A ground-up engineering course covering raw materials science, thermodynamics, reaction chemistry, process equipment, product quality, and plant economics — structured as 10 progressive chapters for the plant overseer.
Understanding rice husk at the molecular level — why it contains silica, how combustion temperature determines silica quality, and what "amorphous" means for the entire process.
Rice husk (also called rice hull) is the outermost protective layer of the rice grain — the seed coat that protects the grain from insects, fungi, and mechanical damage. Botanically it is the palea and lemma of the rice spikelet.
During the rice plant's growth, it absorbs monosilicic acid (Si(OH)₄) dissolved in soil water through its roots. Silicon is transported via the xylem to the leaves and outer grain layers, where it polymerises and deposits as amorphous hydrated silica (SiO₂·nH₂O) in the epidermal cell walls. This process is called biosilicification.
The silica serves a structural function in the living plant — it hardens the husk and creates a physical barrier. When the rice is milled, the husk is separated from the grain. At Fluxara, this agricultural waste is the primary feedstock.
| Component | % |
|---|---|
| Cellulose | 35–40% |
| Hemi-cellulose | 18–22% |
| Lignin | 22–26% |
| Silica (SiO₂) | 15–22% |
| Protein, minerals, ash | ~4% |
As-received moisture: ~10%
After burning, cellulose/hemi-cellulose/lignin oxidise completely to CO₂ + H₂O. Only the mineral fraction remains:
| Component | % |
|---|---|
| SiO₂ (target) | ≥92% |
| K₂O, Na₂O (alkali) | 2–4% |
| CaO, MgO | 1–2% |
| Fe₂O₃, other metals | <1% |
Fluxara requires ≥92% SiO₂ — contract clause
Conventional precipitated silica is made from quartz sand (crystalline SiO₂) dissolved in sodium hydroxide at high temperature and pressure in autoclaves. This requires 150–200°C, 5–10 bar pressure, and expensive energy.
RHA silica is already amorphous and reactive — it dissolves in NaOH at 90°C, atmospheric pressure. This is a fundamental thermodynamic advantage: amorphous SiO₂ has higher Gibbs free energy than crystalline quartz (it is metastable), so it dissolves far more readily. The entire Fluxara cost structure depends on this property.
All silica (SiO₂) is made of silicon atoms, each bonded to four oxygen atoms in a tetrahedral arrangement (SiO₄ tetrahedra). The difference between amorphous and crystalline silica is how these tetrahedra are arranged in space.
Crystalline silica (quartz, cristobalite, tridymite): tetrahedra are arranged in a perfectly regular, long-range ordered lattice. Every atom is in a predictable position. This order makes the structure thermodynamically stable and chemically resistant. Quartz does not dissolve in NaOH at room temperature or 90°C.
Amorphous silica (RHA silica, precipitated silica, fumed silica): tetrahedra are connected randomly — there is short-range order (each Si is still bonded to 4 O) but no long-range order. The structure looks like "frozen liquid glass." This disorder means the surface energy is higher — the material is less stable — and it dissolves much more readily in alkaline solutions.
Figure 1.1 — Amorphous (disordered) vs crystalline (ordered) SiO₂ at atomic scale. Disorder = higher reactivity.
Rice husk combusted per day: 20,000 kg/day
Husk is ~20% ash by mass (as-burned): 20,000 × 0.20 = 4,000 kg ash/day
Ash contains ~92% SiO₂: 4,000 × 0.92 = 3,680 kg SiO₂
But we also buy 11,432 kg/day of RHA @ 92% SiO₂:
SiO₂ from purchased RHA: 11,432 × 0.92 = 10,517 kg
Bonus RHA from combustion: 3,600 kg @ 92%
SiO₂ from bonus RHA: 3,600 × 0.92 = 3,312 kg
Total SiO₂ available: 10,517 + 3,312 = 13,829 kg/day ✓
At 88% extraction: 13,829 × 0.88 = 12,170 kg SiO₂ extracted/day ✓
Purchased RHA comes from third-party rice mills that burn husk for their own purposes (boilers, steam generation). They have no incentive to maintain a strict ≤700°C combustion temperature. If they over-burn, they deliver partially crystallised ash. You cannot tell by looking at it — white powder looks the same whether amorphous or crystalline.
