Tag: purity

  • The 3.58% of the Shell That Caps Your Purity

    The 3.58% of the Shell That Caps Your Purity

    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.

  • 98.5% Hydroxyapatite. The Other 1.5% Came From the Shell.

    98.5% Hydroxyapatite. The Other 1.5% Came From the Shell.

    Indonesia produced 67,201,200 kg of eggshell waste as the world’s eighth-largest egg-producing nation in 2017. Most of it goes to landfill, where it decomposes into ammonia, hydrogen sulfide and amines, and attracts what that attracts. Converting that stream into a biomaterial is a genuinely good idea, and it is the idea this entire industry rests on.

    It also means the feedstock is an agricultural product, with an agricultural product’s variability. So the useful question is not whether eggshell makes good hydroxyapatite. It is what else comes along.

    What a shell is made of

    El Milla, Hartami, Priyanto and Adrian, working at Universitas Brawijaya in Malang, published exactly that measurement in the Malaysian Journal of Medicine and Health Sciences in April 2023.

    They start with the shell itself. Calcium carbonate makes up 94–97% of an eggshell’s chemical components, with 3–4.5% organic content. Alongside it sit small quantities of other things: MgO at 0.83%, SO₃ at 0.66%, SrO at 0.43%, plus P₂O₅, Al₂O₃, K₂O, SiO₂ and Cl₂O₃.

    Their material was made by the hydrothermal route from Golden Comet chicken eggshells collected in Serang, Banten — milled to 100 mesh, calcined at 1000 °C for five hours to convert the carbonate to calcium oxide, then reacted with diammonium hydrogen phosphate at a Ca/P mole ratio of 1.67 and held at 230 °C for 48 hours. Particle size analysis put the average grain size at 243.4 ± 5 nm.

    98.5, and the remainder

    The X-ray diffraction result is a good one. The synthesised material came out at 98.5% hydroxyapatite.

    The remaining 1.5% was silica — and the paper does not leave its origin ambiguous. The impurity, it says, “came from the egg shell which was used as the starting material for the synthesis.”

    That is the sentence worth keeping. The contaminant was not introduced by poor handling or a dirty vessel. It was in the feedstock, it survived 1000 °C and 48 hours of hydrothermal synthesis, and it arrived in the product. There were also peaks in the diffractogram the authors could not assign — “not compounds of the elements Ca, Si, O, H, and P” — which they marked and reported rather than quietly smoothing away.

    Their own conclusion notes the material is a good candidate “but other strategy needed to eliminate the impurities especially for tissue regeneration material.”

    Why the trace table exists

    Set this beside ISO 13779-1, which caps arsenic at 3 mg/kg, cadmium at 5, mercury at 5 and lead at 30, with a total ceiling of 50 mg/kg for metals with adverse biological reactions.

    Those limits can look like bureaucratic furniture until you have read a paper that traces a product impurity directly back to the raw material it was made from. A biogenic feedstock inherits its source’s composition — the beneficial trace elements everyone advertises, and the ones nobody puts on a label, by the same mechanism. Magnesium and strontium are sold as advantages. Silica arrived by exactly the same route and is sold as nothing at all.

    None of which argues against eggshell as a source. It argues against treating “naturally derived” as though it settled the purity question, when it is the reason the question has to be asked.

    Chickens are fed differently in different places, and their shells reflect it. Which makes the composition of the feedstock a supplier variable like any other — one that a serious producer measures per lot, and an unserious one describes in a marketing sentence.

    Sources

    El Milla L, Hartami E, Priyanto R, Adrian AGW. “Synthesis and Characterization of Eggshell Based Hydroxyapatite Nanoparticles: X-ray Diffraction and Spectroscopy Analysis.” Malaysian Journal of Medicine and Health Sciences, 19(SUPP5):15–18, April 2023 (open access) — medic.upm.edu.my. Trace-element limits from ISO 13779-1:2008(E), Table 1 — iso.org/standard/43826.html.