Preblending: Complete Technical Guide

Preblending is the silent stage of the cement process that keeps the kiln alive: it takes the variable rocks of the quarry and stacks them into the layered stockpiles, then reclaims them at right angles so every ton that leaves the pile is the average of thousands of layers: the limestone of the quarry swings its calcium carbonate between 70% and 95%, but the feed of the raw mill must hold its chemistry within fractions of a percent: the prehomogenization pile is the machine that performs this miracle with nothing but geometry and patience.

The Complete Cement Technical Package (931 files including the books, the courses, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) includes this preblending guide with the bed design spreadsheets, the stacker and reclaimer selection tables, the sampling procedures and the efficiency calculations: this article walks the file: the purpose, the pile types, the machinery, the numbers of the bed and the operation: the reader finishes with the ability to design and audit the stockpile of his own plant.

The mixing power of the modern plant is divided among three stages: the prehomogenization of the raw materials, the continuous blending in the raw meal silo and the mixing inside the grinding circuit: the preblending acts on the coarsest material, and its statistics are the most forgiving and the cheapest to buy: the engineer who masters the pile understands the whole blending philosophy of the plant, because every stage of the mixing repeats the same principle: many layers, one average, and the smallest possible scatter around it.

1. The Definition and the Purpose of Preblending

Preblending, or the prehomogenization, is the stockpiling of the raw material in systematically built layers and its reclaiming across those layers, with the single aim of smoothing the chemical fluctuations of the quarry feed: the definition is operational: a simple heap is not preblending: the discipline of the stacking pattern and the reclaim direction are the entire process:

• The input fluctuation: the natural limestone seam swings the CaCO3 content by 5-15% around the mean: the quarry plan mixes the faces, but the residual scatter remains the daily trouble of the chemist;
• The output target: the preblended material leaves the pile with the fluctuation reduced by the factor of 3 to 10: the raw mill then produces the kiln feed with the LSF controlled within plus-minus 0.5% and the standard deviation of the CaCO3 within plus-minus 0.5 to 1.0%;
• The process insurance: the pile is also the buffer: the weeks of the quarry and the crusher maintenance run on the stock: the typical working stock of the prehomogenization is the 3 to 10 days of the kiln consumption;
• The capital decision: the preblending costs capital and land, but the kiln’s stability, the fuel consumption, the refractory life and the quality of the cement all improve: the plants that skip the preblending buy the fluctuation at the burner;

The language of the preblending is statistics: the goal is not to make the material equal, it is to make the variation smaller and the distribution known: the pile is the factory of the average, and this guide explains how the factory is built, measured and operated.

2. Why the Kiln Demands the Stable Chemistry: The Quality Link

The chemistry of the kiln feed is the chemistry of the clinker, and the clinker’s three moduli define the cement: the link between the pile and the quality runs through the numbers that every cement chemist keeps on the whiteboard:

• The lime saturation factor (LSF): the target of the modern plants between 90 and 100, typically 95-97: the LSF swing of two units changes the burnability, the free lime of the clinker and the strength of the cement;
• The silica and alumina ratios (SM, AM): the silica ratio of 2.0-3.0 and the alumina ratio of 1.3-2.5: the ratios control the liquid phase of the clinkering and the coating of the kiln: the pile stabilizes all three at once;
• The fuel and the refractory: the unstable feed forces the operator to run the kiln hotter to protect the quality: the extra 10-20 MJ/t of the specific heat and the shortened refractory campaigns are the price of the neglected blending;
• The free lime and the strength: the underburned clinker shows the free lime above 1.5-2.0% and the 28-day strength falls: the kiln feed fluctuation prints directly into the strength certificate of the cement;

The consequence is the closed logic: the quarry is naturally unstable, the kiln demands stability, and the gap is filled by the three mixing stages: the preblending is the first and the cheapest of the three, and the plants that starve it pay in the fuel, the refractory and the reputation: the file begins from this chain of the value.