XRF (X-Ray Fluorescence) tells you the elemental composition (% Si, K, Fe, etc.) but not the crystalline state. XRD (X-Ray Diffraction) is needed to confirm amorphous vs crystalline — a diffractometer is the authoritative test. In the absence of XRD, a dissolution test at 90°C in 10% NaOH for 30 minutes is a practical proxy: amorphous RHA dissolves readily, cristobalite does not.
The contract clause "min 90% SiO₂" protects against dilution but not against crystallisation. Fluxara's QC lab must include a periodic dissolution test on each supplier batch.
| Test | What it Measures | Frequency | Pass Criterion |
|---|---|---|---|
| XRF | Elemental composition (% SiO₂, Fe, K, etc.) | Every delivery | SiO₂ ≥92% |
| Dissolution proxy (10% NaOH, 90°C, 30 min) | Reactivity / amorphous state | Every delivery | ≥85% dissolution |
| LOI (Loss on Ignition) | Residual carbon / unburned organics | Weekly | <2% |
| Particle size (laser diffraction) | D50, D90 | Monthly | D90 <100 µm |
| XRD (if available) | Crystallinity index | Quarterly / new supplier | Amorphous halo only, no cristobalite peaks |
How rice husk burns, why the furnace design is critical, how steam is generated, and the thermodynamics from fuel to usable heat.
Rice husk contains cellulose (C₆H₁₀O₅)ₙ, hemicellulose, and lignin — all organic polymers made of carbon, hydrogen, and oxygen. When burned in air (21% O₂), these react with oxygen in exothermic oxidation reactions:
GCV (Gross Calorific Value) — total heat released if you condense all the water vapour produced, recovering the latent heat of vaporisation. Measured in bomb calorimeters at constant volume.
NCV (Net Calorific Value) — GCV minus the latent heat of vaporisation of water in the products. NCV is the realistic heat available in industrial furnaces because flue gases exit hot and water remains as vapour (not condensed). NCV is always lower than GCV.
For rice husk at 10% moisture: NCV ≈ 12,150 kJ/kg. This is the figure used in all Fluxara energy calculations.
Rice husk feed rate: 20,000 kg/day = 20,000,000 g/day
NCV (as-received): 12,150 kJ/kg
Gross thermal = 20,000 kg × 12,150 kJ/kg
= 243,000,000 kJ/day
= 243,000 MJ/day
= 243 GJ/day ✓
Convert to useful rate:
Per hour: 243 GJ ÷ 24 hr = 10.125 GJ/hr = 2,813 kW thermal
Per minute: 243,000 MJ ÷ 1440 = 168.75 MJ/min
A boiler with 82% efficiency means 18% of the heat in the fuel does not transfer to the steam. This heat is lost via four mechanisms:
1. Flue gas sensible heat (major loss, ~10–12%): Hot combustion gases (CO₂, H₂O, N₂, O₂) exit the stack at 150–200°C. Every degree of flue gas temperature above ambient is heat thrown away. An economiser (a flue-gas heat exchanger) preheats boiler feedwater to reduce this loss.
2. Unburned combustible in ash (~2–3%): If combustion is incomplete, carbon particles exit with the ash. This is minimised by adequate air supply and residence time.
3. Radiation and convection losses (~1–2%): Heat radiates off the boiler shell surface.
4. Blowdown losses (~1%): Boiler water accumulates dissolved solids (TDS) from the makeup water. Periodic blowdown (draining) removes concentrated brine — this carries heat out.
Gross thermal: 243.0 GJ/day
Boiler efficiency: 82%
Useful heat = 243.0 × 0.82 = 199.3 GJ/day ✓
Steam parameters (typical for process steam at 6–8 bar):
Saturation temperature: ~165°C
Enthalpy of steam: ~2,763 kJ/kg
Enthalpy of feedwater (90°C): ~377 kJ/kg
Enthalpy rise per kg steam: 2,763 − 377 = 2,386 kJ/kg
Steam generation rate:
199,300,000 kJ/day ÷ 2,386 kJ/kg = 83,529 kg/day
Per hour: 83,529 ÷ 24 ≈ 3,480 kg/hr steam
This steam is used for:
→ Leach reactor heating (90°C NaOH solution)
→ Evaporator (concentrating sodium silicate)
→ Spray dryer heating air
→ Building/process hot water
IBR stands for Indian Boiler Regulations, 1950 — the statutory framework governing design, construction, inspection, and operation of steam boilers in India. Any boiler generating steam above 1 kg/cm² pressure must be IBR-registered.
