypass growth before they cost the energy: the file provides the sampling procedure and the template: the curve is the periodic medical check of the separator.

The dynamic separator costs more: is it worth it?

The third generation reduces the specific energy of the finish grinding by the 10 to 20 percent against the older machines and improves the strength of the cement at the same fineness: the payback of the modern separators is typically a few years at the industrial electricity prices: the file shows the business case so the plant can decide with its own numbers.

Does the file include the calculations of the circulating load?

Yes: the residue method is derived and worked in the examples: the spreadsheet of the package performs the balance, the Tromp curve and the scenario comparisons automatically: the engineer enters the sieve residues of his plant and receives the load of the day: the tool among the 931 files of the package.

What is the first check when the cement turns coarse?

The separator first: the rotor speed, the air flow and the damper settings: then the mill: the ball charge and the coating: then the feed: the clinker quality and the moisture: the file orders the cascade so the plant checks the cheap and the likely causes first: the coarse cement is almost always a classification symptom before it is a grinding symptom.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.