3. The Types of the Stockpile: Chevron, Coning and Windrow

The stacking pattern decides the mixing performance, and the industry uses three classical beds, each with its own statistics and its own machines:

• The Chevron pile: the stacker moves along the axis of the bed and discharges the fixed point at the apex: each pass lays a continuous triangular layer on the bed: the classic pattern of the single-material blending: the performance factor of 4-7;
• The coning (sandwich) pile: the stacker rotates and builds the concentric cones: the material layers follow the surface of the cone: the simple circular pile of the small plants: the factor of 2-4, the simpler machine;
• The windrow (horizontal) pile: the stacker moves along the axis while discharging to the side, so the layers lie as the horizontal strips: the better performance of the Chevron (factor 5-8) but the longer boom and the special stacker;
• The circular stockpile: the stacker and the reclaimer share the circular hall: the continuous operation without the changeover of the longitudinal beds: the preferred solution of the large modern plants: the footprint the smallest per ton;

The choice between the beds is the choice of the plant’s appetite: the longitudinal Chevron beds of 30,000-60,000 tons serve the 3,000-6,000 tpd plants; the circular piles of 20,000-80,000 tons serve the same range with the smaller footprint; the windrow serves the hardest blending tasks: the file compares the three with the tables of the performance, the capital and the maintenance.

4. The Geometry of the Bed: The Layers, the Volume and the Numbers

The mixing power of the pile is written in its geometry, and the file quantifies the five numbers that define the bed:

• The length and the width: the Chevron bed of the 30,000-ton class measures about 250-400 meters by 30-45 meters at the base: the ratio of the length to the width (L/W) of 6-12 is the rule of the blending quality;
• The height: the pile height of 10-18 meters with the angle of repose of the limestone at 35-40 degrees: the height is the economics: the higher the pile, the fewer the square meters of the land;
• The number of the layers: the stacker passes, the layers of the Chevron pile typically number between 200 and 1,000, one per pass: the mixing factor grows with the square root of the layers: 400 layers give the factor of 20 against the 100 layers’ factor of 10;
• The volume and the tonnage: the bulk density of the crushed limestone at 1.4-1.6 t/m³: the bed volume equals the tonnage divided by the density: the working stock of 5-10 days of the kiln consumption;
• The XP factor (performance factor): the ratio of the input standard deviation to the output standard deviation: the XP of the well-designed Chevron bed: 4-7: the XP of the windrow: 5-8: the number the file teaches to measure, because it is the certificate of the pile;

The geometry is the physics of the average: the reclaim cuts through all the layers at once, so the output is the weighted mean of everything the stacker laid: the taller and the longer the bed, the more samples the average contains, and the closer the output approaches the true mean of the batch: the land and the capital buy the statistics, and the file shows exactly what each meter buys.

5. The Stackers: The Machines that Build the Bed

The stacker is the builder of the layers, and the file presents the three standard machines with their parameters:

• The traveling stacker: the bridge structure running on the rails along the bed, with the boom that discharges at the fixed point: the travel speed of 0.2-0.6 m/s, the boom luffing for the height control: the machine of the Chevron bed;
• The portal scraper’s stacker: the stacker and the reclaimer combined in the portal frame: the stacker boom on one side builds the bed while the scraper on the other side reclaims the adjacent bed: the continuous two-bed operation of the large plants;
• The circular stacker: the radial boom rotating around the central column, building the circular pile in the concentric layers: the 180-360 degrees of the sweep, the height controlled by the luffing of the boom;
• The tripper and the belt conveyor: the feed belt runs along the bed top, and the traveling tripper chutes the material at the fixed point: the simple alternative of the medium plants: the low capex, the Chevon pattern;
• The parameters of the duty: the stacking rate matches the crusher (300-2,000 t/h), the boom reach 25-40 meters, the belt speeds of 2-3.5 m/s: the number of the passes per bed equals the stacking rate times the bed-building time divided by the layer tonnage;

The stacker is a simple machine that must move exactly: the precision of its travel and the fixed discharge point make the layers uniform, and the uniform layers are the condition of the statistics: the file documents the automation of the stacking sequence: the number of the passes, the direction reversals and the bed changeover, all executed by the PLC without the operator’s attention.