Before the boiler can raise steam, a Boiler Inspector from the state government (Telangana Boiler Inspectorate) must inspect the completed installation, witness hydraulic pressure testing (typically 1.5× working pressure), and issue a Certificate of Fitness. This process takes 6–10 weeks after mechanical completion. Planning this into the commissioning schedule is mandatory — it cannot be rushed.
The furnace/boiler building must be fire-rated RCC (Reinforced Cement Concrete), not Pre-Engineered Building (PEB). IBR regulations mandate masonry or RCC construction for boiler houses due to the fire and explosion risk.
Figure 2.1 — Furnace, boiler, and steam distribution block diagram
20 MT/day ÷ 24 hours = 0.833 MT/hr average feed rate. With 24/7 continuous operation at stable conditions, the design rate is 1.0–1.4 MT/hr, allowing for some variability in feed moisture and size.
The "NOT 25 MT/hr" note in the DPR exists because batch-mode thinking (processing in large infrequent loads) would cause temperature spikes — the furnace would overheat during high feed periods. Continuous slow feeding is essential for temperature control.
CEMS — Continuous Emissions Monitoring System is a mandatory stack monitoring system. For Fluxara's furnace it monitors: CO₂ concentration (confirms combustion), CO (incomplete combustion indicator), particulate matter (PM), SO₂ (from sulfur in husk), NOₓ, and temperature.
The temperature sensor triggers the auto-shutoff at 720°C. The CEMS data is transmitted to TSPCB in real time — this is a statutory requirement before the Consent to Operate (CTO) is granted. Data must be archived for 5 years.
How sodium hydroxide dissolves amorphous silica, what controls extraction efficiency, reactor design, and why NaOH recovery is the most important economic lever.
Silica is an acidic oxide — it reacts with bases, not acids. The Si–O bonds in SiO₂ are attacked by hydroxide ions (OH⁻), which break the Si–O–Si network and form soluble silicate anions (SiO₃²⁻). This is the inverse of glass corrosion: when glass is exposed to alkaline cleaning agents, it slowly etches.
Strong acids (HCl, H₂SO₄) do not dissolve SiO₂ to any significant extent at atmospheric pressure — quartz sand sits in hydrochloric acid unaffected. Hydrofluoric acid (HF) is the exception (it forms SiF₄), but HF is far too dangerous and expensive for an industrial scale process.
Alkaline leaching (NaOH) is therefore the only practical chemistry at ambient pressure and ≤100°C. This is the chemistry at the heart of every precipitated silica plant in the world.
10% NaOH concentration: Below 8%: dissolution rate too slow, poor extraction efficiency. Above 15%: solution becomes too viscous, difficult to pump and mix; also increases scaling risk. 10% is the industrial optimum for RHA silica.
90°C temperature: The dissolution rate of amorphous silica in NaOH increases approximately 2× for every 10°C rise (Arrhenius relationship). At 70°C, residence time would need to be 4+ hours. At 90°C (just below boiling), 2 hours is sufficient. Going to 100°C or above requires pressurised reactors — not justified economically. 90°C is achievable with steam-jacketed vessels at atmospheric pressure.
2-hour residence time: Derived from the dissolution kinetics of amorphous RHA silica at 90°C. This must be validated in the lab trial before engineering freeze — the actual time depends on the specific RHA particle size and amorphous content.
SiO₂ extracted per day: 12,170 kg/day (from mass balance Ch.1)
Stoichiometric NaOH needed:
12,170 kg SiO₂ × 1.331 kg NaOH/kg SiO₂ = 16,198 kg NaOH/day
But NaOH is recycled via causticisation (82% recovery rate):
NaOH recovered = 16,198 × 0.82 = 13,282 kg/day
NaOH lost (18%): 16,198 − 13,282 = 2,916 kg/day ← fresh makeup needed
Wait — why does the DPR show 4,375 kg/day fresh NaOH?
