The unwanted by‑product biuret can spike when urea lingered hot for too long. Plants are fighting back with lower temperatures, shorter residence times, careful ammonia management — and lab analysis that actually proves it.
Biuret (C₄H₆N₂O₂) forms when two urea molecules condense and release ammonia (2CO(NH₂)₂ → biuret + NH₃). It’s undesirable in fertilizer: it ties up nitrogen and can harm plants if excessive. Global fertilizer specifications typically limit biuret to 0.9–1.5 wt%—for example, FAO guidelines set ≤1.5% (www.fao.org), and many agronomic ureas target <0.9% (patents.google.com). Indonesian standards likewise specify ~1% biuret for 46% N urea (www.researchgate.net).
Because biuret forms via a reversible equilibrium, high temperature and long residence time (time the liquid spends in equipment) accelerate production, while free ammonia can suppress it (www.bcinsight.crugroup.com) (www.researchgate.net). Minimizing biuret, then, is a game of operating discipline—and accurate, reliable analysis of the final product.
Where biuret forms and why it spikes
Industrial urea plants condense a urea solution into a concentrated melt, then prill or granulate it (prilling: spraying melt to form solid beads; granulation: forming larger granules). The hottest parts—evaporators, strippers, and prilling lines—are biuret hot spots as two urea molecules disproportionate into biuret and ammonia. Recent studies and patents indicate much of the biuret is generated in the final concentration/stripper section (patents.google.com) (patents.google.com) (Fig. 1).
Temperature is the first lever. Laboratory studies suggest the forward reaction (2 urea → biuret + NH₃) dominates roughly between 140–160 °C; at higher temperatures the reaction may partially reverse (www.researchgate.net). Urea-stripper and evaporator temperatures, often 150–180 °C, therefore favor biuret formation. Lowering processing temperatures where feasible—such as trimming stripper pressure/temperature by a few degrees—can measurably cut biuret.
Residence time is the second lever. Uneven or long liquid hold‑up in evaporators increases biuret, and conventional vertical‑tube concentrators can create high‑recirculation zones so some fluid dwells far longer than average. Patent disclosures argue that most biuret arises from these long‑tail residence times (patents.google.com). At reduced throughput, fixed vessel volume versus lower flow stretches average residence time; even the suction leg to a melting pump matters—e.g., a conventional pump requiring a ~3 m suction head can hold significant hot melt and promote biuret (www.bcinsight.crugroup.com).
Ammonia concentration is the third lever. Because biuret formation is the reverse of amination (urea + ammonia ⇌ biuret), low free ammonia in the melt favors biuret (www.bcinsight.crugroup.com). Controlled additions of ammonia, as gas or liquid, shift the equilibrium toward urea, with patents documenting this effect (www.freepatentsonline.com) (patents.google.com). For example, injecting liquid ammonia into hot urea melt to about 500–5,000 ppm NH₃ (ppm: parts per million) at ~130–140 °C can significantly reduce net biuret (www.freepatentsonline.com).
Downstream handling matters too. While melt temperatures at granulation are usually 60–80 °C, hot transfer lines and distribution nozzles can prolong high‑temperature exposure. Patent literature emphasizes minimizing melt transport time from the last concentrator to the prilling tower to avoid extra heat exposure (patents.google.com). Low‑shear, low‑suction pumps near the evaporator outlet help keep hold‑up minimal (www.bcinsight.crugroup.com) (www.bcinsight.crugroup.com).
Concentration section operating parameters
Maintaining high flow and short residence in evaporators is central. Plants can minimize liquid inventory via constant‑volume evaporators or by adding surge/buffer tanks to decouple throughput from hold‑up. Patent literature describes mechanically varying evaporator volume (e.g., movable walls) or inserting an intermediate buffer upstream to smooth pulses (patents.google.com) (patents.google.com). At lower loads, recycling a portion of purified solution back to the feed keeps effective velocity up, avoiding “too‑slow” operation that spikes biuret formation.
Gas injection in evaporators is another lever. Injecting a partly insoluble gas—typically an ammonia‑containing stream—creates fine bubbles that mix and accelerate flow, effectively shortening residence time without changing liquid flowrate (patents.google.com) (patents.google.com). The ammonia also shifts equilibrium back toward urea (patents.google.com) (www.freepatentsonline.com). Quantitatively, adding ammonia to achieve roughly 0.05–0.5 wt% in the melt (i.e., 500–5,000 ppm) at ~130–140 °C has been found effective (www.freepatentsonline.com). Metering this addition with a dosing pump supports precise control of that 500–5,000 ppm NH₃ window.
Temperature optimization rounds it out. Running multiple‑effect evaporation lets each effect operate as low as vacuum allows; replacing a single high‑pressure unit with several lower‑temperature stages reduces thermal stress. Immediately quenching the final concentrate to just above the melt point—e.g., by flashing in an arrow tower—minimizes time at ~170–180 °C. In short, avoid extra heating beyond what is needed for the target concentration.
Ammonia recovery management also counts. Over‑stripping the melt removes the very reagent that suppresses biuret; maintaining a few hundred ppm free ammonia in the concentrated stream (via controlled recovery/neutralization) passively limits formation.
