Mill Questionnaire: Complete Technical Guide

The mill questionnaire is the structured set of technical questions that defines the duty of a grinding mill before it is selected, quoted, supplied, or audited, and file 413826362 in the Complete Cement Technical Package is a complete mill questionnaire of the type issued by cement plants to equipment suppliers, and used internally to document the design basis of a new mill or the operating basis of an existing one. The questionnaire covers every parameter that determines the mill selection: the raw materials and their characteristics, the product requirements, the capacity and the operating schedule, the power supply, the site conditions, the grinding technology preference, the auxiliary equipment, the maintenance and the spares, and the quality control requirements. This article explains the complete mill questionnaire section by section, with the reason for every question, the expected answer range for the typical applications, and the way the answers drive the mill design, so that the reader can complete the questionnaire for his own project, can answer a supplier’s questionnaire, or can use the questionnaire as the audit checklist for an existing mill. The article covers the ball mill and the vertical roller mill applications, the raw mill, the cement mill, and the coal mill cases, and the interpretation of the grindability, the moisture, the abrasiveness, the fineness, and the process data that the questionnaire collects.

1. The Purpose of the Mill Questionnaire

The mill questionnaire serves the same function as the datasheet for the kiln: it is the specification instrument of the grinding equipment. For the procurement, the questionnaire is issued to the suppliers with the project’s design basis, and the supplier’s quotation is only comparable if every supplier answers the same questionnaire with the same assumptions, because the mill design, the mill size, the motor power, and the price all depend on the raw material characteristics, the product fineness, and the operating conditions that the questionnaire documents. For the engineering, the questionnaire is the checklist that ensures the completeness of the design basis, because a mill quoted without the feed moisture or the grindability data will be either oversized and expensive or undersized and incapable, and the questionnaire forces the collection of the missing data before the commitment. For the audit, the same questionnaire serves as the structured interview of the existing mill operation, because the answers of the plant’s own team to the questionnaire reveal the gaps between the design basis and the actual operation.

The structure of the mill questionnaire follows the logic of the mill design: the general project information, the material to be ground with its complete characteristics, the product specification, the capacity and the scheduling, the power and the site conditions, the technology preferences, the auxiliary and the interfacing equipment, the instrumentation and the control, the environmental requirements, and the documentation and the spares. The questionnaire of file 413826362 contains these sections in the tabular layout with the columns of the parameter, the unit, the requested value, and the remarks, and the completed questionnaire is the contract annex of the supply contract, because the guaranteed performance of the mill is defined by the questionnaire data. The rest of this article explains each section in the order of the questionnaire.

2. The Material to Be Ground: The First and Most Important Section

The material section of the questionnaire is the section where the mill design starts, because the grindability, the moisture, the abrasiveness, and the chemistry of the feed determine the mill type, the size, the power, and the wear part design. The grindability is the measure of the energy required to grind the material, and the questionnaire requests the standard test values: the Bond work index in kWh per tonne for the ball mill design, the Hardgrove index for the coal mills, and for the vertical roller mills the specific grindability determined by the VRM manufacturers’ test mills. The Bond work index of the typical cement raw materials is 10 to 14 kWh per tonne for the limestone, 15 to 25 for the clinker, 25 to 40 for the slag, and the questionnaire must state the index of the actual feed, because a change of 1 kWh per tonne changes the mill power requirement by 5 to 10 percent.

The moisture section requests the feed moisture, the maximum, the minimum, and the average values, and the drying requirement, because the moisture determines the ventilation and the drying system of the mill: a raw mill fed with the 3 to 8 percent moisture limestone requires the hot gas from the kiln or a separate hot gas generator, and a finish mill fed with the hot clinker at 80 to 120 °C must handle the heat release without exceeding the temperature limits. The abrasiveness section requests the wear test data, the silica content, and the abrasion index of the feed, because the abrasive quartz and the hard inclusions determine the liner and the grinding media life, and the questionnaire records the expected consumption of the media and the liners in grams per tonne. The chemistry and the size distribution sections complete the material data: the full chemical analysis, the loss on ignition, the free silica, the feed size distribution with the maximum lump size, and the feed temperature, because the material is the boundary condition of every subsequent design decision.

