How to measure grinding rod wear rate and compare supplier trials

Grinding rod wear rate: define the kg/t question first

When a mill starts using more rods, the first question is where the steel went. It may have worn away. It may have left as broken pieces or bent rods. Or the mill may simply be carrying a heavier charge than it did at the start of the month. The purchasing ledger records all three as demand for more steel.

For a useful grinding-rod consumption figure, follow one mill over one defined period. Reconcile the mass entering and leaving its working charge, then divide the replacement burden by the dry metric tonnes of new feed processed in that same period. Keep transfers, charge changes and discarded rods visible.

This is also the starting point for comparing suppliers. Sepúlveda’s full-plant work concerned ball media, but its campaign discipline is useful here: establish a repeatable measurement routine before changing the media. Aldrich’s review explains why the ore, slurry and operating conditions have to travel with the result.

Sources:[1][2]

Calculate grinding rod consumption with a kg/t mass balance

Start with four mass records: the opening charge, rods added, serviceable rods transferred out, and the closing charge. An increase in the steel still inside the mill is not consumption. A transfer of usable rods to another mill is not consumption by this mill either.

Gross rod consumption = rods added − serviceable transfers out − (closing charge − opening charge). Divide by dry metric tonnes of new feed. Discarded broken and bent rods remain part of this gross replacement burden; record them separately to explain it.

Use the same survey method at both ends of the window. Align the charging log, ore scale, moisture correction and stoppage record. If the uncertainty in charge inventory is larger than the apparent supplier difference, another decimal place in kg/t will not settle the comparison.

Use the grinding rod consumption trial ledger to enter opening and closing charge, additions, removals and dry feed for the same block. It calculates gross kg/t and offers a repeated-block Excel template; neither the calculation nor the illustrative example is a published JOTAIN trial result.

Sources:[7]

A worked example: the charge increase that looks like wear

Two published consumption figures—and the conditions behind them

The measurement belongs in the operating mill

This 1988 field investigation measured corrosion in operating grinding equipment. Ore and water conditions were part of the experiment.

Select an image to open the full-size original and inspect its labels.

Sources:[3][4]

Consumption has risen. What should the plant check first?

Start with the change visible in the records. These checks narrow the investigation; they do not diagnose a failure from one symptom.

For the material review, compare the surface-to-core hardness profile with the recovered rod dimensions and the reason for removal.

Sources:[2][3][7]

Wear, breakage, bending, removals, and handling loss

A broken rod and a rod that gradually wears thin can cost the same kilograms but tell very different stories. Weigh the recovered pieces and record the fracture position, approximate diameter, date, operating event and identifiable lot. For bent rods, record the shape, remaining dimensions and reason for removal.

Photograph recovered rods before cutting or disposal, following the site’s handling and isolation procedures. Include a scale, the full length where practical, and close views of the damaged area. Keep unidentified pieces in a separate category; mixed charges make supplier attribution difficult.

Subtracting weighed discarded removals from gross consumption leaves a mass-balance residual. That residual can include progressive wear, unrecovered fragments, carryout and measurement error. Retaining this distinction makes the next investigation much more useful than labelling every missing kilogram “abrasion.”

If the removal pattern points to geometry, review rod diameter, length and mill fit alongside the material and operating records.

Sources:[3][7]

A comparable operating window

Compare like operating periods before comparing the rods. Record the ore campaign, feed-size distribution, dry throughput, product target, slurry condition, mill speed, liner state and charge mix. “Normal operation” is too broad to be a useful entry in the log.

A coarser product target or a change in ore can alter the duty enough to overwhelm a media difference. Keep product size, dry tonnes per hour and specific energy alongside kg/t. Zeng and Forssberg’s rod-mill experiments found that feed size, rod diameter and charge composition affected grinding response; they did not establish a universal correction factor for dissimilar mills.

Keep upset periods in the record and mark them clearly. Decide which conditions belong in the comparison before reviewing the supplier results. The historical RI 7826 investigation itself warns that ore variation and the time needed to reach circuit equilibrium limited interpretation.

Use the grinding rod operating-data checklist to freeze mill, feed, charge, wear, breakage, and trial fields before comparing kg/t.

