Natural Calcium Carbonate

d50 is the size below which half the mass lies, and it is the first number a catalogue uses to define a grade. Yet most field problems come not from the mean but from the top end of the distribution. In a 30 µm filled film a single 15 µm particle starts a puncture; the same particle in paint lowers gloss and makes brush marks visible. Asking for d97 (top cut) alongside d50 is therefore not a technical nicety but part of troubleshooting.

Distribution width is the second factor. Of two grades with the same d50, the narrow one gives lower packing density, higher viscosity and a smoother surface; the broad one fills voids with fine particles and allows a higher loading, but raises surface roughness. Construction chemicals usually want a broad, controlled distribution in the 5–40 µm band, while thin film and masterbatch want a narrow distribution with a low top cut.

The measurement method also changes what the number means. A d50 measured by laser diffraction (ISO 13320) can differ markedly from one measured by sedimentation on the same sample; dispersion time, ultrasound and the refractive index setting shift the result too. When comparing suppliers, use the same method and the same dispersion protocol — otherwise the two reports cannot be read against each other.

In practice whiteness is discussed through three separate quantities: Ry, measured as blue-light reflectance to ISO 2469; L* (lightness) in the CIE Lab system; and b* (yellowness). A grade may show Ry 95 and still bring a visible yellow cast into a white masterbatch if its b* is 2.5. TiO₂ does not correct that cast, it only covers it — masking bought at pigment prices.

Yellowness comes from iron and manganese oxides at ppm level. Holding Fe₂O₃ below 0.05% depends on selective extraction of the ore and on preventing metal pick-up in the grinding circuit; iron shed by wear parts looks like a small number in chemical analysis but reads directly in colour.

Measurement conditions must be fixed. The pressure applied to the pressed powder tablet, the moisture of the sample and the instrument's calibration standard can shift the result by several points. Writing whiteness ≥ 95 into a specification is not enough; the standard and the sample preparation belong in the specification too.

Oil absorption is the amount of liquid needed to saturate 100 g of filler under ISO 787-5, expressed in g/100 g. It matters far more than it looks: every gram the filler absorbs is spent from the most expensive item in the recipe — binder, plasticiser or resin. Choosing a calcite at 16 g/100 g instead of 22 g/100 g lowers total cost per tonne even when the filler price per kilogram is identical.

The value depends mainly on specific surface area and particle shape. As grinding gets finer the BET surface grows and oil absorption rises; angular, platy particles demand more binder than near-spherical ones. In paint this shifts the critical pigment volume concentration (CPVC) directly — running a high oil-absorption filler at the same PVC increases film porosity, staining and scrub loss.

Stearate coating makes the surface hydrophobic and typically lowers oil absorption by a few units, cutting plasticiser demand visibly in PVC compounds. Coated grades naturally cost more per kilogram; the decision is made by checking whether the coating premium is smaller than the plasticiser saving. A coating decision taken without that calculation is usually just extra cost.

Surface coating means covering the polar surface of the calcite particle with a monolayer of stearic acid or calcium stearate. The amount required is not a fixed percentage but a function of surface area: one gram of stearic acid covers roughly 400–450 m² as a monolayer. On a grade with 4 m²/g BET that is about 0.9%; on an ultrafine grade at 9 m²/g it is closer to 2%. Applying 1% to every fineness means under-coating the fine grade and over-coating the coarse one.

The benefit in polymer systems is clear: agglomerates break down, extruder torque and energy consumption fall, higher loadings become possible at the same mechanical values, and moisture pick-up — hence bubble risk in extrusion — drops. In polyolefin compounding, running uncoated calcite usually comes back as a dispersion defect, gels and surface faults.

The harm shows up on the side that gets ignored. In waterborne paint a hydrophobic surface resists wetting and lengthens dispersion time; in reactive systems such as polyurethane and unsaturated polyester, free stearic acid can retard cure. With over-coating, unbound stearate migrates to the mould surface (plate-out) and raises cleaning frequency. So the question is not only the coating level but the coating efficiency and the free stearate content.

For a compounding line on a twin-screw extruder, coated grades at d50 1.5–3 µm and d97 8–12 µm are typical. Finer calcite needs higher shear and longer residence time to disperse, which costs line output. In a filler masterbatch at 70–80% loading, uncoated material tends to bridge in the feed zone and make feeding unstable.

On the coatings side a high-speed disperser opens the 5–15 µm band without trouble; grades below 2 µm realistically need a bead or ball mill. In primers, fillers and mastics a d50 of 20–40 µm is both sufficient and economical — finer material there buys binder consumption, not performance.

Asphalt filler is selected on a completely different logic: the criterion is sieving, not microns, and what counts is the fraction passing 0.063 mm together with the void content of the filler. The coarse fraction from the same quarry is used as aggregate without micronising; both products come from one ore, but their specifications do not overlap at all.

Natural ground calcium carbonate comes straight from the ore. Precipitated calcium carbonate (PCC) is made by carbonating, with carbon dioxide, the milk of lime obtained from slaking quicklime. The real difference is not purity but morphology control: in PCC the crystal habit — scalenohedral, prismatic or aragonite needle — and the particle size can be set within the process.

In practice PCC brings higher specific surface, higher opacity and lower bulk density; the price is higher oil absorption, higher binder and plasticiser demand, and a markedly higher cost per tonne. GCC offers low oil absorption, high loading and a cost advantage, and is the economical choice wherever tight morphology is not required.

The right question is not which is better but where the bottleneck sits in the recipe. If the bottleneck is opacity, volume or surface area, PCC wins; if it is cost, loading level or binder demand, GCC does. In many paper and paint recipes the two are used together, in different roles.

In paper, calcite is both filler and coating pigment; it sets ash content, opacity and the printing surface. The critical constraint is chemical: carbonate dissolves and releases CO₂ in acid stock, so it can only be used in neutral or alkaline processes. On the coating side, gloss is governed less by d50 than by the steepness of the distribution and particle shape.

In paint, calcite usually acts as a TiO₂ extender: by spacing pigment particles apart it delivers the same hiding power with less TiO₂. The gain does not come from volume alone but from choosing the right particle size; an over-fine filler that lands in the same size range as TiO₂ can cause flocculation instead of spacing.

In glass and ceramics calcite is not a filler but a raw material, serving as the CaO source in the melt. Here fineness is not decisive — chemical purity is. Because iron carries into glass colour even at ppm level, the specification states an Fe₂O₃ limit rather than a whiteness figure. Likewise in agriculture and livestock use the discussion is about neutralising value and sieve analysis, not microns.

Calcite is not hygroscopic, but fine powder holds physical moisture on its surface. Shipping moisture is kept below 0.2%; exceeding that shows up as bubbles and streaks on extruded film, and as viscosity drift in paint. On coated grades the hydrophobic surface slows moisture pick-up noticeably and tolerates opened packaging better.

Silo and feeding behaviour follow fineness directly. Grades below 2 µm flow poorly; bridging and ratholing risk rises. Cone angle, aeration layout and feeder type therefore belong in the same conversation as grade selection — forcing a fine grade into an existing silo creates a production loss that comes from the plant, not from the product.

In long-haul sea shipments the real risk is not rain but condensation inside the container. In a box that warms by day and cools at night, moisture condenses on the roof and drips onto the big bags, causing local caking. That is why a desiccant and a container liner are standard practice on our container shipments.