Evaporator Foaming: 7 Causes of Carryover and Product Loss

Last updated: 2026-11-06

Evaporator foaming can push condensate TDS from below 50 mg/L to more than 800 mg/L in a single shift, sending dissolved metal values straight into the condensate system and off the product balance sheet. This guide shows process engineers how to separate foam-driven carryover from mechanical entrainment, match the antifoam to the liquor chemistry, and size the vapor separator so foam never escalates to a shutdown.

Key Takeaways

• Evaporator foaming and mechanical entrainment produce identical condensate symptoms but different mass-balance signatures.
• A TDS rise paired with falling steam economy signals foam; a TDS spike with a stable economy signals a damaged demister.
• Antifoam selection is chemistry-specific: polyether types survive hot alkaline liquors where silicone oils hydrolyze and fail.
• Separator sizing and disengagement height decide whether a foam event stays an upset or becomes a plant trip.
• Design margins and equipment options are summarized in our evaporator and crystallizer systems guide.

What Is Foaming in an Evaporator, and How Is It Different From Entrainment?

Evaporator foaming is a stable dispersion of vapor bubbles in the boiling liquor that resists coalescence and swells the liquid volume far beyond its working level. Carryover is the physical transport of that liquid phase — droplets or foam lamellae — out of the boiling zone into the vapor head, from which a fraction reaches the condensate. Entrainment is the mechanical lift of droplets by vapor velocity alone, a phenomenon that occurs with or without foam and is governed by separator design limits rather than solution chemistry. The distinction matters because the remedies differ.

Three interfacial concepts explain the behavior. A surface tension gradient is a spatial variation in surface tension along the liquid surface that drives film thinning and stabilizes bubble walls against rupture. Dissolved surfactants — flotation collectors, frothers, amine residues — adsorb at the bubble interface and create exactly this gradient, which is why a few ppm of frother in the feed can convert a calm evaporator into a foam column within minutes. An antifoam is an insoluble surface-active additive that displaces those stabilizing species at the interface and locally collapses the film; it works by being chemically incompatible with the foam film, not by neutralizing the surfactant in bulk solution.

Note that evaporator foaming is not itself a fault — transient foam at the boiling surface is normal in any vacuum unit. The fault is uncontrolled foam height: when the foam occupies the vapor head disengagement space, bubbles reach the demister intact, the pad floods, and liquid passes through in sheets, so the condensate reflects the boiling liquor, TDS and all.

How Do You Tell Foaming From Entrainment?

The two failure modes produce the same headline symptom — dirty condensate — so diagnosis has to come from paired measurements. The most useful pairing is condensate TDS against steam economy: foam adds liquid volume to the vapor stream and wets the demister, raising boiling-side pressure drop and stealing heat-transfer area, so economy falls with the TDS. A broken or blinded demister passes droplets just as efficiently but leaves the boiling process untouched, so economy stays flat while condensate goes dirty. This pairing separates the two modes faster than any lab test.

Table 1 — Diagnostic matrix: foaming versus mechanical entrainment

Two practical notes. First, the economy comparison only means something if you meter steam accurately; with only a condensate-flow estimate, use the widening temperature approach across the calandria instead. Second, vacuum oscillation is ambiguous: genuine foaming produces slow, feed-correlated oscillation, while air ingress produces rapid high-frequency noise. If the vacuum signature confuses the picture, work through the dedicated evaporator vacuum loss checklist before touching antifoam; if economy has drifted for weeks rather than hours, suspect fouling — the evaporator steam economy audit is the faster route.

Which Seven Causes Produce Evaporator Foaming?

1. Flotation reagents and amine surfactants in the feed. Symptom: foam onset correlated with an upstream flotation or solvent-extraction campaign, arriving in waves that match concentrate campaigns. Cause: xanthate and frother residues, amine collectors, and reagent carryover on filter cake survive thickening and reach the evaporator at ppm levels — enough to stabilize bubbles at the boiling surface. Action: audit every reagent touching the liquor upstream and treat the evaporator feed as a reagent sink; the durable fix belongs in the flotation circuit, not in antifoam consumption.

2. Salting-out of high dissolved salts. Symptom: foaming that worsens steeply as the evaporator approaches crystallization density. Cause: near saturation, dissolved salts push surface-active impurities out of solution and onto the bubble interface — classic salting-out — so a liquor that foamed mildly at 20% solids can foam violently at 28%. Sodium sulfate liquors are notorious for this step change near the crystallization point, covered in our sodium sulfate crystallization resource. Action: identify the density window where foaming escalates and hold the working point below it, or accept a standing antifoam dose for the final effect.

