Most industrial salt contracts ask for between 98.5% and 99.5% sodium chloride on a dry basis. The right figure depends on your process, and the limits on calcium, magnesium, sulphate and insoluble matter often matter more than the headline number.
So do not buy on purity alone. Two cargoes at the same sodium chloride content can behave very differently in your plant.
Why the impurities matter
Every impurity costs you something in treatment, waste or downtime. These are the ones buyers usually limit:
- Calcium and magnesium must be removed before brine enters a membrane cell, so high levels raise your chemical use and sludge.
- Sulphate builds up in recycled brine and has to be purged or treated.
- Insoluble matter, mostly clay and sand, settles as sludge and can block equipment.
- Moisture adds weight you pay freight on and can cause caking in storage.
- Iron, heavy metals and other trace elements matter for some downstream uses.
Dry basis or as received
Purity can be stated on a dry basis or as received. Dry basis means the moisture has been removed before calculation, so the figure looks higher. As received includes the water. A cargo with 99% sodium chloride dry basis and 3% moisture contains less salt per tonne than the headline suggests.
Your contract should state the basis for every figure, the moisture limit, and how a claim is calculated if moisture is over the limit. Without that, a dispute is hard to settle.
Matching purity to use
Membrane cell chlor-alkali plants usually need the cleanest salt, because their membranes are sensitive to hardness. Soda ash plants and water softening can often accept somewhat lower grades, as long as insolubles and moisture are controlled. Washed solar salt from Gujarat can meet many industrial specifications, but not every producer washes and drains it to the same standard.
Ask your process engineers for the limits they really need. Over-specifying costs you money, while under-specifying moves the cost into your plant. If you are unsure, compare the analysis of salt you already use with the offer. Grain size can matter too. Very fine salt dissolves fast but packs tightly and cakes, while coarse salt drains better in storage. State the size range you need, so a producer cannot meet the purity and miss the handling.
How to check it
Ask for a full analysis, not just sodium chloride. A useful certificate shows the test method for each figure and the lot it covers. Then have samples drawn at loading and tested by an independent laboratory. Our article on what tests belong on a salt COA lists them, and reading a lab certificate explains how to judge one.
Next step
We check samples and documents, and have lots tested before they sail. To set a purity specification that fits your plant, book a free sourcing review.