→ The 82% recovery is circuit efficiency, including losses in wash water,
incomplete causticisation, and drag-out. The actual makeup accounts for
these practical losses. The 4,375 kg/day is the validated design figure
from the process engineer's material balance.
NaOH as 48% lye (commercial form):
4,375 kg/day (100% basis) ÷ 0.48 = 9,115 kg/day ≈ 9.114 MT/day lye ✓
At ₹18,240/MT: 9.114 × 18,240 × 330 days = ₹5.49 Cr/yr ✓
The theoretical maximum extraction assumes all SiO₂ in the RHA is amorphous and accessible. In practice, four factors limit extraction:
1. Partially crystallised silica (~5%): Even from good-quality RHA, a small fraction of silica may have been over-burned at the supplier and is semi-crystalline. It dissolves slowly or not at all in 2 hours at 90°C.
2. Silica encapsulated by inert minerals: K₂O, Fe₂O₃ and other ash components can form a glassy shell around silica particles, blocking NaOH access.
3. Particle size: Larger RHA particles have lower surface-area-to-volume ratio. The NaOH must diffuse to the particle interior. Grinding RHA to <100 µm before leaching improves extraction to 92–95%.
4. Equilibrium limitations: As Na₂SiO₃ builds up in solution, the silicate concentration creates back-pressure on dissolution.
88% is an engineering estimate. The lab trial (Months 2–4) must measure actual extraction efficiency with the specific RHA suppliers and process conditions.
| Improvement Lever | Extraction Impact | Cost Impact | Practical? |
|---|---|---|---|
| Grind RHA to <75 µm (ball mill) | +4–7% efficiency | +₹0.3 Cr/yr (energy + CAPEX) | Marginal |
| Increase NaOH concentration 10%→12% | +2–3% | +₹0.5 Cr/yr NaOH | Net negative |
| Increase temperature to 95°C | +1–2% | Minimal | Yes — easy |
| Extend residence time 2hr→3hr | +1–2% | +1 more reactor (₹40L CAPEX) | Consider at Phase 2 |
| Improve RHA supplier quality (XRD audit) | +3–5% | Minimal | Yes — priority |
Table 3.1 — Extraction efficiency improvement options
NaOH solutions are strongly alkaline (pH 12–14). At 90°C, they attack and corrode mild steel rapidly, contaminating the product with iron (Fe³⁺ ions turn the solution yellow/brown, introducing impurities that poison downstream reactions). Stainless steel (SS304/316) resists NaOH better but can still be etched at high concentrations over time.
Polypropylene (PP) is chemically inert to NaOH at all concentrations up to 90°C. The reactor shell is stainless steel for structural strength, with a 6–8 mm PP inner lining. This combination gives mechanical integrity + chemical resistance. The lining is the critical life-limiting component — inspect for cracks and delamination every 2 years.
Type: Mechanically agitated, steam-jacketed batch/continuous
Volume: 10,000 L (10 KL) per vessel
Quantity: 3 reactors (2 operating, 1 standby/parallel)
Material: SS316 shell + 6mm PP inner lining
Agitator: Anchor/paddle type, ~30 rpm
Operating temp: 90°C (steam-jacketed)
Operating pressure: Atmospheric
pH: 11–13 during leach
Residence time: 2 hours
CAPEX: ₹120 Lakhs (3 reactors)
How CO₂ converts water glass into precipitated silica, the physics of nucleation and particle growth, how pH controls product grade, and what CTAB surface treatment does.
Any acid can neutralise the alkaline silicate solution and cause silica to precipitate. But CO₂ has three unique advantages:
1. Free: CO₂ comes from the furnace flue gas at zero cost. Using mineral acids would add ₹3–5 Cr/yr in chemical costs.
2. Clean byproduct: CO₂ + Na₂SiO₃ → SiO₂ + Na₂CO₃. The Na₂CO₃ (sodium carbonate, "soda ash") is directly usable in causticisation. If HCl were used, the byproduct NaCl (salt) cannot be economically processed — it would be a waste stream.
3. Gentle pH drop: CO₂ is a weak acid. It lowers pH gradually and controllably, allowing precise endpoint control. Strong acids would overshoot pH instantly and produce coarse, poorly controlled particles.
When pH drops in the sodium silicate solution, the silica becomes supersaturated — there is more dissolved silica than can remain in solution. To reduce this excess, silica must come out of solution as a solid. This happens in two stages: nucleation then growth.
