Chemical & Pharmaceutical Industry

Before a material is selected, what the material legally is has to be defined. The same calcium carbonate may be a plain reagent on one line, a processing aid on the second, a food additive on the third, a pharmaceutical excipient on the fourth and an active substance on the fifth. Each role answers to a different text: a technical specification, food-additive purity criteria, a Ph. Eur. or USP monograph, excipient GMP guidance, or ICH Q7 on the active side.

This is an expensive decision to revisit. Changing the excipient class or the supplier in an authorised product triggers change control, comparative dissolution work and, more often than not, fresh stability data. The class therefore has to be fixed in the first week of formulation design, not at pilot scale; deciding later to move from technical to pharmaceutical grade usually means a new development cycle.

The same step produces the document list to be requested from the supplier: monograph compliance statement, certificate of suitability or master file reference where applicable, TSE/BSE statement, allergen and GMO declarations, elemental impurity data, microbiological monitoring results and the excipient GMP audit record. That list belongs in the contract annex — it is not something you start assembling when the first delivery arrives.

Natural carbonate is a mined product; its impurity profile drifts with the quarry face, the seam and the season. An ICP-MS scan of one batch is a photograph, whereas what you need is the film. A dataset of at least 10–20 batches gives a mean and a standard deviation for each element; the specification is then written against the upper tail of the distribution rather than the mean, because the batch that gets rejected is never the average batch.

Which elements to track depends on the finished product. Barium and strontium travel with calcium in carbonate deposits and are not easily separated from it. Iron and manganese impart colour and behave catalytically next to oxidation-sensitive actives, shortening shelf life. Magnesium shifts both the assay value and the rate of dissolution in acid. Lead and cadmium are geological in origin; grinding, washing or calcination do not reduce them meaningfully, so the control point is the choice of quarry.

The calculation starts from the acid load. A stream of 500 kg/h of 30% sulfuric acid is 150 kg of pure H₂SO₄; the stoichiometric requirement is 114 kg Ca(OH)₂, or 125 kg with a 10% excess. As a 15% milk of lime that is roughly 830 kg/h of slurry, and storage, agitator, pump and pipe diameter are sized against that flow. Doing the same duty with CaO means 95 kg/h of solids feed, and the slaker is selected accordingly.

Heat management is the second leg of the design. Strong acid–strong base neutralisation is exothermic, and where CaO is used the heat of slaking is added on top. Above 70 °C the crystal habit of gypsum changes, shifting towards fine acicular growth, and filterability drops noticeably. If the jacket or external exchanger is sized on average rather than peak load, temperature excursions at shift start-up are inevitable.

Sludge character is largely set by dosing rate. Fast dosing creates high supersaturation, and the result is a fine, gelatinous crystal population that blinds the filter cloth. Slow dosing combined with partial recycle of seed sludge to the reactor gives coarse, well-draining crystals. Two plants with identical chemistry end up with two different filter cycle times and two different cake moistures on this difference alone.

In organic calcium salt synthesis the reaction typically runs at 60–90 °C with a pH end point of 6.5–8.0. The specification is bounded from both sides: residual free acid spoils hygroscopicity and odour, residual free alkali spoils taste and solution clarity. The end point is therefore not a single pH value but a narrow window in which both the free-acid and the free-alkali limits are met.

Fineness and reactivity of the calcium source act directly on that window. Finely ground carbonate or hydroxide shortens reaction time but raises filter load and foaming tendency. Coarse grains that never dissolve give a gritty mouthfeel in the finished product and show up in the acid-insoluble matter test. In practice reaction time and particle fineness are optimised together; improving one alone degrades the other.

At the crystallisation step, cooling rate and nucleation control determine particle size distribution. Drying temperature then determines the water of hydration; drying a pentahydrate salt too hot causes partial dehydration and pushes the assay above specification. Dryer outlet temperature here is not an energy parameter but a quality parameter.

The monograph asks for assay and impurities; the tablet press asks for flow and compressibility. That second group appears nowhere in the monograph: D10/D50/D90, bulk and tapped density, Carr index, Hausner ratio, specific surface area. A powder that meets the monograph but will not flow passes QC and then stalls production; in projects that fail to separate the two, the problem is always found late.

Direct-compression lines call for granulated carbonate, with D50 typically held in the 100–250 µm band and a Carr index kept below 18. On a wet granulation line finely ground carbonate is acceptable, because the granulation step solves the flow problem. The wrong choice comes back as tablet weight variation, capping and hardness scatter.

With derivative salts, fineness turns one compound into two different products for two different industries. In calcium stearate, particle fineness governs both tablet lubricant performance and the acid-scavenging and lubricating behaviour on a plastics recycling line; chemically the same substance is sold at different fineness and different bulk density for the two applications.

The ICH Q3D calculation is done component by component, not raw material by raw material. The elemental contribution of each component at the daily dose is computed in micrograms per day, all components — and where relevant contributions from equipment and packaging — are summed, and the result is compared with the PDE. The ppm value of a single raw material is only one term in that sum.

For high-dose products the picture changes: in a 1250 mg calcium carbonate tablet the carbonate fills almost the entire elemental impurity budget on its own. There the raw material specification is written far tighter than the monograph limit and that limit is written into the supply contract. In the opposite case, a product using 20–50 mg of filler per day is comfortably covered by the monograph limit, and full screening of every batch is unnecessary cost.

The control strategy follows from the same arithmetic. Where the risk assessment shows a wide safety margin, periodic verification (say every tenth batch) plus supplier data is enough; where the margin is narrow, batch-by-batch measurement becomes mandatory. That decision is documented, because what an auditor asks about is not the result itself but why the result is trusted.

Calcium hydroxide slowly carbonates with atmospheric CO₂, and calcium oxide takes up both moisture and CO₂. Storage practice is therefore directly a question of protecting the assay value: a big bag left open or a silo whose lid does not seal produces a measurable loss of active content within weeks. In pharmaceutical grade that loss is not merely a yield issue, it is a non-conformity.

For desiccant duty the calculation rests on four variables: the volume to be protected, the target relative humidity, the water vapour transmission rate of the packaging and the exposure time. The theoretical 32% capacity of CaO is used at around 20–25% in practice; the remainder becomes inaccessible as the reaction product seals the grain surface. The safety margin is set against the uncertainty of transit time, not against that gap.

In the pharmaceutical and food chain, the validity of a batch rests not on the analytical result but on the traceability of that result. The certificate of analysis must link through the batch number to the production record, the production record to the raw material lot, and the raw material lot to the quarry and shipment records. Retention samples and batch records are kept for a period that outlasts the product's shelf life.

Change control is the link most often overlooked. A change of quarry face, grinding line, screen configuration or packaging material may be invisible on the certificate of analysis and highly visible in the finished product. The obligation to notify such changes in advance belongs in the contract. Specification alignment work, sampling and batch-level data sharing can be run as a project through the contact page.