6. The Reclaimers: The Machines that Harvest the Average

The reclaimer is the more demanding of the two machines, because it must cut the full face of the bed evenly: the file dedicates its machinery chapter to the four standard types:

• The bridge scraper reclaimer: the portal bridge spans the bed width, the scraper chain or the harrow drags the material from the face to the central belt: the full-face reclaim in one pass: the classic of the Chevron beds: the reclaim rate 300-2,000 t/h;
• The portal scraper with the slewing boom: the boom swings across the face while the scraper flights cut the wedges: the flexible reclaim of the variable rates: the standard companion of the portal stacker-reclaimer systems;
• The bucket wheel reclaimer: the rotating wheel of the buckets on the end of the slewing boom digs the face continuously: the rates to 3,000 t/h and above: the machine of the largest plants and the circular piles;
• The bridge and the harrow: the harrow bar moves along the face loosening the material, the scraper follows: the full-face action of the highest mixing: the subtle king of the windrow and the Chevron quality;

The reclaim direction is the soul of the blending: the machine moves along the bed axis and cuts the whole triangular cross-section, so the output belt receives the mixture of all the layers in the proportion of their cross-section: the file explains why the diagonal reclaim of the harrow type is superior to the sloped reclaim of the wheel: the mixing factor of the same bed can double with the correct reclaim direction, and the file shows the curves.

7. The Blending Efficiency: The Mathematics of the Scatter

The performance of the pile is measured in the standard deviations, and the file teaches the complete statistical tool, from the sampling to the certificate:

• The definition: the standard deviation of the CaCO3 (or the LSF) of the crusher output is measured as the sigma-in; the sigma of the reclaimer output is the sigma-out; the performance factor XP = sigma-in / sigma-out;
• The measured expectations: the sigma-in of the blasted limestone typically 3-6% CaCO3; the sigma-out of a good Chevron bed 0.7-1.5%; the XP of 4-7: the sigma-out of the windrow beds 0.5-1.0%: the numbers of the file’s audit tables;
• The square-root law: the ideal mixing of the random layers reduces the variance by the square root of the layers: 100 layers give the ideal factor of 10, but the practical correlations, the segregation and the reclaim reduce the realized factor to 4-7;
• The sampling discipline: the samples of the crusher belt hourly and the reclaimer belt hourly: the X-ray analysis of the CaCO3: the paired data of the input and the output computed monthly: the XP is a living number, not a design certificate;
• The improvement logic: the XP below 3 means the bed is not doing its job: the causes: the layer thickness variations, the segregation in the stacking, the sloped reclaim, the broken stacker path: the file’s checklist isolates the cause;

The statistics of the pile are the language between the quarry and the kiln: the geologist measures the deposit, the chemist measures the belt, and the engineer computes the sigma: the file’s Excel tool performs the whole analysis from the raw sample lists to the XP chart, and the plant that runs it monthly sees its blending improving year after year.

8. The Segregation: The Natural Enemy of the Layers

The perfect layer system is disturbed by one physical phenomenon: the segregation of the particles as they fall and roll: the coarse particles roll to the foot of the cone, the fine stay at the top: the consequence is that each layer is not uniform, and the file devotes its physics chapter to the countermeasures:

• The mechanism: the discharging stream of 0-80 mm material forms the cone: the large, heavy and round particles roll down the slope, the fine particles remain at the apex: the radial segregation of every stacker discharge;
• The vertical consequence: the coarse material accumulates near the bed axis (the Chevron apex), the fines at the outer wings: the reclaim cuts the coarser core and the finer edges at different moments, distorting the average;
• The countermeasures: the windrow stacking (the side discharge reduces the fall height), the lower stacker boom (the smaller the cone, the smaller the segregation), the intermediate stockpiles of the pre-crushed material;
• The sampling of the reality: the plant that suspects the segregation checks the particle size distribution of the reclaimer output against the stacker input: the difference of the average sizes is the proof;

The segregation is not the end of the blending but its correction: the design answers it with the geometry, the operation answers it with the pattern: the file’s rules of thumb: the discharge height under 5 meters, the reclaim direction parallel to the axis, and the separate beds for the coarse and the fine fractions when the separation is severe: the physics, contained.

