A certificate of analysis, commonly called a COA, is one of the most important documents in a silicon metal purchase. For grades such as 1101, 2202, 3303 and 441 it records the chemical values that define the grade, and it is the first document a buyer checks before shipment and again before acceptance at the works. A COA should not, however, be treated as the only inspection basis. It states reported results for a specific lot, and it does not by itself confirm that the material in the container matches that lot, that the size range is correct, or that the material has survived transport without moisture pick-up or contamination.
Check the Lot Identity First
Start with traceability. The COA must carry the lot or heat number, the product grade, the production date and the quantity it covers, and those details must match the packing list, the container loading plan and the marks on the bags. A certificate without a lot number cannot be tied to the material, and a certificate whose quantity is larger than the actual lot dilutes the value of the analysis. Where a shipment contains several lots, each should have its own certificate rather than one averaged document, because a single blended figure hides variation between lots.
Read the Chemical Values Correctly
The grade digits encode the maximum iron, aluminium and calcium content in tenths of a per cent. Grade 553 allows Fe 0.5%, Al 0.5% and Ca 0.3%, while grade 3303 allows Fe 0.3%, Al 0.3% and Ca 0.03%. Read the reported values as measured numbers, not as the limits. A lot of grade 441 reporting Fe 0.38%, Al 0.36% and Ca 0.08% is inside the specification but leaves very little margin, and that matters for a process that is sensitive to aluminium or calcium.
| COA item | What to verify | Why it matters |
|---|---|---|
| Lot or heat number | Matches packing list, bag marks and container plan | Traceability and claim validity |
| Silicon content | Consistent with the grade and with the balance of impurities | Yield per tonne of charge |
| Fe, Al, Ca | Measured values against the grade limits, with margin noted | Product quality and catalyst performance |
| Sampling method | Stated procedure, sample size and whether the sample is representative | Reliability of the reported values |
| Size range and fines | Screen analysis or at least the declared range and fines limit | Feeding behaviour and oxidation loss |
| Analysis method | Instrument or wet chemistry method identified | Comparability of results between laboratories |
Confirm Sampling and Analysis Method
Values are only as good as the sample behind them. The COA should state how the sample was taken, from what point in the process, and how many increments were combined. Sampling from the top of a container is not the same as sampling across a conveyor or across the crushed stock, and coarse silicon metal segregates by size, so a sample drawn from one fraction cannot represent the whole lot. The analysis method matters as well, because optical emission or X-ray fluorescence results are not automatically identical to wet chemical values at the same concentration. Where a formal inspection scheme applies, ISO 2859-1 provides sampling plans indexed by acceptance quality limit that can be written into the contract.
Inspection at Arrival
Arrival inspection is a short, repeatable routine rather than a repeat of the full analysis. Check container seals and the condition of the bags, look for wet or torn bags and for water staining inside the container, weigh a sample of bags against the declared net weight, take a screen sample to confirm the size distribution and fines level, and draw a composite sample for Fe, Al and Ca analysis. Record the container number, the lot numbers present, the sample identity and the results, and keep the records with the shipping documents. Where a result is outside the specification, the composite sample should be retained until the claim is settled.
Common COA Problems and How to Handle Them
The most frequent issues are a missing lot number, values quoted as ranges or as the grade limits rather than as measurements, a certificate covering a different quantity from the shipment, and a certificate issued long before shipment with no statement that it still applies. Each of these can be handled in the contract rather than after arrival: require the lot number and measured values, require a certificate per lot, require the analysis to be issued within an agreed period before loading, and agree in advance which laboratory result prevails in the event of a difference, together with the retention period for the sample.
Frequently Asked Questions
Q: What should a silicon metal COA contain?
A: The lot or heat number, the grade, the quantity covered, measured values for silicon, iron, aluminium and calcium, the sampling method, the analysis method, the size range and the date of issue, all matching the shipment documents.
Q: Is a COA enough to accept a silicon metal shipment?
A: No. It confirms reported chemistry for a lot but not the identity, size range, packing condition or moisture state of the delivered material. Arrival checks on weight, size and a composite sample are still required.
Q: What does grade 3303 mean on a COA?
A: It means iron not more than 0.3%, aluminium not more than 0.3% and calcium not more than 0.03%. Values reported well below those limits indicate a tighter product than a lot sitting at the limits.
Q: Which sampling standard can be written into the contract?
A: ISO 2859-1 provides sampling procedures for inspection by attributes with plans indexed by acceptance quality limit, and it can be referenced alongside the product standard for silicon metal, GB/T 2881.
Q: Why do two laboratories report different values for the same lot?
A: Sampling, sample preparation and the analytical technique all contribute. Coarse material segregates by size, and different instruments respond differently at low concentrations, so the contract should name the method and the laboratory that prevails.
Q: How long should arrival samples be kept?
A: Until the lot has been fully consumed or until the claim period in the contract expires, whichever is later. Samples should be stored dry, labelled with the lot number and sealed against contamination.