3. Fine particles and colloids stabilizing foam. Symptom: persistent, cream-colored foam with slow drainage that survives antifoam doses that worked last month. Cause: sub-micron precipitates — silica, iron hydroxides, gypsum fines — adsorb at the bubble interface and form a mechanical armor that slows film drainage by an order of magnitude. Action: clarification or fine filtration upstream is the durable fix; antifoam only buys time.

4. Carbonate and bicarbonate decomposition releasing CO2. Symptom: foaming that starts suddenly when liquor crosses a threshold temperature, often with visible gas puffing and vacuum oscillation. Cause: bicarbonate decomposes above roughly 80–90 °C (176–194 °F), releasing CO2 as a second gas phase; a liquor holding only 0.3% w/w bicarbonate as CO2-equivalent releases about 3 kg of gas per tonne of feed — enough to double the apparent vapor load locally. Action: pre-decarbonate with an atmospheric degassing stage or acid dosing ahead of the evaporator. This is a feed-preparation fix, and it also protects downstream pH control.

5. Mechanical factors: high level, violent boiling, vapor velocity above separator design. Symptom: foaming complaints that disappear when the operating point changes, independent of chemistry. Cause: liquid level too close to the vapor outlet, excessive heat flux, or vapor velocity beyond the separator’s design envelope all mimic — or amplify — chemical foam. Pushing a unit to 120% of design evaporation rate raises superficial vapor velocity by the same 20%, which can push droplet lift-off past the demister’s capture ceiling even with clean liquor. Action: restore design level and heat flux first; re-measure before adding chemicals.

6. Wrong antifoam selection — silicone oil failing in strong alkali. Symptom: antifoam that worked at commissioning stops working after a pH shift, while consumption climbs. Cause: silicone emulsions hydrolyze in hot alkaline liquor above roughly pH 12, losing interfacial activity and sometimes leaving silicate deposits on heat-transfer surfaces. Action: match antifoam chemistry to liquor chemistry, as detailed below; the product that worked in water will not necessarily work in a 250 g/L sodium sulfate liquor.

7. Demister or wire-mesh fouling and installation defects. Symptom: gradual TDS creep over weeks, rising pad pressure drop, occasional liquid showers in the vapor duct. Cause: a blinded or partially collapsed mesh pad has reduced free area, so local velocity through the remaining lanes exceeds the entrainment limit and the pad re-entrains the liquid it collects; bypass gaps at the pad perimeter and missing hold-down grids produce the same signature from day one. Action: inspect the pad at the first sustained ΔP rise; the measures in our evaporator scaling control resource reduce how often you have to.

What Antifoam Works in Which Service?

Three antifoam families cover almost all hydrometallurgical evaporation services, and they are not interchangeable — choosing wrong is itself a cause of evaporator foaming (Cause 6 above).

Silicone oils (polydimethylsiloxane emulsions), 5–50 ppm active. The strongest general-purpose knockdown agents: extremely low surface tension, fast collapse of existing foam, effective at tiny doses. Their weakness is chemical: in hot alkaline liquor above pH 12, the siloxane backbone hydrolyzes, activity decays within hours, and hydrolysis products can contribute to silicate scaling on the calandria. Silicones also carry a product-contamination caution in any liquor feeding electrolysis.

Polyether and polyglycol types (EO/PO block copolymers). The workhorse for alkaline and high-salt service. Less aggressive knockdown than silicones — expect 10–50 ppm — but chemically stable in hot caustic and sulfate brines, tolerant of high ionic strength, and largely water-soluble, so overdosing does not leave an insoluble film on heat-transfer surfaces. For hot alkaline sulfate liquors, a polyether-based antifoam is preferred because silicone emulsions hydrolyze and lose activity within hours at pH above 12.

Mineral-oil-based blends. Still effective where the liquor is neutral to mildly acidic and the foam is particle-stabilized; they work by spreading on the bubble surface and displacing the stabilizing film. Drawbacks: higher dose rates (20–100 ppm), insoluble films that can coat heat-transfer surfaces, and oil residues unacceptable in many product liquors. For low-pH, colloid-stabilized froths, a mineral-oil spreader type is preferred because it de-wets the particle interface directly, whereas polyether activity there is modest and short-lived.