9. The Design Example: The Preblending Station of the 3,000 tpd Plant

The file’s worked example is the design of a complete preblending station, and the abbreviated version shows the method:

• The requirements: the kiln of 3,000 tpd consumes about 420 t/h of the raw material: the quarry reports the sigma-in of 4.2% CaCO3: the target sigma-out of 1.0%: the required XP of 4.2;
• The bed selection: the longitudinal Chevron bed with the bridge scraper reclaimer: the working stock of 7 days: the 420 t/h times 24 hours times 7 days equals about 70,000 tons: the two beds of 35,000 tons each for the changeover;
• The geometry: the width of 36 meters, the height of 13 meters, the length of 260 meters: the cross-section of 234 m², the volume of 60,800 m³, the 35,000 tons at 1.55 t/m³: the L/W of 7.2;
• The stacking: the traveling stacker of 500 t/h, the 800 passes per bed, the layer tonnage of 44 tons per pass: the stacking time of 70 hours per bed;
• The reclaim: the bridge scraper of 420 t/h average with the 1.5 peak factor: the full-face reclaim of the 36-meter width: the XP expected of 5-6 against the required 4.2: the margin of the design;

The example closes with the verification: after the commissioning the plant samples both belts for two weeks, computes the actual XP and compares with the 4.2: the acceptance test of the station: the same method serves the audit of the existing piles: the file gives the complete calculation sheet with the formulas of the cross-section, the volume and the tonnage, the sheet the consultant copies into his own project.

10. The Operation of the Beds: The Changeover and the Stock Management

The daily operation of the prehomogenization is the management of the rhythm between the stacking and the reclaiming, and the file’s operations chapter covers the routines:

• The two-bed cycle: the stacker builds the bed A while the reclaimer empties the bed B: at the completion the machines switch: the sequence of the changeover is programmed, the belts never stop: the buffer of the process;
• The stock tracking: the weigh-belt and the volume scanners track the tonnage of each bed: the CMMS and the planning share the data: the working stock below 3 days triggers the quarry acceleration;
• The bed end effects: the ends of the bed have the fewer layers: the last and the first meters of the reclaim carry the poorer average: the plants waste the ends or blend them with the better middle: the file documents both strategies;
• The weather protocols: the rain wets the pile surface, the snow freezes the crust: the reclaim chokes the feeders with the lumps of the frozen material: the winter protocol of the file: the breaker, the pre-crushing and the stock rotation;
• The housekeeping: the spillage along the rails, the dust on the beams, the belt alignment: the weekly inspection route of the stacker and the reclaimer is the printed checklist of the file: the small machines, the small discipline;

The operation of the beds is the invisible service of the plant: when it runs, nothing appears in the daily report; when it stops, the raw mill starves in hours: the file’s operators’ manual makes the invisibility the subject of the checklist, the shift handover and the stock report: the discipline of the beds is the discipline of the kiln, written in the layers.

11. The Troubleshooting of the Prehomogenization System

The symptoms of the blending department are the slow, statistical ones, and the file maps them to the causes:

• The sigma-out too high: the XP too low: the checks: the stacker pattern (the fixed point?), the segregation (the discharge height?), the reclaim direction (the full-face?), the bed end contamination: the diagnosis in the order of the file;
• The reclaimer rate swings: the bridge stalls in the coarse wedge or the frozen crust: the harrow pressure rises: the speed control of the scraper reacts: the manual intervention at the problematic bands;
• The belt jams at the reclaim: the oversized lumps from the bed (the stockpile scavengers, the weather crusts): the grizzly and the breaker before the mill: the reclaimer avoids the lumps: the pile management;
• The stacker breakdown: the bed grows unbalanced and the changeover delayed: the planning falls back to the direct bypass: the process survives on the single bed with the reduced stock: the maintenance priority of the plant;
• The chemistry complaints of the raw mill: the LSF of the kiln feed drifts: the blame is shared: the pile audit first, the silo second: the file’s decision tree divides the responsibility between the preblending and the homogenizing silo;

The troubleshooting chapter of the file is written as the monthly audit: the plant collects the sigma data, the stacker log and the reclaimer log, and the worksheet isolates the weak link of the blending chain: the preblending fails rarely, but when it does, the entire kiln feed feels it: the audit habit is the insurance.

