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.<\/p>\n
A 2024 study from Hanoi University of Science and Technology put numbers on the question. In their material, \u201cthe composition of the eggshells was 94.63% inorganic salts and 3.58% eggshell membranes.\u201d That 3.58% is protein \u2014 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 \u201c9.531 million tones of eggshell by-products\u201d produced yearly.<\/p>\n
The study\u2019s real contribution is measuring what separation costs and what it returns. Their best chemical route \u2014 sodium hydroxide at \u201c3-3.5% concentration, a reaction time of 60 minutes at 70 \u00b0C\u201d \u2014 stripped 90.63\u201392.26% of the membrane. A combined mechanical-plus-chemical method did better: \u201ca good separation efficiency of 95.04% within 2.25 hours, and the recovered calcium purity reached 99.69%.\u201d<\/p>\n
Read as a process specification rather than a lab result, the numbers say three things. First, purity above 99.5% from eggshell is achievable \u2014 but it is earned at the separation step<\/em>, 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 \u2014 aggressive alkali treatment \u201ccould compromise the membrane\u2019s bioactivity,\u201d which matters to processors who sell the membrane as a second product rather than discarding it. Separation is where the shell\u2019s two value streams part ways, and running it hard for the mineral can cheapen the protein.<\/p>\n The economics compound the chemistry. The recovered membrane itself sells \u2014 but the paper notes its constraint (\u201cthe maximum solubility of EMS is only 62%, limiting its exploitation and utilization\u201d), and harsher alkali buys mineral purity at the price of membrane quality. A processor is therefore always choosing which of the shell\u2019s 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.<\/p>\n So the buyer\u2019s 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 \u2014 \u201c95% separation, residual organics below X, verified by loss-on-ignition per lot\u201d \u2014 has a process. A supplier who answers \u201cthe shells are washed\u201d 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.<\/p>\n Purity claims are made at the end of a process. This one is decided at the beginning.<\/p>\nThe question for your supplier<\/h2>\n
Sources<\/h2>\n