Material Data Requested by the Mill Questionnaire (file 413826362, typical ranges)

Parameter	Unit	Raw mill (limestone)	Cement mill (clinker)	Coal mill
Bond work index	kWh/t	10 – 14	14 – 18	50 – 70 (Hardgrove 40 – 60)
Feed moisture	%	3 – 8	0.5 – 2	8 – 15
Feed temperature	°C	Ambient	80 – 120	Ambient
Max feed lump	mm	75 – 100	25 – 40	25 – 50
Feed size (d80)	mm	10 – 20	5 – 10	5 – 15
Abrasion (quartz content)	% SiO2	1 – 10	1 – 3	5 – 20

3. The Product Specification Section

The product section of the questionnaire defines what the mill must produce, and its central parameter is the fineness with the target value and the tolerance. For the cement mill the fineness is specified by the Blaine specific surface in cm2 per gram, typically 3200 to 3800 for the ordinary Portland cement and up to 5000 for the high early strength grades, and by the residue on the 45 micron sieve, typically 5 to 15 percent, and for the raw mill the fineness is specified by the residues on the 90 and 200 micron sieves, typically 12 to 16 percent on the 90 micron sieve. The questionnaire also requests the particle size distribution or the target residue curve, because the different applications require the different distribution slopes, and the mill and the separator are designed for the specific distribution.

The product quality requirements complete the section: the cement type and the standard, the composition of the product mix with the clinker, the gypsum, the fly ash, the slag, and the limestone proportions, the temperature of the product at the discharge, and the product temperature limit for the storage, typically 90 to 110 °C to protect the gypsum and the cement quality. The questionnaire also records the quality control requirements: the sampling points, the frequency of the fineness measurements, the automatic sampler and the analyzer requirements, and the reporting, because the design of the mill circuit must include the sampling and the control equipment. The product section is completed with the range of the product recipes that the mill must handle, because a mill designed for one recipe only is a mill that fails when the recipe changes.

4. The Capacity and the Operating Schedule Section

The capacity section of the questionnaire defines the annual and the hourly duty of the mill, and it contains the parameters that determine the mill size: the required production rate in tonnes per hour, the annual production, the operating hours per year and per day, the availability target, and the peak and the turndown requirements. The plant’s operating schedule determines the design capacity: a plant that runs 330 days per year at 24 hours requires a lower hourly capacity than a plant with the shorter schedule, and the questionnaire records both the average and the guaranteed capacity. The turndown requirement, the minimum capacity at which the mill must still operate stably, is a frequently underestimated parameter: the vertical roller mills have a minimum operating load of 30 to 60 percent of the rated capacity, below which the grinding pressure and the bed stability become critical, and the questionnaire must state the required turndown.

The future expansion requirement closes the capacity section: whether the mill must be capable of the future capacity increase, whether the drive and the classifier have the margin, and whether the plant building and the foundations allow the future modification. The suppliers price the expansion capability into the offer, so the questionnaire must state the requirement explicitly rather than leaving it to the interpretation. The capacity section also records the feed supply conditions: the availability of the feed material, the variations of the feed, and the buffer storage between the upstream process and the mill, because the mill design assumes the steady feed supply, and the questionnaire documents the actual supply conditions so that the design includes the appropriate feed control and the storage.

5. The Power, the Site, and the Utilities Section

The power and the site section of the questionnaire records the boundary conditions of the installation: the electrical supply voltage, the frequency, the short-circuit capacity, the power quality, and the power tariff structure; the site altitude, the ambient temperature range, the humidity, the wind and the seismic conditions; and the available utilities of the cooling water, the compressed air, the hot gas for the drying, and the inerting gas for the coal mills. The altitude and the ambient temperature affect the fan and the motor ratings, and the questionnaire records them so that the supplier corrects the equipment for the site conditions. The power tariff structure, with the peak and the off-peak prices, influences the mill operating strategy and sometimes the flywheel design of the grinding operation, and the questionnaire records the relevant data of the tariff.

