Crude triazinamide directly from hydrazinolysis ring expansion only reaches HPLC 96%–97%, mixed with free hydrazine, ring-cleavage triazinone tar, inorganic salts, dimeric byproducts and colored impurities. Industrial standardized multi-stage purification removes all pollutants to hit ≥99.0% assay, single unknown impurity ≤0.15%, fully compliant for pymetrozine TC manufacturing.

1. Pre-Purification In-Situ Impurity Removal (Before Primary Filtration)

This step cuts major toxic impurities at the reaction stage to reduce recrystallization load

1. Capture residual free hydrazineAfter ring expansion reaction completes at 80–105 ℃, add cyclohexanone (0.1–0.2 eq vs hydrazine hydrate), hold 1–1.5 h to convert free hydrazine into insoluble hydrazone precipitate.
2. Negative pressure dehydration to eliminate water-soluble saltsVacuum reflux at 75–85 ℃ to separate water layer; soluble inorganic salt (NaCl, sulfate) concentrates in aqueous phase and separated out.
3. Gradient slow cooling for crude crystal precipitationCool from 100 ℃ to 30 ℃ over 2 h, then cool to 0–5 ℃ and hold 2 h; slow crystal growth reduces impurity entrapment inside crystal lattice.
4. Centrifuge & cold solvent rinsingHorizontal peeler centrifuge separates solid; rinse filter cake with cold isobutanol / n-amyl alcohol to wash away surface mother liquor carrying tar and dissolved impurities.Output crude triazinamide: HPLC ~96–97.5%.

2. First-Stage Recrystallization: Decolorization & Removal of Heavy Tar

Solvent Selection (Industrial Preferred Mixed Alcohol System)

Main solvent: Isobutanol / n-amyl alcohol; auxiliary co-solvent: small amount of ethanol to adjust solubility window

Operation Steps

1. Charge crude triazinamide + hot mixed solvent into decolorization reactor, heat to 70–80 ℃ under stirring until full dissolution (use minimal solvent to maximize yield).
2. Add 0.5–1% activated carbon (acid-washed food grade carbon, low heavy metal) for thermal decolorization, stir 30–40 min to adsorb dark tar, polymer dimers and trace metal ions.
3. Hot pressure filtration (critical step)Maintain temperature above 65 ℃ to prevent premature crystallization; filter out carbon and insoluble polymeric impurities completely. Insoluble tar will cause unknown impurity peaks in HPLC if not removed here.
4. Natural cooling to room temperature, then cool to 0–5 ℃ for crystal precipitation, centrifuge to separate intermediate refined crystal (HPLC ~98.2–98.7%).

3. Second-Stage Precision Recrystallization: Hit HPLC ≥99.0% Target

This step lowers single unknown impurity below 0.15%, removes residual micro-salts and incomplete ring-opening derivatives.

1. Dissolve once-recrystallized solid in fresh purified alcohol solvent, heat to reflux, full dissolution with slight solvent excess.
2. Static sedimentation 30 min to separate tiny inorganic salt suspended particles at bottom.
3. Ultra-slow gradient cooling (cooling rate ≤0.5 ℃/min) to generate large, regular flaky crystals with minimal impurity wrapping.
4. Low-temperature centrifugation, rinse crystal cake with ice cold pure solvent twice to displace residual high-impurity mother liquor.After second recrystallization: HPLC ≥99.0%, single unknown impurity ≤0.15%, hydrazine residue undetectable by HPLC.

4. Post-Treatment: Vacuum Drying to Avoid Degradation

High temperature triggers partial deacetylation of triazinamide, reducing purity back down. Strict drying control:

1. Load crystal into vacuum tray dryer, vacuum -0.06~-0.08 MPa, temperature controlled ≤80 ℃, drying 4–6 h.
2. End-point test: moisture ≤0.3% by Karl Fischer titration.
3. Sieve through 80-mesh screen to remove agglomerated lumps formed by residual solvent; obtain uniform white low-dust crystal.

5. Key Impurity Types & Targeted Removal Solutions

Impurity Type	Harm to downstream pymetrozine	Purification Elimination Method
Free hydrazine	Reacts with nicotinaldehyde to form useless hydrazone, increases raw material cost	Cyclohexanone capture before filtration + two recrystallization solvent washing
Triazinone ring-cleavage byproducts	Reduces condensation yield, generates black tar	Two-stage carbon decolorization + hot filtration
Inorganic chloride/sulfate salts	Corrode reactors, raise unknown impurity peaks	Negative pressure water separation + cold solvent rinsing
Dimer polymer tar	Blocks filtration lines, causes batch yield fluctuation	Activated carbon adsorption + hot pressure filtration
Heavy metal (Pb/As/Cd)	Fails EU pesticide registration limits	Acid-washed activated carbon chelating adsorption

6. Critical Process Control Parameters to Guarantee 99% Purity

1. Solvent dosage: Only use minimum hot solvent for full dissolution; excess solvent lowers total recovery yield below 90%.
2. Cooling speed: Fast cooling forms microcrystals wrapped with impurities; slow gradient cooling mandatory for high purity.
3. Drying temperature: Never exceed 80 ℃; overheating triggers deacetylation and new unknown impurities.
4. Mother liquor recycling: Distill recovered solvent for recirculation; concentrated waste mother liquor treated separately to avoid impurity accumulation.
5. Online HPLC monitoring: Test crystal after each recrystallization stage to adjust solvent ratio and cooling time.

7. Final QC Standard After Full Purification

• HPLC Assay: ≥99.0%
• Maximum single unknown impurity: ≤0.15%
• Moisture: ≤0.3%
• Residual hydrazine: Not detected
• Heavy metals (Pb, As, Cd): ≤10 ppm
• Appearance: Uniform white crystalline powder, no yellow discoloration