The overdose problem. Antifoam is itself surface-active, and too much of it is its own failure mode. Excess polyether adsorbs on heat-transfer surfaces and reduces the boiling heat-transfer coefficient; excess mineral oil builds an insulating film that shows up as falling economy and fouling; and in crystal-product service, overdosing risks contaminating the saleable salt itself. Start at the low end of the dose range and re-baseline after every upstream reagent change.

Dosing practice. Inject antifoam continuously into the feed or onto the boiling surface through a quill — never as a periodic slug into the vapor head, which gives a knockdown spike followed by a foam rebound. Meter with a small positive-displacement pump, keep the stock diluted per the supplier’s instructions (typically 5–20% active in water), and record dose against foam height. If dose demand keeps rising week over week, the cause is upstream.

How Do You Size the Vapor Separator to Survive Foam?

Chemistry manages foam generation; the separator decides what evaporator foaming costs you. The sizing logic has three parts: compute the volumetric vapor load, keep superficial velocity inside the separator’s entrainment limit at maximum evaporation with margin, and preserve enough disengagement height for foam to drain before reaching the demister.

Step 1 — vapor volume flow. The vapor volumetric flow is Q_v = m_v / ρ_v, where m_v is the evaporation rate in kg/s and ρ_v the saturated vapor density at the operating pressure. Worked example: a 30 t/h (66,000 lb/h) forced-circulation evaporator boiling near atmospheric pressure, saturated vapor density ρ_v ≈ 0.52 kg/m³:

Q_v = (30,000 / 3,600) / 0.52 ≈ 16.0 m³/s.

Pressure matters enormously: the same mass flow at deep vacuum (65 °C / 149 °F boiling) has a vapor density around 0.16 kg/m³, tripling the volumetric load to roughly 52 m³/s. This is why vacuum evaporators need either much larger vapor bodies or a recompression step — the trade-off is explained in our MVR working principle resource.

Step 2 — velocity and diameter. Apply a 1.2 multiplier for maximum evaporation and margin, then divide by the allowable superficial velocity: Q_design = 1.2 × 16.0 ≈ 19.2 m³/s; at 1.5 m/s allowable vertical velocity, D = √(4 × 19.2 / (π × 1.5)) ≈ 4.0 m.

Table 2 — Vapor separator types for evaporator service

Step 3 — disengagement height. Keep at least 0.8–1.2 m (2.6–3.9 ft) of clear vapor space between the highest operating boiling surface — including the foam layer — and the demister face. Foam height control is fundamentally a freeboard problem: if the foam column can reach the demister, no velocity calculation saves you.

For high-foaming services with colloid-stabilized froth, a cyclone or vane-type separator is preferred because wire-mesh pads blind quickly and re-entrain collected liquid once foam films bridge the mesh openings. Conversely, for clean liquors with only occasional mild foaming, a wire-mesh demister is preferred because it achieves the lowest condensate TDS of any option at modest cost. Both the MVR evaporation crystallization unit and the triple-effect evaporation crystallization unit carry foam margin in the vapor body from the process data sheet onward.

What Does Carryover Actually Cost?

Carryover is priced as a nuisance until someone runs the numbers. Worked example, every assumption stated: evaporation duty 30 t/h (66,000 lb/h); condensate produced 28 t/h, the balance going to blowdown and losses; condensate TDS during foaming episodes 900 mg/L against a permitted 50 mg/L (limits vary by reuse application — 50 mg/L is a representative boiler-feed and product-wash figure); product value US$400/t; availability 8,000 h/y.

At 900 mg/L, the condensate carries 28,000 kg/h × 900 mg/kg = 25.2 kg/h of dissolved solids — liquor that belongs in the product stream. At the permitted 50 mg/L the loss would be 1.4 kg/h, so the net excess loss is 23.8 kg/h: 0.57 t/d, roughly 0.6 t/d, or about 210 t/y of dissolved product value leaving through the condensate line. Valued at $400/t, evaporator foaming at this severity writes off approximately $84,000 per year.

The hidden second cost: if the condensate returns to boilers, 900 mg/L TDS is far beyond any boiler-feed specification, so the water needs demineralization or the boiler lives with elevated blowdown — at that TDS, holding steam drum chemistry can double or triple blowdown rate. That secondary penalty commonly adds a five-figure annual sum, site-specific and to be confirmed by test work.

The third cost is downtime risk: foam-driven vacuum loss and demister flooding do not just dirty the condensate; they trip the plant, and one unplanned wash-and-restart per month costs more shifts than the antifoam budget. Where condensate recovery feeds a zero-liquid-discharge commitment, the same carryover threatens the water balance — see our zero liquid discharge application page.