The layout and the interfacing data complete the section: the available space and the building dimensions, the foundation conditions, the feed and the product transport systems, the silo and the storage interfaces, and the dust collection interfaces. The questionnaire records the elevation and the position of the feed points and the discharge points, because the mill circuit layout, the elevators, and the conveying distances all depend on these coordinates. The utilities data are completed with the quality requirements: the cooling water temperature and the chemistry, the compressed air pressure and the dryness, and the hot gas temperature and the volume available for the drying, because each of these values is a design input of the mill ventilation and the auxiliary systems.

6. The Technology Preference Section

The technology section of the questionnaire records the plant’s preferences and constraints for the mill technology, and it is the section where the plant declares the boundaries of the selection: the preferred mill type, the ball mill, the vertical roller mill, or the roller press combination; the preferred drive type; the supplier preferences; and the reasons, which may be the existing spare parts, the operator experience, or the maintenance capability. The questionnaire also records the evaluation criteria of the technology: the specific energy consumption, the wear part life, the maintenance complexity, the drying capability, and the noise, because the plant’s priorities determine which technology wins the selection even at the same price.

The technology section also covers the existing installations of the plant: the performance of the existing mills, the problems experienced, the successes, and the lessons, because the new mill must be selected with the knowledge of the operating experience. The questionnaire finally records the project schedule and the delivery requirements, the erection and the commissioning requirements, the training and the documentation requirements, and the performance tests and the guarantees requested, because the supply contract terms follow from these requirements. The completed technology section gives the suppliers the full context of the selection, which is the basis of the comparable and the useful offers.

7. The Auxiliary Equipment and the Interfaces Section

The auxiliary section of the questionnaire defines the equipment around the mill, and its completeness determines the quality of the whole quotation. The section covers the feed equipment: the weigh feeders with their accuracy and the dosing range, the feed chutes and the air locks, and the metal detectors and the magnetic separators. The classification equipment: the separator type, the dynamic separator with its rotor and its drive, the fineness control range, and the circulating load capability. The transport equipment: the bucket elevators, the air slides, the screw conveyors, and the pumps, with their capacities and the conveying distances. The ventilation and the dedusting: the mill vent filter, the process gas fan, the hot gas system for the drying, and the dedusting of the transport points. And the cooling equipment: the clinker cooling system, the water injection into the mill, and the product cooling.

The questionnaire requests the capacities, the specifications, and the supplier preferences for each auxiliary, because the auxiliary equipment is frequently supplied by different vendors and the plant must integrate the offers. The section also covers the instrumentation and the control: the sensors, the analyzers, the control philosophy, the DCS interface, and the interlocks, and the electrical equipment: the motors, the switchgear, the MCC, the cabling, and the earthing. The environmental equipment closes the section: the noise limits, the dust emission limits, and the required environmental monitoring, because the environmental permits of the plant define the equipment requirements. The complete auxiliary section of the questionnaire is the specification basis of the entire grinding plant, not only of the mill itself.

8. The Operation, the Maintenance, and the Documentation Section

The operation and the maintenance section of the questionnaire records the plant’s operating and maintenance capability, because the mill design must match the plant that will run it: the number of the operating shifts, the staffing of the control room and the operators, the maintenance crew capability, the workshop facilities, and the site for the heavy repairs. The questionnaire records the spare parts philosophy: the critical spares list, the holding levels, the lead times, and the budget, because the supplier’s documentation and the training must enable the plant to maintain the equipment with its own resources. The documentation section requests the complete documentation set: the drawings, the manuals, the spare part lists, the recommended spare parts with the prices, the training courses, and the commissioning support, with the language and the format requirements.