12. The Circular Pile and the Multi-Component Blending

Beyond the single-material bed, the modern plants blend several components in the same pile, and the file covers the extended architectures:

• The circular prehomogenization: the stacker and the reclaimer in the dome: the continuous operation, the smallest footprint per ton, the 20,000-120,000-ton piles: the standard of the new large plants: the dust control inside the dome;
• The multi-component stacking: the separate weigh-feeders dose the limestone, the clay and the correction materials on the common belt to the stacker: each layer is already the proportioned mix: the pile blends the proportions themselves;
• The intermediate stockpiles: the clay and the correction are stored in the small piles of their own: the quality control of the proportioning begins at the feeders: the files of the batching and the belt scales belong to the package;
• The homogenizing silo as the partner: the preblending reduces the coarse fluctuations, the silo finishes the fine ones: the division of the labor: the 60-80% of the smoothing by the pile, the remainder by the silo: the two chapters of the file complete each other;

The architecture of the blending is the capital decision of the raw department: the circular pile with the multi-component stacking reduces the number of the machines and the land while improving the quality, and the file’s comparison tables show the total cost of ownership of the alternatives: the consultant of the new plant reads this chapter with the flowsheet in hand.

13. The Maintenance of the Stackers and the Reclaimers

The blending machines are the outdoor steel of the plant, exposed to the weather and the dust, and the file’s maintenance chapter is the schedule of their long life:

• The rails and the wheels: the weekly inspection of the rail gauge, the wheel flanges and the rail alignment: the 3-6 monthly laser survey of the straightness: the wear of the rails is the silent killer of the smooth travel;
• The chains and the flights of the scraper: the wear of the flight tips at the bed face: the chain tensioning and the lubrication: the replacement of the flights at the planned tonnages: the apron plates of the 400-600 HB steel;
• The drives and the gears: the slew drives, the travel gearboxes and the belt drives: the oil analysis quarterly: the vibration routes monthly: the predictive readings of the package’s CMMS template;
• The belts and the pulleys: the reclaim belt misalignment, the pulley lagging wear, the idler replacement: the belt of the reclaimer is the most stressed belt of the raw department;
• The electrical and the PLC: the cable reels, the limit switches, the encoder feedback: the PLC program of the stacking sequence backed up: the sensors of the position fail in the dust: the spare strategy of the file;

The maintenance of the blending machines is the unglamorous half of the blending quality: a stacker that drifts from its path by a meter degrades the layers and the XP: the file’s checklists turn the long-term statistics into the weekly actions, and the plants that follow them report the twenty-year lives of the bridges and the reclaimers: the steel of the pile, well kept.

14. The Safety of the Stockpile Yard

The stockpiles and their machines carry their own hazards, and the file lists the rules that the yards enforce without exception:

• The falling faces: the reclaim faces of the Chevron beds can slip and fall: the personnel stay out of the undercut zone: the barriers and the inspection ladders: the “no man in the pile” rule of the yard;
• The dust and the visibility: the stacking and the reclaiming raise the dust clouds: the water sprays and the dust extraction: the operators use the dust masks and the yard traffic drives with the lights: the wind speed limit of the stacking at the dusty days;
• The moving machines: the stacker and the reclaimer move on the rails: the pedestrian routes cross only at the marked gates: the horn and the flashing of the movements: the radio discipline of the yard;
• The maintenance access: the work on the boom and the wheel at height: the harnesses, the platforms and the permit systems: the lock-out of the travel and the slew before the entry to the mechanism zones;

The yard is a quiet place with large moving machines, and the quiet makes it dangerous: the file’s safety chapter is the template of the yard’s procedures, the walkways, the signage and the toolbox talks: the statistics of the industry say the yards claim their share of the accidents, and the file answers with the procedures the plants adopt as the policy.

