Lead and Zinc Ingot Purity Grades, and What the Extra Decimals Change

Why lead grades cluster just below 100%

Refined lead is sold by purity within a very narrow band — commonly 99.97%, 99.985%, 99.99% and 99.994%. What remains is bismuth, copper, antimony, arsenic, tin and silver in small quantities, and each of those has an effect that varies with the end use.

Bismuth is the impurity most often watched, because it is difficult to remove and it degrades the behaviour of lead in several applications. A grade described as 99.994% is not three thousandths better than 99.97% in some general sense; it is a metal with a materially smaller bismuth allowance.

That is why lead is not bought on the headline percentage alone. Two lots both described as 99.99% can differ in their individual impurity levels, and where the metal is going into battery manufacture, radiation shielding or a chemical process, it is the individual limits that decide acceptance.

Zinc SHG and the meaning of 99.995%

Zinc of 99.995% purity is conventionally called SHG — Special High Grade. It is the standard grade for galvanising, for die casting alloys and for the production of zinc oxide and other zinc chemicals.

As with lead, the useful information lies in what the remaining 0.005% consists of, chiefly lead, cadmium, iron and copper. Cadmium is regulated in many markets, both in the metal and in products made from it, so a buyer purchasing zinc for a regulated application will be checking cadmium specifically rather than relying on the overall figure.

Zinc is also supplied below SHG, commonly around 99.95% and 99.5%, for uses where the tighter limits are not required. The price difference between SHG and a lower grade is usually larger, in proportion, than the difference in analysis, because galvanising quality is sensitive to exactly those impurities.

What the extra purity costs, and what it buys

The premium for high-purity lead and for SHG zinc is real, and it is easiest to justify where the impurity does measurable harm: reduced corrosion resistance, poor coating adhesion in galvanising, contamination of an alloy, or failure of a downstream product test.

Where it usually does not pay is in applications that are themselves re-melted into a tolerant alloy. There, a lower grade with a documented analysis will perform identically, and paying for the last decimals buys nothing measurable.

The judgement is therefore not which grade is better, but which impurity, at which limit, matters in this process. That question is answerable from a specification — and it is worth answering before the quotation is requested rather than after the lot has landed.

Specify by analysis, not by grade name

Grade names such as SHG are convenient shorthand, but they are not definitions of impurity limits. A purchase specification that states the grade and stops there leaves the impurities to the supplier's discretion.

A better specification sets the minimum purity and then the maximum limits for the elements the process is sensitive to — bismuth for lead, cadmium and iron for zinc — and requires those figures to be reported on the certificate of analysis for each lot.

This costs nothing to write and removes nearly all of the argument at the receiving end, because it turns a discussion about names into a comparison of numbers.

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