Sources:[2][4][5]

Matched supplier trials and stop rules

First record several stable baseline periods with the current rods. Identify the trial rods by heat, treatment lot, diameter and length, and agree how the existing charge will be handled. A mixed charge is a transition, not a clean trial of the new supplier.

Use repeated comparable operating blocks. Where the plant allows it, balance the supplier sequence across important conditions such as ore campaigns and liner stages. NIST’s guidance on blocking explains the principle: group similar conditions so that the difference being tested is easier to see. ^[8]

Choose the trial duration from charge turnover, normal process variation and the size of the improvement worth acting on. A calendar promise of “one week” or “one month” says little without that context. Keep the individual block results, not just the final average.

Agree the site’s safety and process stop rules before charging: abnormal fracture or bending, tangling, unusual vibration, discharge problems, or unacceptable product and power changes. A stopped trial is still a result. Report the exposure and reason for stopping, without projecting an unobserved full life.

When the plant has reconciled a baseline and candidate trial window, connect the result to the grinding rod application and quotation route so material, dimensions, inspection, packing, destination, and trial quantity are quoted together.

Sources:[1][7][8][9]

Cost per tonne, uncertainty, and operating outcomes

The recurring media cost per dry tonne is gross kg/t multiplied by the delivered rod price per kilogram. Put both offers on the same currency and delivery basis. Freight, duty and inland transport matter; so does the treatment of scrap credit.

Then look beyond the invoice. Record the labour and downtime associated with removals, the achieved product size, throughput and energy use. Add costs only where a documented event and an agreed allocation support the amount. Keep the one-off trial and changeover bill separate from recurring operation.

A lower average is interesting; a repeatable difference under comparable conditions is a stronger purchasing basis. Show the operating-block results and their variation. If the apparent gain is smaller than the uncertainty in the measurement, extend or refine the trial before committing routine supply.

Sources:[1][2][7]

What laboratory abrasion can—and cannot—tell you

Inside the laboratory

Connect a test report to its sample, heat and treatment lot. The equipment photograph documents the visit; the lot report supplies the readings.

Select an image to open the full-size original and inspect its labels.

A laboratory test is most useful when it answers a specific question. ASTM G65 compares scratching-abrasion resistance under a controlled dry-sand/rubber-wheel condition. A rod mill adds a different combination of contact, impact, slurry and corrosion. The G65 result cannot be turned directly into a mill kg/t figure.

Use chemistry, hardness, microstructure and mechanical tests to describe and qualify the material being offered. Ask for the specimen location, actual readings and heat or treatment-lot link. Then use the mill trial to establish performance in service. The two records should complement each other.

Send the rod specification and the trial brief together. JOTAIN can review the proposed material, treatment and inspection requirements against the rod dimensions and service problem. Include the baseline consumption record and photographs of used rods so the discussion starts with the mill, not just the price per tonne of steel.

Item: Grinding rods for a controlled plant comparison at [site and mill]
Candidate rods: [supplier, grade/standard, diameter × length, heat-treatment condition, quantity]
Traceability: [heat, heat-treatment lot, bundle marks, certificate, delivery record]
Trial question: [what the plant is comparing and the result that will change the buying decision]
Mass balance: [additions, opening/closing charge, recovered pieces, bent rods, planned removals, unidentified media]
Throughput basis: [dry new feed, scale, moisture method, synchronized trial window]
Comparable duty: [ore/feed, size distribution, throughput, speed, power, product target, slurry/water, liner and charge state]
Sequence: [baseline blocks, transition, stabilization, candidate blocks]
Measurements: [scale checks, inventory method, raw-data contact, missing-data rule]
Event ledger: [breakage, bending, removals, handling, downtime, traceability]
Stop rules: [site safety, mill condition, breakage/tangling, product, power, throughput]
Cost basis: [delivered price, freight/duty boundary, scrap credit, recurring and one-time trial costs]
Decision: [block inclusion, uncertainty, commercial margin, proceed/repeat/stop authority]
Documents: [ISO 10474 type if ordered, actual results, manufacturing/inspection records, lot map, deviations]
The order and plant protocol define the comparison. Laboratory abrasion results remain screening evidence and are not converted into plant kg/t or savings.

Sources:[6][9]