Nucleation is the formation of the very first tiny solid clusters (nuclei) from the dissolved silica. It requires overcoming a surface energy barrier — creating a new solid surface has an energy cost. The rate of nucleation is extremely sensitive to the degree of supersaturation. At high supersaturation (rapid pH drop, low final pH), nucleation rate is very high → many nuclei form → many small particles in the final product. At low supersaturation (slow pH drop, higher final pH), nucleation rate is low → few nuclei form → each grows large → fewer, bigger particles.
Growth is the attachment of dissolved silica molecules to existing nuclei. Growth rate is proportional to available surface area and supersaturation. If nucleation is fast (many nuclei), growth per particle is limited — final particles are small. If nucleation is slow (few nuclei), all the supersaturation is consumed by growth — final particles are large.
Figure 4.1 — pH control during CO₂ addition determines particle size and product grade
The same furnace, leach reactors, and precipitation vessels can produce Standard, HDS, or Dental grade silica. The only difference is the pH profile during CO₂ addition. This means Fluxara can dynamically adjust its product mix based on market demand and customer qualification status — a major operational advantage.
In Year 1 (before HDS qualification), produce Standard grade. In Year 2 (after HDS approval), shift the majority to HDS. The physical plant doesn't change — only the process control setpoints.
CTAB — Cetyltrimethylammonium Bromide — is a cationic surfactant (MW: 364.5 g/mol). It is added to the precipitated silica slurry after filtration and before (or during) spray drying. Each CTAB molecule has a positively charged head group that binds to the negatively charged silica surface, and a long hydrocarbon tail (C₁₆) that points outward.
The effect: CTAB molecules form a uniform monolayer on the silica surface, creating a hydrophobic coating. In the tyre rubber matrix, the hydrocarbon tails interact favourably with the polymer chains — this improves dispersibility and creates strong polymer-silica bonding, which reduces rolling resistance.
CTAB value (mg CTAB/g silica) is measured by a standardised adsorption test. A higher CTAB value means more surface accessible to polymer chains. ≥175 mg/g is the HDS specification. This is controlled by: (a) particle size (smaller = more surface area = higher CTAB capacity), (b) silica purity (no contamination blocking surface sites), (c) CTAB concentration and contact time.
Consumption: 0.289 MT/day (Ph1A)
Price: ₹1,80,000/MT (imported)
Annual cost: ₹1.72 Cr/yr
Revenue enabled: shifts PS from ₹26/kg to ₹45–50/kg
Revenue gain: +₹19–24/kg on HDS fraction
CTAB adds ~₹0.39/kg to production cost but enables +₹19/kg premium — 49× return on the CTAB cost
CTAB is imported (no domestic manufacturer at commercial scale). Lead time: 4–6 weeks from order to delivery.
A stockout means HDS production stops → revert to Standard grade → revenue drops from ₹45/kg to ₹26/kg on that volume.
4-week buffer stock = 0.289 × 28 days = ~8.1 MT CTAB on hand at all times.
Storage: cool, dry, away from moisture (hygroscopic). Shelf life: 2 years if stored correctly.
How NaOH is recovered from sodium carbonate using quicklime, the thermodynamics of CaO slaking, and why PCC is an obligatory co-product, not an afterthought.
After precipitation (Ch.4), the liquid remaining is a sodium carbonate (Na₂CO₃) solution. If this Na₂CO₃ were simply discarded, you would need to buy fresh NaOH for every leaching cycle — at ₹38,000/MT NaOH (100% basis), this would add ₹6+ Cr/yr to OPEX, making the plant uneconomical.
The solution is causticisation — converting Na₂CO₃ back to NaOH using calcium hydroxide (Ca(OH)₂). This is exactly how the kraft pulp and paper industry regenerates NaOH in its chemical recovery cycle. Fluxara borrows this well-established industrial chemistry.
The recovered NaOH is recycled back to the leach reactors (Ch.3). Only the makeup NaOH (to replace the 18% lost in process losses) needs to be purchased — 4.375 MT/day instead of the full circuit requirement of ~24.3 MT/day.