Melt transfer and granulation parameters
Pumping and transfer design can make or break biuret control. The run from the last evaporator to the granulator benefits from being as short and cool as practical. Insulating lines and pumps reduces heat gain. Placing a self‑regulating centrifugal pump directly under the evaporator outlet keeps suction head near zero, trimming melt hold‑up; one industry example contrasted this with a conventional pump requiring 2–3 m NPSH (net positive suction head), which can trap many liters of hot melt for seconds to minutes, raising biuret (www.bcinsight.crugroup.com) (www.bcinsight.crugroup.com).
Melt temperature and flow at finishing should be kept to the lowest compatible levels, often ~70–80 °C for prilling. Avoid excessive superheat. Precise, rapid dispensing into the prilling tower or fluid‑bed granulator ensures the liquid does not linger at high temperature. In batch operations, minimizing recirculation of melted urea helps; some processes intermittently spray a small amount of ammonia into a static melt tank to reconvert any biuret back to urea during hold periods, with the added ammonia carried into product (www.freepatentsonline.com).
Process uniformity reduces spikes. Start‑ups, shutdowns, or capacity changes can elevate biuret, so tuning control valves and feed rate control is material. When down‑rating, adjusting steam input and gas injection keeps velocities similar to full‑load through evaporators. Modern designs use variable‑speed recirculation or bypass lines so that even at 50% load, effective velocity and residence time mimic full‑load conditions (patents.google.com) (patents.google.com).
Performance benchmarks and case data
With the above controls, modern plants typically achieve ~0.3–0.7% biuret in final product, comfortably under a 1% limit. One Indonesian study reported batches averaging 0.52% biuret (www.researchgate.net). By contrast, poorly tuned operation—or a high‑suction setup—saw values approaching 1% or above. Engineering updates such as variable evaporator volume, gas injection, and improved pumps can halve biuret levels relative to legacy designs.
Capacity effects are real. Many plants guarantee biuret specifications only at full capacity; at reduced throughput, biuret often climbs. Patents show that dynamically controlling volumetric residence time—via gas or volume changes—maintains specification even at 50% load (patents.google.com) (patents.google.com).
Operating data underline the trend. In a 2019 stimulus, adding 5,000 ppm ammonia cut biuret from ~0.9% to 0.4% (www.freepatentsonline.com). Casale noted that replacing a conventional pump with a zero‑suction pump “kept the NPSH at ~0 m, and as a consequence… the residence time of the melt is kept to a minimum, leading to less biuret formation” (www.bcinsight.crugroup.com). Across cases, empirical data link shorter hold times and higher NH₃ to lower biuret, with modernization often cutting levels by a factor of 2–4.
Analytical methods and quality control
Accurate measurement of biuret in final urea underpins quality control. Regulatory limits are clear: for Indonesian urea (46% N), SNI 2801:2010 caps biuret at 1.0% (www.researchgate.net); FAO specifications allow up to 1.5% (www.fao.org). Automotive‑grade urea (DEF) requires much lower levels (often <0.3%).
Labs rely on several methods. Traditional spectrophotometry uses the “biuret test” via copper complexation around 545–555 nm, while titrimetric procedures such as AOAC 976.01 or 960.04 are established. Modern HPLC methods include a 2014 single‑lab validation that extended AOAC 2003.14 to urea, achieving a linear range of 0.01–0.05% (mass) with an LOQ ~0.03%; instrument precision showed intralab RSDs ~1% and average recovery ~97%, and interlaboratory RSD (at 0.2–0.9% levels) averaged ~21% (academic.oup.com) (academic.oup.com).
Given specifications near 1%, analyses must resolve differences of a few tenths of a percent. An Indonesian UV‑Vis example (biuret–CuSO₄ at 555 nm) measured 0.52% ±0.06% biuret at 95% confidence (www.researchgate.net). The ±0.06% expanded uncertainty (~11% relative) set the range to 0.46–0.58%. Calibrating with known‑blend standards and managing systematic error are therefore critical.
Sampling and QC complete the loop. Representative sampling (e.g., per SNI 0428) from blended product, batch or daily composites, and dual‑method checks (colorimetric vs. chromatography) improve confidence. Because biuret is nongaseous, samples are stable, but dissolution must be complete. Injecting a standard urea–biuret mixture in each run helps ensure accuracy. Accurate biuret data let operations correlate process conditions with product quality: if analysis trends upward (e.g., 0.2%→0.4%), checks on evaporator hold‑up or melt lines are indicated; consistently low biuret (<0.5%) can justify modest energy‑saving adjustments elsewhere.
Bottom line
Combining optimized operating parameters—lower temperature, shorter residence time, sufficient ammonia in the melt—with rigorous QC analysis enables consistent minimization of biuret. The result is urea that meets crop safety standards and regulatory limits, validated by numbers.
Sources: Industry and patent literature on urea production (e.g., Stamicarbon patents patents.google.com patents.google.com), Casale technical reviews (www.bcinsight.crugroup.com www.bcinsight.crugroup.com), fertilizer specifications (FAO, SNI) (www.fao.org www.researchgate.net), and analytical method studies (academic.oup.com) (www.researchgate.net). Each cited source is acknowledged in the text.
