The hidden profit in galvanizing’s zinc ash: squeeze, smelt, sell

Hot‑dip galvanizing (immersion of steel in a molten zinc bath to form a corrosion‑resistant coating) is a massive zinc consumer—roughly 14 million tonnes/yr globally, with galvanizing accounting for about 50% of zinc demand (www.mdpi.com). But the process also generates “dross” or zinc ash (a mix of Zn, ZnO and iron compounds) that locks away value. Despite high zinc content—“hard zinc” dross (bottom skimmings) can be 91–96% Zn (www.researchgate.net)—about 12–15% of the zinc metal fed into a bath is lost as slag/dross (www.researchgate.net).

That waste is often classified as B3 (hazardous) due to heavy metals, but it’s also an economic resource if recovered. The core question in 2025: how much of that 12–15% can be clawed back—and can the leftover ash be sold?

Zinc ash generation and composition

In zinc‑bath galvanizing, dross forms unavoidably at the bath surface and bottom, capturing metallic zinc alongside zinc oxide and iron‑bearing compounds. Because “hard zinc” dross can reach 91–96% Zn (www.researchgate.net), it represents tied‑up working capital. With galvanizing taking about 50% of global zinc demand out of an ~14‑million‑tonne market (www.mdpi.com), the stakes are significant if 12–15% of feed becomes slag/dross (www.researchgate.net).

Mechanical separation and compression

On removal, dross is commonly crushed and sieved so larger metallic zinc globules can be separated from finer oxide‑rich ash; gravity separators or magnets then recover coarse zinc pieces, with screening of >1 mm zinc lumps yielding material that’s ~90%+ Zn (znorecyle.com). Mechanical compaction or briquetting is also used.

One further step now drawing attention is pressing semi‑molten ash to squeeze out molten zinc metal. While published, press‑specific data are scarce, the concept parallels “squeezing oil from sludge.” Laboratory filtration under high G‑force using graphite felt recovered ~93.7% of zinc from slag (www.researchgate.net), suggesting that adding mechanical force could materially lift yields. Any mechanical step reduces waste volume and concentrates zinc for later processing.

Pyrometallurgical smelting routes

Industrial practice often turns to heat. Dross is fed into diesel‑ or gas‑fired rotary or reverberatory furnaces so zinc (and ZnO) can volatilize or pool separately; under heat, metallic zinc melts or boils off to be collected as liquid metal, while iron and other impurities form slag (znorecyle.com). A study reported that melting coarse dross at 500 °C for 30 minutes recovered about 88% ±2% of zinc; lower temperature or shorter time gave less (only ~68% Zn at 480 °C/30 min), while above ~550 °C losses rose (www.researchgate.net).

In practical plants—including Melt Systems, Induction Furnaces and MZR units—recoveries range widely, often 50–90%, depending on equipment design and operating conditions (www.researchgate.net) (www.researchgate.net). Some plants even use vacuum or distillation: heating ash under vacuum vaporizes Zn, which is then condensed, yielding very pure metal but requiring tight process control (znorecyle.com). By whatever heat method, the metallic zinc is reclaimed for reuse (often returned to the galvanizing bath), while the leftover solid is a ZnO‑rich ash.

Chemical leaching and Waelz processing

Hydrometallurgical routes dissolve zinc into solution for downstream recovery. One study used acetic acid (glacial) at ~130–170 °C to convert galvanizing dross into zinc acetate; the resulting zinc acetate was ~75.4% Zn (www.researchgate.net). In practice, sulfuric or other acids can dissolve zinc, after which the solution is purified (electrowinning or precipitation) to yield Zn salts or metal. In such acid dosing regimes, precise metering can be handled by equipment like a dosing pump.

Another route is the Waelz kiln process (commonly used for electric arc furnace dust): at ~1000 °C, ZnO is reduced to Zn vapor and reconverted to ZnO; the Waelz route produces a concentrated ZnO powder suitable for pigment or rubber uses. These chemical/thermal processes typically generate recoveries on the order of 80–90+% (depending on reagent and conditions), though corrosion, residues and emissions must be managed. Where solution polishing is required, operators may deploy ion exchange systems or separate suspended solids with a clarifier before final recovery steps.