The section closes with the performance and the guarantee requirements: the guaranteed production rate, the specific energy consumption, the fineness and the quality at the guarantee conditions, the availability, and the test procedures, because the contract acceptance is based on these guarantees. The questionnaire also records the liquidated damages and the bonus conditions that the plant applies to the guarantees, and the insurance and the banking conditions of the supply contract. The completed operation, maintenance, and documentation section ensures that the mill purchase is complete: the equipment, the documentation, the training, and the spares that the plant needs to operate the mill for the years, and its completeness is the difference between the successful projects and the projects where the mill is delivered but the plant cannot run it.

9. From the Questionnaire to the Mill Selection: The Technology Comparison

Once the questionnaire is complete, the answers are used to compare the mill technologies, and the comparison is the decision that the questionnaire has prepared. The ball mill is the classical technology: the proven reliability, the tolerance of the feed variations, the simple operation, the ability to produce the narrow particle size distributions, and the low maintenance complexity, but the specific energy consumption of 25 to 40 kWh per tonne of cement and the heavy wear parts make it the technology of the mature plants and the small capacities. The vertical roller mill offers the specific energy of 17 to 28 kWh per tonne of cement, the combined drying and grinding in one machine, the compact footprint, and the capacity to handle the moist feeds, but it demands the higher process control quality, the bed stability, and the specialist maintenance of the hydraulic system and the grinding table. The roller press, used with a ball mill in the semi-finish or the finish grinding circuits, offers the highest energy efficiency of the pre-grinding, and its application is decided by the plant’s requirements for the flexibility and the quality.

The questionnaire’s answers determine the recommendation by the decision matrix: the specific energy and the power cost, weighted by the tariff; the drying requirement, weighted by the feed moisture and the available hot gas; the product fineness and the distribution requirements, weighted by the product portfolio; the availability and the maintenance capability of the plant, weighted by the local skills and the spares logistics; the expansion capability; and the capital cost. The matrix is completed with the suppliers’ guarantees from their questionnaire responses, and the selected technology is the one that maximizes the weighted value over the life cycle, not necessarily the one with the lowest price. The documented recommendation, with the decision matrix and the evidence from the questionnaire, is the deliverable of the selection, and it is the same document that the plant presents to the management for the approval, because it shows that the selection was made on the data and not on the preference.

10. Using the Questionnaire for the Mill Audit

The same questionnaire that specifies the new mill serves as the audit instrument of the existing mills, and this double use multiplies its value. The audit version of the questionnaire is filled by the plant team from the actual operation: the actual feed characteristics from the daily sampling, the actual capacity and the availability from the production records, the actual fineness and the quality from the quality records, the actual specific energy from the power meters, and the actual maintenance and the wear part consumption from the stores. The comparison of the audit answers with the original design basis reveals the deviations: a mill designed for the 5 percent moisture fed with the 8 percent moisture, a mill designed for the Blaine 3400 running at the Blaine 3800 for the market requirements, a mill designed for the 24-hour operation running 16 hours, and the wear part consumption above the design by a factor of two.

The audit findings are then translated into the improvement program: the feed preparation improvements to restore the moisture and the size distribution, the separator optimization to achieve the fineness at the lower specific energy, the operating schedule revision to capture the off-peak power, the ventilation and the water injection improvements to control the temperature, and the wear part material upgrades to reduce the consumption. The audit also produces the updated design basis for the spare part strategy, the refurbishment decisions, and the future capacity planning, because the measured data replace the original assumptions. The mill audit performed with the questionnaire of file 413826362 is therefore the same tool as the kiln condition chart and the downtime register in the maintenance philosophy of this package: the structured collection of the facts, the comparison with the design basis, and the closed loop of the improvement actions, all supported by the questionnaire that defines the truth of the mill.

10. Frequently Asked Questions

Q1. Who fills the mill questionnaire and when?

The plant’s project team fills the questionnaire for a new mill before the enquiry is issued, and the suppliers confirm or clarify the values in their offers. For the audit, the plant’s process and mechanical engineers fill the questionnaire from the actual operation data, usually on the annual basis or before a major modification.