The causticisation reaction is driven to completion by two thermodynamic factors:
1. CaCO₃ is insoluble: The product CaCO₃ has very low solubility in water (Ksp ≈ 3.3 × 10⁻⁹ at 25°C). As it precipitates, it is continuously removed from the equilibrium — Le Chatelier's principle shifts the reaction to the right, producing more CaCO₃ and more NaOH.
2. Ca(OH)₂ is a stronger base than Na₂CO₃ is an acid: The Ca²⁺ ion has higher affinity for CO₃²⁻ than Na⁺ has. The Ca²⁺ effectively "pulls" the carbonate out of solution, liberating OH⁻ which combines with Na⁺ to give NaOH.
The 82% recovery efficiency (not 100%) reflects: incomplete dissolution of CaCO₃ at the operating temperature, NaOH drag-out in the washed CaCO₃ cake, and equilibrium losses. This is the most critical parameter to validate in the lab trial.
ΔH = −63.7 kJ/mol means 63.7 kJ of heat is released per mole of CaO slaked. At 13,364 kg/day:
13,364 kg/day ÷ 56.08 g/mol × 1000 = 238,320 mol/day × 63.7 kJ/mol = 15,181,000 kJ/day = 15.2 GJ/day of heat released just from slaking. This heat must be managed — the slaker vessel must have cooling water jackets, otherwise the Ca(OH)₂ slurry temperature can exceed 100°C, causing quality degradation and safety risks (steam flashing).
The slaking exotherm is actually an asset — it preheats the Ca(OH)₂ slurry before feeding to the causticisation vessel, reducing the external heating requirement slightly.
Economics: CaO at ₹6,000/MT contains more Ca per kg than Ca(OH)₂ (MW 56 vs 74). Pre-slaked Ca(OH)₂ would cost ~₹8,000–9,000/MT for the equivalent Ca content — 33–50% more expensive.
Logistics: Ca(OH)₂ absorbs CO₂ from the air (carbonation: Ca(OH)₂ + CO₂ → CaCO₃), degrading in quality over days to weeks if stored in open containers. CaO is more stable in storage.
Freshness: Freshly slaked Ca(OH)₂ (warm, highly reactive) gives better causticisation efficiency than pre-slaked material that has partially carbonated.
CaO purchased: 13,364 kg/day @ ₹6,000/MT
Daily CaO cost: 13.364 × 6,000 = ₹80,184/day = ₹2.65 Cr/yr ✓
Slaking:
Ca(OH)₂ produced: 13,364 × 1.122 = 14,994 kg/day Ca(OH)₂
Causticisation:
Na₂CO₃ available: ~21,477 kg/day (from precipitation)
Ca(OH)₂ required: 21,477 × 0.699 = 15,012 kg/day → matches ✓
NaOH recovered: 21,477 × 0.755 × 0.82 (82% eff.) = 13,303 kg/day
NaOH makeup: 16,198 − 13,303 = 2,895 kg/day ← matches mass balance ✓
CaCO₃ (Nano-PCC) produced:
21,477 × 0.944 = 20,274 kg/day ≈ 20.272 MT/day ✓
Annual: 20.272 × 330 = 6,690 MT/yr ✓
At average PCC price ₹20/kg (blended domestic):
Revenue: 6,690 MT × 20,000 kg/MT × 20 = ...
Note: PCC pricing in the model is grade-blended. See Section 10 of DPR for revenue model.
The CaCO₃ particle size (d50 target: 0.5–2 µm for PCC grades) is controlled by three variables:
Temperature: Higher temperature (85–95°C) produces smaller, more reactive CaCO₃ nuclei → finer PCC. Lower temperature → larger, denser crystals (less desirable).
Agitation speed: Higher agitation maintains suspension and prevents agglomeration — finer effective particle size. Inadequate agitation allows particles to settle and form large agglomerates.
Ca(OH)₂ concentration: Dilute Ca(OH)₂ slurry (lower [Ca²⁺]) → slower crystal growth → finer PCC. This is a tool — by diluting the slurry entering causticisation, particle size can be tuned.
After causticisation, the CaCO₃ slurry is classified using hydrocyclones — centrifugal classifiers that separate by particle size. Coarse particles (d50 >2 µm) are sent back for re-precipitation or to bulk/coatings grade. Fine particles proceed to stearic acid coating (for sealant grade) or OCC coating (for plastics grade).