How Do You Fix Evaporator Foaming in Six Steps?

1. Sample the condensate and the feed. Pull condensate at the sample point and feed from the last process vessel ahead of the evaporator, during a foaming episode, not between them. Measure TDS, pH, and — if the lab allows — surface tension. Without paired samples you will diagnose the wrong stream.

2. Run surface tension and foaming tests. A shake or sparge test on both samples, ideally with a Ross-Miles-style foam height measurement, answers the key question in an afternoon: is the feed intrinsically foam-prone, or only the concentrated liquor? Test the concentrate too — salting-out means foam behavior changes with density, and a feed-basis test alone can be reassuringly wrong.

3. Audit upstream reagent residuals. List every collector, frother, flocculant, antiscalant, and cleaner that touched the liquor upstream, with dose dates, and correlate foaming episodes against the list. This finds the Cause-1 signature that no evaporator-side fix will permanently cure.

4. Adjust the antifoam program. Select chemistry per the service discussion above, start at the low end of the dose range, meter continuously, and track foam height against dose. If demand escalates over weeks, stop escalating the dose and return to steps 2 and 3 — rising antifoam consumption is a symptom, not a solution.

5. Verify vapor velocity and separator condition. Reconcile actual evaporation rate and operating pressure against the separator data sheet: compute Q_v = m_v / ρ_v at current duty, compare superficial velocity with Table 2, and inspect the demister at the first sustained ΔP rise. Many chronic foaming problems end here with a fouled pad or an oversized operating point.

6. Change the process where chemistry and hardware both check out. Remaining options, in escalating capital order: pre-decarbonate or degas the feed to remove the CO2 driver (Cause 4); pre-concentrate in a separate stage so the final effect never crosses the salting-out density window; lower the working level and reduce heat flux to restore freeboard; or, where crystal quality allows, shift final concentration into an Oslo crystallizer arrangement that tolerates the mother-liquor chemistry better than a boiling surface does. If the mother liquor itself is the troublemaker, dedicated mother liquor recovery logic — bleed versus treatment — belongs in the same study.

Three Operating Snapshots (indicative, synthetic-typical basis)

The following composites reflect typical operating patterns across hydrometallurgical evaporation plants; figures are indicative, synthetic-typical basis, not from any named site.

Snapshot 1 — flotation reagent carryover. A base-metal concentrator upgraded its flotation reagent scheme on a Monday; by Tuesday afternoon the adjacent evaporation circuit’s condensate TDS had risen from 45 mg/L to 1,150 mg/L, and steam economy fell about 8% in the same shift. A paired shake test showed feed surface tension dropping from 68 mN/m to 52 mN/m. A 20 ppm standing dose of polyether antifoam restored clean condensate within two days.

Snapshot 2 — carbonate decomposition. A sodium sulfate evaporation circuit began showing ±3 kPa vacuum oscillation and puffing foam whenever liquor temperature crossed 88 °C (190 °F). Feed analysis showed 0.35% w/w bicarbonate as CO2-equivalent, releasing roughly 3.5 kg CO2 per tonne of feed at boiling. An atmospheric pre-degasser on the feed tank eliminated the oscillation and cut antifoam demand by about 60%.

Snapshot 3 — demister blinding misdiagnosed as foaming. A zinc-leach residue liquor evaporator ran with condensate TDS creeping from 60 to 600 mg/L over six weeks while steam economy stayed within 2% of baseline. The flat economy — the tell from Table 1 — pointed away from foaming. Inspection found the wire-mesh pad blinded to roughly 40% free area with gypsum scale, ΔP up from 0.4 to 2.3 kPa; pad replacement plus the site’s existing scale-control routine resolved the TDS without a kilogram of antifoam.

FAQ

What condensate TDS level indicates evaporator foaming rather than normal carryover?

Healthy evaporators on clean liquor produce condensate below roughly 50 mg/L TDS. Sustained readings above 100 mg/L justify investigation, and readings above several hundred mg/L indicate liquor is physically reaching the condensate — the signature of foaming, demister damage, or gross entrainment. TDS alone cannot distinguish the three; pair it with steam economy and demister pressure drop as in the diagnostic table above. Limits depend on the condensate’s reuse application, so confirm your own specification before acting.

Can I dose antifoam continuously, or only during upsets?

