Understanding a Lime Certificate of Analysis

Two documents travel with industrial lime, and buyers often treat them as one. The datasheet (or specification sheet) describes the product as the manufacturer offers it: limits such as "CaO 85% minimum" or "95% passing 200 mesh". The certificate of analysis describes one lot: the figures a laboratory actually recorded on the material that was loaded. The datasheet is a promise; the certificate is the evidence for a particular consignment.

This guide walks through a typical lime certificate, what each parameter tells you about the material, and how to compare it with the specification in your purchase order. It describes test parameters and methods in general terms; the methods used for a given order are those stated on the certificate itself.

Identification first

Before any numbers, a certificate should identify what was tested:

• Product and grade — quick lime lumps, quick lime powder or hydrated lime, and the size fraction or grade ordered.
• Lot or batch number — the production lot the sample represents. One certificate covers one lot; a consignment drawn from two lots should carry two.
• Consignment reference — the invoice, dispatch or vehicle number, so the paper can be tied back to the truck.
• Quantity and date — how much was dispatched and when the sample was taken.
• Specification column — the limits the lot was tested against. Without this column a result is just a number; with it, a result is a pass or a fail.

If any of these are missing, ask for them before the material is used. A certificate that cannot be matched to a lot cannot be relied on later.

Chemical parameters

Calcium oxide (CaO)

The headline figure for quick lime. It reports total calcium expressed as oxide, and for a high-calcium lime it is what most of the material consists of. The specification sets a minimum; the certificate reports what the lot achieved. A lot comfortably above the minimum is the normal case. A lot just at the minimum deserves a look at the other parameters — a high loss on ignition, for instance, usually explains a lower CaO.

Available lime

Total CaO counts every calcium atom, including those still locked in carbonate or already combined with silica. Available lime counts only the calcium oxide (or hydroxide, for hydrated lime) that will actually react — which is what a dosing plant, a melt shop or a soil-stabilisation crew is paying for. It is determined by titration methods described in standards such as IS 1514 and ASTM C25. For hydrated lime the Indian standard IS 1540 (Part 2) is commonly referenced. When a specification quotes available lime rather than total CaO, compare like with like.

Magnesium oxide (MgO)

Limestone from a high-calcium belt carries a little magnesium; a dolomitic stone carries a lot. For most duties a low MgO is what a high-calcium specification is asking for, and the certificate reports it against a maximum. Where the process wants magnesium — some refractory and agricultural uses — a dolomitic lime is specified instead, and the certificate will show MgO as a major constituent rather than an impurity.

Silica (SiO₂) and R₂O₃

Silica and the combined iron and aluminium oxides (reported together as R₂O₃) are the mineral impurities carried over from the stone. They do not react usefully, and in a melt shop silica consumes lime to form slag. Both are reported against a maximum.

Loss on ignition (LOI)

A sample of quick lime is heated to a high temperature and weighed again. The weight lost is mostly carbon dioxide from carbonate that was not fully burnt, plus water from any hydration since the kiln. A low LOI indicates well-burnt, well-protected lime. A rising LOI across a sequence of certificates is the earliest sign of material picking up moisture in storage.

Physical parameters

Reactivity

Quick lime reacts with water and releases heat. Reactivity tests measure how quickly: the temperature rise over time when a set mass of lime is slaked in a set volume of water (the approach used in EN 459-2 and ASTM C110), or the volume of acid consumed in a fixed time (used by some Indian steel plants). The certificate states the method and the result; the two are not interchangeable. A specification such as "350 mL minimum" only makes sense alongside the method it refers to, which is why the method should be agreed at order and printed on the certificate.

Moisture

For quick lime, moisture is both an impurity and a warning: the material has begun to hydrate. For hydrated lime a small free moisture figure is normal. Both are reported against a maximum.

Size fraction and sieve analysis

Lump lime is reported as the fraction ordered — for example 10–50 mm — usually with the percentage outside that band. Powders and hydrated lime are reported as the percentage passing a stated sieve, such as 200 mesh (75 µm). Size governs how the material behaves in a feed system and how fast it reacts, so a certificate for a ground product without a sieve figure is incomplete.

Appearance

A one-line description — colour, lump or powder, any visible contamination. It is the only parameter the receiving yard can check without a laboratory, so it is worth reading.

How to check a certificate

1. Match the identification to the delivery note and the vehicle.
2. Compare each result with the specification column. "Min." means the result must be at or above the figure; "max." means at or below.
3. Check the methods named against the ones in your purchase order, particularly for reactivity and available lime.
4. Look at the pattern, not only the pass. A lot that passes everything but shows LOI and moisture both higher than the last three certificates is telling you something about storage or transit.
5. Keep the certificate with the lot, so a question raised when the material is used weeks later can be answered.
6. Keep the retained sample. A sealed portion of the loading sample, held by each side, is what a retest is run on if the certificate is questioned. A scoop from the stockpile a week later, after the material has taken up moisture, is not evidence of what was dispatched.

Who issues it, and what it is not

A certificate of analysis is issued by the manufacturer's laboratory, or by an independent laboratory the parties agreed on, for a lot the manufacturer tested. It is not a certification of the firm, a quality-system certificate or a guarantee for future lots; it is a record of one lot's results against the methods it names. Where a buyer wants independent confirmation, the purchase order can require third-party sampling and testing of the consignment, at a laboratory and against methods both sides accept.

When a figure is outside the band

A certificate should be issued before loading, so an out-of-specification lot is held at the plant, not argued about at the destination. If a result on a certificate is outside the agreed band, the normal course is to raise it with the supplier before unloading, referencing the lot number, and to agree a retest from a retained sample or a replacement. What was agreed about retesting and rejection belongs in the purchase terms, not in a phone call after the truck has arrived.

Where Deshwali Minerals fits

Every consignment from the Borunda plant is drawn from a tested lot and goes out with its batch certificate, and the parameters on it are the ones on the product specification. The quality page describes how testing is organised, and the resources page lists what a certificate from the plant contains. For a specification for your duty, request a callback or request a sample and test the material in your own laboratory first.

Sources

• Bureau of Indian Standards, IS 1514:1990 — Methods of sampling and test for quicklime and hydrated lime: BIS standards portal
• Bureau of Indian Standards, IS 1540 (Part 1) quick lime and (Part 2) hydrated lime for chemical industries: BIS standards portal
• ASTM C25 — chemical analysis of limestone, quicklime and hydrated lime; ASTM C110 — physical testing of quicklime, hydrated lime and limestone: ASTM International