Q2. Why is the Bond work index so important in the questionnaire?

Because it is the direct measure of the energy required to grind the material, and the mill power and size are calculated from it. A wrong work index produces a mill that is either undersized, with the capacity below the guarantee, or oversized, with the investment and the power losses, so the index must be measured on the actual feed by the standard Bond test.

Q3. What is the difference between the raw mill and the cement mill questionnaires?

The sections are the same, but the values differ: the raw mill questionnaire emphasizes the feed moisture, the drying capability, the feed size, and the kiln gas interface, while the cement mill questionnaire emphasizes the fineness and the Blaine, the product recipe flexibility, the temperature control, and the separator performance. The coal mill questionnaire adds the safety sections of the inerting, the explosion protection, and the fire detection.

Q4. What is the turndown ratio of a mill and why does the questionnaire ask for it?

The turndown ratio is the ratio of the maximum to the minimum stable capacity. The vertical roller mills operate down to 30 to 60 percent of the rated capacity, below which the bed and the grinding pressure become unstable, and the plant that must operate at the low loads for the night or the maintenance windows must specify the turndown so that the mill design includes the capability.

Q5. How accurate must the questionnaire data be?

The data that determine the mill size, the feed grindability, the moisture, the capacity, and the fineness must be measured with the standard methods and the documented basis, because they become the contractual guarantee conditions. The data that describe the preferences and the interfaces can be indicative, but every value that the supplier uses in the design should be confirmed in writing.

Q6. What happens if the actual material differs from the questionnaire data?

The mill performance will deviate from the guarantee: a harder feed reduces the capacity, a wetter feed reduces the drying capability, and an abrasive feed shortens the wear part life. The questionnaire therefore requests the range of the material characteristics, and the plant must re-issue the questionnaire whenever the raw material source or the product mix changes permanently.

Q7. Can the questionnaire be used to compare supplier offers?

Yes, this is one of its main purposes. Every supplier answers the same questionnaire, the offers are normalized on the same design basis, and the comparison covers the price, the guaranteed performance, the specific energy, the wear part life, the documentation and the spares, and the deviations, which makes the selection transparent and the contract conditions enforceable. The comparison must also cover the delivery schedule, the erection and the commissioning services, the training, and the local support of each supplier, because the total cost of ownership includes the project execution risks as well as the equipment price.

Q8. What is the typical specific energy of the modern finish grinding systems?

The ball mill closed circuit consumes 25 to 40 kWh per tonne of cement, the vertical roller mill 17 to 28 kWh per tonne, and the roller press with the ball mill combination 18 to 26 kWh per tonne, depending on the fineness, the clinker grindability, and the blend composition. The specific energy is one of the most important comparison criteria of the questionnaire, because it is the operating cost that the plant pays for the entire life of the equipment.

Q9. Why does the questionnaire ask about the site altitude?

Because the altitude changes the air density and with it the fan and the motor performance: at 2000 meters the air density falls by about 20 percent, and the ventilation systems of the mill must be corrected for the actual site conditions. A mill designed with the sea level fan data and installed at the high altitude will struggle with the ventilation, the drying, and the transport, so the questionnaire records the altitude explicitly.

11. Summary

The mill questionnaire of file 413826362 is the complete specification and audit instrument of the grinding equipment: the material section with the grindability, the moisture, the abrasiveness, and the chemistry; the product section with the fineness and the quality requirements; the capacity and the operating schedule section; the power, the site, and the utilities section; the technology preference section; the auxiliary equipment and the interfaces section; the operation, the maintenance, and the documentation section; and the use of the same questionnaire for the audit of the existing mills. The article has explained every section with the typical values for the raw, the cement, and the coal mills, and with the reasons why each question matters to the design and the selection. The questionnaire is the instrument that ensures the complete design basis before the commitment, the comparable offers in the procurement, and the honest audit of the existing operation, and the plant that fills it completely, verifies its data, and uses it consistently buys the right mill the first time and runs the grinding department on the documented facts.

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.