Mechanical press gains and economics

Simply melting dross can leave zinc—especially as ZnO—trapped in residual ash. Adding a hydraulic or screw press to crush the hot ash cake can [squeeze out remaining liquid metal](https://beta.co.id/en/blog/galvanizers-are-squeezing-profit-from-zinc-ash); alternately, partially melt the ash so it’s plastic, then press. Though press‑specific field data are limited, supergravity/filtration analogs have achieved >93% Zn recovery (www.researchgate.net), and the 88% figure from melting at 500 °C/30 min (www.researchgate.net) provides a baseline. Any press (or centrifugal) process must handle high temperatures and molten metal safely. The upside is direct: at ~$2,500/ton Zn, recovering an extra 5% saves ~$125 per tonne of zinc originally in the bath.

ZnO‑rich ash as marketable product

After metal recovery, the remaining solid—roughly 30–60% of the original ash mass—is typically rich in zinc oxide (ZnO) and other oxides. This ZnO‑rich ash has value in its own right. Galvanizing‑equipment vendors note that “leftover zinc ash can be sold to the zinc chemical plant” (hbannuo.en.made-in-china.com).

The global ZnO market was about $5.51 billion in 2023 and is projected to reach ~$8.51 billion by 2032 (CAGR ~5.6%), with Asia‑Pacific at ≈56% share (www.fortunebusinessinsights.com) (www.fortunebusinessinsights.com). Key applications include rubber and tires—the largest and fastest‑growing ZnO segment—ceramics and glass (glaze strengthening), paints and coatings (white pigment, UV protection and corrosion resistance), agriculture (micronutrient fertilizer, e.g., “Afox 72–75”), and other industries such as cosmetics, pharmaceuticals and electronics (www.fortunebusinessinsights.com) (www.fortunebusinessinsights.com) (www.fortunebusinessinsights.com). As one equipment supplier puts it, using a zinc ash furnace “lowers zinc consumption and enhances utilization” because the remaining ash is marketable (hbannuo.en.made-in-china.com).

Trade flows and Basel compliance

Trade data show active zinc ash markets. In 2023–2024, Indonesian exporters sent roughly 25 shipments of “zinc ash,” with India receiving 88% (~22 shipments) and South Korea the remainder (www.volza.com). Globally, China, the USA and Ukraine are also major exporters of zinc ash.

Under Basel‑Convention frameworks, zinc‑bearing galvanizing waste can be traded for recycling. For example, other countries classify Zn ash with ≥65% Zn as allowable if impurities are controlled (fr.scribd.com). In Indonesia, specific regulations on galvanizing sludge are sparse, but zinc ash would generally fall under hazardous waste (B3) rules. The Ministry of Environment (KLHK) actively encourages innovation in B3 waste reuse, stating B3 wastes “should not just be incinerated or landfilled; they are expected to be used beneficially” (ppid.menlhk.go.id). Firms can obtain permits to repurpose B3 waste streams, enabling sales of recovered Zn or ZnO‑rich ash rather than disposal.

What the numbers imply for galvanizers

Across mechanical and pyro‑ routes, reclaiming the majority of zinc—often 85–90%+ recovery (www.researchgate.net)—is feasible. Implementing a molten‑dross pressing step could lift yields further (supergravity analogs >93% suggest headroom), while the residual ZnO‑rich ash—30–60% of original ash mass—can be sold into large end‑markets. For galvanizers, especially in Indonesia, that makes a case to invest in ash‑recycling equipment (furnaces, presses, leaching units) to capture up to ~15% more zinc and monetize the remaining ash. Key performance metrics—percent Zn recovery, cost per tonne Zn saved, and revenue per tonne of ash sold—can be derived from the figures above.

With supportive policies and active export channels, transforming zinc ash into saleable materials can improve resource efficiency and profitability in a sector that already takes about half of global zinc demand (www.mdpi.com).

Sources

Metallurgical research on zinc ash processing: (www.researchgate.net) (www.researchgate.net) (www.researchgate.net); Market analyses of zinc oxide demand: (www.fortunebusinessinsights.com) (www.fortunebusinessinsights.com); Equipment supplier literature: (hbannuo.en.made-in-china.com); Trade and regulatory information: (www.volza.com) (ppid.menlhk.go.id); Process overviews: (znorecyle.com) (znorecyle.com); Basel classification: (fr.scribd.com).