Paper & Pulp Industry

Smelt from the recovery boiler is dissolved in weak wash to make green liquor. Three quantities measured here set the tone for the whole line: total titratable alkali (TTA, 110–130 g/L as Na₂O), sulphidity (25–35%) and density. If TTA is too high the causticizer is overloaded and efficiency drops; if it is too low the white liquor going to the digester is weak and the target kappa is missed.

Dregs removal is the real job of this step. At the outlet of the clarifier or green liquor filter, suspended solids should be brought to 20–50 mg/L. Every day run above that limit means permanently loading inerts into the loop. The separated dregs are washed counter-currently and taken out of the cycle; the wash water returns to the weak liquor line so alkali loss stays bounded.

A common field mistake is to watch the dregs filter only for whether it “looks clear”. Green liquor is dark to begin with; the eye cannot tell 30 mg/L from 300 mg/L. Clarity has to be followed by regular suspended solids measurement, and the precoat regime and wash frequency set against that number.

In the slaker CaO reacts with water and the heat of reaction carries the temperature into the 95–103 °C band. That band is not arbitrary: below it slaking slows and stays incomplete, while too close to boiling brings steam blowback and overflow risk. The temperature rise curve is the most practical indicator of the reactivity of the delivered batch — it tells you how the lot will behave without waiting for a laboratory t60.

A retention of 20–30 minutes in the slaker is typical. Within that time, coarse unslaked grains, sand and inert fragments are rejected by the grit classifier. Grit rate is one of the health indicators of the plant: on a normal line it runs at 1–3% of the lime charge. Above 5% either the delivered lime is hard-burnt or the top size is too coarse for the slaker retention; the third possibility is slaking water that is too cold.

Grit is not only a loss item. The free CaO it carries reacts with water wherever it is dumped, heats up and raises dust. The design of the grit line is therefore a mass balance question and a safety question at once; enclosed conveying, wet quenching and controlled disposal are standard.

Causticizing usually runs in 3–4 reactors in series with a total retention of 90–150 minutes. As the reaction approaches equilibrium its rate falls; the efficiency gained in the last stages is small but matters for crystal maturation of the lime mud. Cutting that time costs only a point or two of causticizing efficiency, while filterability degrades visibly — the real price is paid in cake dryness, not in efficiency.

The lime charge is set at 1.05–1.15 mol/mol against Na₂CO₃, calculated on available CaO. The only reliable way to find the right point is to step the charge at constant green liquor composition and measure efficiency together with mud settling rate and filter cake dryness. That maps the peak of the curve and shows which side of it you are working on; a single efficiency number never carries that information.

Causticizing efficiency depends not only on charge but also on green liquor temperature, TTA and sulphidity. High TTA gives lower efficiency at the same charge — which does not mean the lime is bad. Efficiency should therefore always be reported alongside TTA, temperature and sulphidity, and shift-to-shift comparisons made with those three held steady.

The slurry leaving the causticizer is split into white liquor and lime mud. Suspended solids in the white liquor directly affect the quality of the chemistry going to the digester; modern lines with pressurised disc filters reach below 10 mg/L, while classic clarifiers sit several times higher.

Lime mud is washed counter-currently to recover the sodium compounds it holds. Insufficient washing is paid for twice: as alkali loss, and as the ringing and balling that sodium triggers in the kiln. Wash water returns to the loop as weak liquor; the mill's overall water balance, condensates and surplus streams are built on the same logic used in water and wastewater treatment.

Filter cake dryness is the output indicator of this step and should be held at 70–80%. A fall in dryness is almost always about the mud itself: overliming, a rising phosphorus load or shortened causticizing time. Changing the filter cloth corrects none of the three; it only postpones the symptom by a few weeks.

In the rotary lime kiln the mud is dried, heated and recalcined in a 1150–1250 °C burning zone. The aim is to bring residual CO₂ below 1.5% while preserving the pore structure of the lime. Overburnt lime densifies, loses specific surface and behaves sluggishly in the slaker; underburnt lime never takes part in causticizing and returns to the mud, circulating as dead load.

Ring and ball formation is the most expensive kiln failure. Its triggers are well known: sodium in poorly washed mud, free lime left in the mud, high SiO₂ and Al₂O₃ load, and fluctuating feed moisture. None of the four is decided in the kiln itself — they are all set upstream. When chasing a ring problem, read wash efficiency and lime charge data first and go to the burner last.

Kiln flue gas is not only a waste stream but a resource: the CO₂ it carries feeds the carbonation reactor of a satellite PCC plant and becomes the raw material of the filler that ends up in the sheet. The dust and reduced sulphur side of the same stream is a classic gas cleaning problem, solved with an electrostatic precipitator and scrubber combination.

Grit, dregs, mud purge and unavoidable losses mean make-up lime enters continuously, typically 5–15% of the circulating lime. That stream is also the only real means of diluting inerts: the cleaner the make-up, the lower the equilibrium impurity level in the loop. Make-up lime is therefore not a “topping up” line item but a lever that tunes the chemistry of the whole cycle.

Make-up lime is not chosen on price alone. Available CaO ≥ 92%, P₂O₅ ≤ 0.05%, MgO ≤ 1.5%, SiO₂ + Al₂O₃ ≤ 1.5%, loss on ignition ≤ 3% and fast reactivity (t60 of 1–4 min) is the profile a typical kraft line looks for. Asking for a certificate per batch is not enough; verifying in your own laboratory for the first months is the only way to see a supplier's real distribution and batch-to-batch variability.

Two points decide storage: CaO picks up both moisture and CO₂ from the air. Partly hydrated or recarbonated lime may still flow out of the silo cleanly yet fail to give the expected temperature curve in the slaker, pushing grit up. Enclosed silos, dry-air venting and a sensible stock turnover are essential; in packaged goods and spare-part warehouses the same problem is solved at a different scale with the methods described under industrial moisture and humidity control.

A filler decision is made by reading three quantities at once: target opacity, acceptable strength loss and the fines load the retention system can carry. Filler content runs at 10–30% as ash in the sheet. Thanks to the high light scattering of PCC the same opacity can be reached at lower ash, which directly relieves the strength budget and cuts sizing demand.

In coating, the steepness of the particle size distribution decides the result: 60–90% below 2 µm is a typical range, and the narrower the distribution, the better the smoothness, gloss and ink receptivity. Abrasivity is the hidden cost here; a pigment with high residual quartz shortens blade and screen life and takes back the per-tonne saving.

On a machine running alkaline sizing (AKD/ASA), wet-end pH is held between 7.5 and 8.5. Below that band the carbonate starts to dissolve; above it, sizing efficiency and retention deteriorate. Grades of the same precipitated carbonate produced under a different purity regime are used on the chemical and pharmaceutical industry side; in paper, what decides performance is crystal habit and distribution rather than purity.

Before switching to a new filler source or a new make-up lime supplier, establish a baseline with the current line's ash, opacity, strength, retention and grit data; only then is a comparison meaningful. For samples, analysis and planning a mill trial, reach the technical team through the contact page.