Both strategies exist. Continuous metering at 5–50 ppm active, depending on chemistry, is the default for liquors with a known persistent surfactant load, because it prevents foam from ever reaching the demister. Interrupted dosing is acceptable where foaming is episodic and tied to specific feed batches, provided someone watches the sight glass or a foam sensor triggers the dose. Avoid slug dosing into the vapor head: it produces knockdown-rebound cycles that stress the demister.

Why did my silicone antifoam stop working after a pH change?

Silicone (polydimethylsiloxane) antifoams hydrolyze in hot, strongly alkaline liquor above roughly pH 12: the siloxane backbone breaks down, interfacial activity decays within hours, and hydrolysis byproducts can even contribute silicate to scale deposits. If your liquor crossed into that range, switch to a polyether or polyglycol antifoam, which is chemically stable in hot caustic and high-salt service. Expect a different dose rate — polyethers usually need 10–50 ppm active.

How much freeboard does a foaming evaporator need?

Maintain at least 0.8–1.2 m (2.6–3.9 ft) of clear vapor space between the highest boiling surface — measured including the foam layer, not just the nominal liquid level — and the demister face. Foam occupies volume, so a level that looks safe at commissioning can lose half its freeboard when the liquor turns foam-prone. If liquor chemistry is uncertain at design time, size freeboard for the foaming case and verify foam height at the sight glass during commissioning at maximum evaporation rate.

Does foaming affect steam economy, or only condensate quality?

Genuine evaporator foaming affects both: the foam layer wets and loads the demister, raises boiling-side pressure drop, and effectively steals heat-transfer area, so steam economy typically drops 5–10% alongside the TDS rise. This pairing is the most useful diagnostic split: a condensate TDS spike with flat steam economy points to mechanical entrainment or a damaged demister, not foam. If economy has drifted over weeks rather than hours, suspect fouling first.

Can I size a demister for foaming service the same as for clean service?

No. A wire-mesh demister in foaming service should be designed at the low end of the 0.8–2.0 m/s velocity window — roughly 1.0–1.3 m/s — and with extra disengagement height, because foam blankets the mesh and re-entrains collected liquid once local velocity rises. Alternatively, select a vane or cyclone separator, whose open passages tolerate foam slugs far better. Verify the separator at 1.2 times maximum evaporation rate, not just at nameplate duty.

How fast can evaporator foaming damage a product balance?

Immediately and continuously. In the worked example above, a 30 t/h evaporator with condensate at 900 mg/L against a 50 mg/L specification loses about 23.8 kg/h of dissolved solids net of the permitted baseline — roughly 0.6 t/d, or about 210 t/y, worth approximately $80,000–85,000/y at $400/t product value. The loss accrues every operating hour the foam persists, so payback on fast diagnosis is days to weeks.

Conclusion

Evaporator foaming is not mysterious once you separate the two questions it hides: what is generating stable bubbles in the liquor, and what is letting them reach the condensate. The first is chemistry — reagent residuals, salting-out, colloids, carbonates — answered by paired sampling and shake tests before anyone buys chemicals. The second is hardware — vapor velocity, disengagement height, demister condition — answered by the sizing arithmetic in this guide. Plants that respond to rising condensate TDS with paired economy data and a demister inspection, before reaching for antifoam, routinely cut carryover losses on the order of $80,000 per year back to rounding error. If your circuit is fighting foam or unexplained condensate contamination, send your liquor analysis and operating data through the request-a-quote page, and our process team will review the separator and antifoam program against your actual operating point.

References

1. U.S. Geological Survey — Mineral Resources Program, hydrometallurgy and mineral-processing research publications. https://www.usgs.gov/
2. CSIRO Mineral Resources, froth flotation and surface chemistry research. https://www.csiro.au/
3. Society for Mining, Metallurgy & Exploration (SME), SME Mineral Processing & Extractive Metallurgy Handbook, sections on flotation reagents and brine chemistry.
4. Wills, B.A. & Finch, J.A., Wills’ Mineral Processing Technology, 8th ed., chapters on flotation reagents and frothers.
5. Green, D.W. & Southard, M.Z. (eds.), Perry’s Chemical Engineers’ Handbook, 9th ed., sections on evaporation, vapor-liquid separation, and entrainment.
6. TMS (The Minerals, Metals & Materials Society) / MetSoc of CIM, proceedings on evaporation and crystallization in hydrometallurgical circuits.
7. Supplier technical data on silicone, polyether, and mineral-oil antifoams in alkaline brine systems (typical ranges, to be confirmed by test work).

Changelog

• 2026-11-06: Initial publication.