15. The Sampling and the Quality Control of the Preblending

The blending statistics are only as honest as the samples, and the file devotes its quality chapter to the art of the representative sampling:

• The crusher belt sampling: the automatic sampler cuts the falling stream every 15-30 minutes: the incremental samples of 5-10 kilograms composite the hourly bag: the belt sampler of the crusher discharge is the ear of the sigma-in;
• The reclaimer sampling: the same discipline on the reclaim belt: the paired samples of the input and the output over the same window make the XP computation possible: the file provides the sample schedule of the acceptance tests;
• The laboratory preparation: the reduction of the composite to the laboratory sample by the riffle splitting and the crusher: the quartering protocols avoid the analyst bias: the moisture of the sample corrected in the CaCO3 calculation;
• The XRF and the wet chemistry: the pressed pellets and the fused beads of the X-ray fluorescence report the CaCO3 and the oxides: the calibration standards of the plant: the reference samples exchanged with the independent laboratories quarterly;
• The trend reporting: the daily means, the moving averages and the sigma of the week: the control chart of the CaCO3 with the action limits of plus-minus 2 sigma: the plant watches the drift of the pile quality like the drift of the kiln feed;

The sampling loop closes the circle of the preblending: the stacker builds, the reclaimer averages, the sampler measures and the chemist reports: the plant that masters the loop knows its pile exactly, and the knowledge is the power to adjust the stacking plan, the quarry mix and the kiln feed in advance: the file’s Excel data sheets take the raw sample lists and produce the sigma-in, the sigma-out and the XP charts in minutes, the weekly ritual of the quality engineer.

16. The Frequently Asked Questions

What is the difference between preblending and the homogenizing silo?

Preblending acts on the crushed rock of 0-80 millimeters in the stockpile, with the performance factor of 4-7, and its purpose is the coarse smoothing of the quarry variation: the homogenizing silo acts on the fine raw meal of below 1 millimeter with the blending factors of 4-10: the plant needs both because the two stages face different statistics and the silo alone cannot erase the quarry’s daily swings: the division is written in the design books and practiced by every modern plant.

How many days of stock should the preblending pile hold?

The typical design is 3-10 days of the kiln consumption: the 3-5 days protect against the short crusher and quarry interruptions, the 5-10 days allow the quality campaigns and the weekend maintenance of the crushing: the larger the pile, the longer the stacking cycles and the higher the capital: the file’s sizing chart balances the working stock against the quarry reliability and the land price.

What is the expected blending factor (XP) of a Chevron bed?

The measured performance factors of the well-designed Chevron beds lie between 4 and 7: the windrow beds reach 5-8, the circular piles 3-6: the theoretical square-root-of-layers limit is higher, but the segregation and the reclaim effects reduce the realized factor: the plant should measure its own XP monthly: the file gives the sampling and the calculation procedure, and the audit table of the industry values.

Can the existing pile be improved without new machines?

Often yes: the lower discharge height of the stacker boom reduces the segregation; the reclamation from the full face with the harrow improves the mixing; the bed end management and the multi-component stacking upgrade the performance: the file’s improvement checklist ranks the low-cost changes by their effect on the XP: the plants typically recover a factor of 1-2 with the geometry corrections alone.

How does the rain and the moisture affect the preblending?

The rain wets the surface of the pile and raises the moisture of the reclaimed material, increasing the drying load of the raw mill: the frozen crusts jam the feeders in the winter, and the wet fines pack the belts: the stock rotation and the drainage of the yard are the first defenses: the file’s weather protocol covers the seasonal operation of the beds.

Is the windrow stacking worth its higher cost?

For the single hardest blending tasks (the high-fluctuation deposits and the strict clinker quality) the windrow pattern adds about one point of the performance factor at the cost of the longer boom and the more complex stacker: the file’s decision chart recommends the windrow only when the required XP exceeds what the Chevron geometry can deliver, which happens in the deposits with the sigma-in above 6%.

17. Conclusion

The prehomogenization is the arithmetic of the many layers: the quarry delivers the noise, and the pile turns it into the average: the Chevron, the windrow and the circular beds, the stackers that build and the reclaimers that harvest, the sigma of the input and the sigma of the output: every number of the blending is the promise of the steady kiln and the consistent cement: the stage costs little, works long and pays every hour of the plant’s life.

The Complete Cement Technical Package includes this preblending guide with the bed design spreadsheets, the machine selection tables, the XP calculation tools and the operating and maintenance checklists: the one-time 249.99: the instant download of the 931 files: the blending knowledge of the modern plant, organized and ready: order the package and let your piles do the mathematics they were built to do.

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.