The lot fails on loss-on-ignition. Nothing in the calcination went wrong; nothing in the precipitation drifted. The problem walked in with the feedstock, glued to the inside of every shell fragment: the membrane nobody removed.
How much of an eggshell isn’t mineral?
A 2024 study from Hanoi University of Science and Technology put numbers on the question. In their material, “the composition of the eggshells was 94.63% inorganic salts and 3.58% eggshell membranes.” That 3.58% is protein — and any calcium product made from unseparated shell inherits it as an organic impurity, along with its discolouration on firing and its odour in storage. The scale of the feedstock makes the fraction consequential: from world egg production, the paper counts “9.531 million tones of eggshell by-products” produced yearly.
The study’s real contribution is measuring what separation costs and what it returns. Their best chemical route — sodium hydroxide at “3-3.5% concentration, a reaction time of 60 minutes at 70 °C” — stripped 90.63–92.26% of the membrane. A combined mechanical-plus-chemical method did better: “a good separation efficiency of 95.04% within 2.25 hours, and the recovered calcium purity reached 99.69%.”
What does 99.69% actually buy?
Read as a process specification rather than a lab result, the numbers say three things. First, purity above 99.5% from eggshell is achievable — but it is earned at the separation step, hours before any furnace is lit. Second, the step has a cost line: alkali, heat, time, and an effluent stream. Third, there is a trade the paper flags honestly — aggressive alkali treatment “could compromise the membrane’s bioactivity,” which matters to processors who sell the membrane as a second product rather than discarding it. Separation is where the shell’s two value streams part ways, and running it hard for the mineral can cheapen the protein.
The economics compound the chemistry. The recovered membrane itself sells — but the paper notes its constraint (“the maximum solubility of EMS is only 62%, limiting its exploitation and utilization”), and harsher alkali buys mineral purity at the price of membrane quality. A processor is therefore always choosing which of the shell’s two products to optimise. Buyers on either side of that choice deserve to know which one their supplier made, because the same plant, run two ways, ships two different powders under one name.
The question for your supplier
So the buyer’s question is specific: how is the membrane removed, at what measured efficiency, and where does the residual organic fraction show up on the certificate? A supplier who answers with a number — “95% separation, residual organics below X, verified by loss-on-ignition per lot” — has a process. A supplier who answers “the shells are washed” has a hope. The 3.58% does not negotiate: whatever fraction of it survives preparation is in the drum, lot after lot, setting a ceiling that no amount of downstream skill can raise.
Purity claims are made at the end of a process. This one is decided at the beginning.
Sources
Phan Thanh Tam, Nguyen Thi Thuy, Nguyen Hai Van. “Research on Eggshell Membrane Separation for High-Purity Calcium Carbonate (CaCO₃) Recovery.” JST: Engineering and Technology for Sustainable Development (Hanoi University of Science and Technology), 2024;34(4):001–008 (open access) — jst.vn/index.php/etsd/article/view/382/344.