Copper Raffinate Storage & Tank Compatibility
Storing Copper Raffinate? Start Here
<p><strong>Copper raffinate</strong> is the copper-depleted aqueous stream that leaves the solvent-extraction (SX) circuit of a copper SX-EW (solvent extraction–electrowinning) operation. After pregnant leach solution (PLS) gives up its copper to an organic oxime extractant, the leftover aqueous phase — the raffinate — is recycled back to the heap or leach pad. It is essentially a <strong>dilute sulfuric-acid brine</strong> carrying only trace copper (commonly under 0.5 g/L Cu) along with dissolved iron, aluminum and magnesium sulfates, and a small amount of entrained organic (oxime in a kerosene-type diluent).</p><p>Because near-equilibrium SX exchanges acid for copper, raffinate is typically more acidic than the PLS — on the order of 5–15 g/L H<sub>2</sub>SO<sub>4</sub> at a pH around 1.5–2.5, sometimes re-fortified with acid before it returns to the heap. Material of construction matters because this low-pH, sulfate-laden, chloride-bearing liquor is aggressive to bare metals but well within the comfort zone of the right plastics, which is why raffinate ponds, day tanks and launders are dominated by polyethylene and FRP.</p>Polyethylene (HDPE / XLPE) Compatibility with Copper Raffinate
<p><strong>Verdict: Compatible (S).</strong> Copper raffinate's dominant chemistry is dilute sulfuric acid plus dissolved metal sulfates, and polyethylene resists sulfuric acid well across the dilute-to-moderate range (rated excellent to roughly 50% H<sub>2</sub>SO<sub>4</sub> at ambient temperature). Copper sulfate and the other metal-sulfate salts are aqueous and benign to poly. HDPE and crosslinked polyethylene (XLPE) are the workhorse materials for raffinate storage, day tanks and transfer in SX-EW plants.</p><p>The one honest caveat is the <em>trace entrained organic</em> — oxime extractant carried in a kerosene-type diluent. As a tiny, emulsified fraction of the aqueous raffinate it is far too dilute to swell or soften polyethylene in normal service. If a stream is unusually organic-rich (e.g., upstream of a coalescer) or runs warm, confirm against your supplier's resistance data and consider XLPE for the extra wall integrity. For the standard dilute-acid raffinate duty, polyethylene is the correct, proven choice.</p>Material compatibility at a glance
Copper raffinate is a dilute, strongly acidic copper-sulfate brine, so the controlling factor is dilute sulfuric acid plus dissolved metal sulfates — not an organic solvent or oxidizer. Polyethylene (HDPE/XLPE), PP, PVC/CPVC and vinyl-ester FRP all handle this duty and are the industry-standard materials for raffinate ponds, tanks and piping. Metals (carbon steel especially, and to a lesser degree 316 SS) are the weak point because of the low pH and possible residual chloride. Match elastomer seals to both the acidity and any entrained SX organic.
| Material | Rating | Note |
|---|---|---|
| HDPE / XLPE | S | Dominant driver is dilute sulfuric acid, which polyethylene resists well to ~50% at ambient temperature; standard choice for raffinate ponds, day tanks and transfer. Trace entrained organic is too dilute to swell poly in practice. |
| Polypropylene (PP) | S | Resists dilute sulfuric acid and copper/metal sulfates; common for pumps, fittings and small vessels. |
| PVC / CPVC | S | Widely used for raffinate and PLS piping; resists dilute H<sub>2</sub>SO<sub>4</sub> and metal-sulfate brines. |
| FRP (vinyl-ester lined) | S | Standard for large raffinate/PLS launders and tanks; vinyl-ester resin handles the acidity and any organic carry-over. |
| 316 Stainless Steel | C | Pitting/crevice risk from low pH plus any residual chloride; often avoided in favor of lined steel or non-metallics. |
| Carbon Steel (unlined) | U | Rapidly corroded by dilute sulfuric acid; use only with acid-resistant rubber/FRP lining. |
| EPDM elastomer | S | Good for gaskets/seals in dilute acid service; verify against any entrained organic. |
| Viton (FKM) | C | Good acid/organic resistance but verify against the specific oxime/diluent; EPDM often preferred for the dilute-acid duty. |
Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.
The safety that actually matters
<ul><li><strong>Acidic liquid (pH ~1.5–2.5):</strong> dilute sulfuric acid causes skin and eye irritation; wear chemical goggles, face shield, acid-resistant gloves and apron.</li><li><strong>Eye hazard:</strong> splashes can cause serious eye irritation — keep eyewash/safety shower accessible at transfer and sampling points.</li><li><strong>Residual copper and metals:</strong> harmful to aquatic life; contain spills and prevent discharge to storm drains or surface water.</li><li><strong>Entrained organic / mist:</strong> trace oxime-in-diluent can be irritating; provide ventilation and avoid generating aerosols.</li><li><strong>Do not mix with bases or strong oxidizers</strong> without controls — neutralization is exothermic and can liberate heat/gas.</li><li><strong>Always defer to the site SDS:</strong> exact acidity, metal content and entrained-organic load vary by ore body and circuit — the operating facility's SDS is the authority.</li></ul>Common questions
- Can I store copper raffinate in a polyethylene (HDPE or XLPE) tank?
- Yes. Raffinate is essentially a dilute sulfuric-acid brine with trace copper, and polyethylene resists dilute-to-moderate sulfuric acid and metal sulfates very well. HDPE and XLPE are the industry-standard materials for raffinate ponds, day tanks and transfer in SX-EW plants. Confirm temperature and any unusually high entrained-organic load against supplier data.
- What makes copper raffinate corrosive — the copper or the acid?
- The acid. Raffinate carries only trace copper (typically under 0.5 g/L Cu); its aggressiveness comes from the residual sulfuric acid (commonly ~5–15 g/L H<sub>2</sub>SO<sub>4</sub>, pH ~1.5–2.5) and any chloride, which attack bare carbon steel and can pit stainless. Plastics like polyethylene and FRP are unaffected by this dilute-acid chemistry.
- Is the entrained SX organic a problem for plastic tanks?
- In normal service, no. The oxime extractant and kerosene-type diluent are present only as a tiny entrained/emulsified fraction — far too dilute to swell or soften polyethylene. Only an unusually organic-rich or warm stream warrants extra caution; in that case verify against supplier resistance data or step up to XLPE.
- What materials should I avoid for copper raffinate service?
- Unlined carbon steel is the main one — dilute sulfuric acid corrodes it quickly. 316 stainless is conditionally acceptable but risks pitting from low pH plus residual chloride. Favor polyethylene (HDPE/XLPE), polypropylene, PVC/CPVC and vinyl-ester FRP, with EPDM elastomer seals for the dilute-acid duty.
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Sources & References
All compatibility ratings, hazard classifications, and chemical identifiers on this page are sourced from authoritative third-party publications. Verify against the original references before final specification.
- NFPA 704: Standard System for the Identification of the Hazards of Materials for Emergency Response — Defines the health/flammability/reactivity diamond used to characterize the representative hazard profile of this aqueous acidic stream; no standardized diamond exists for the process stream, so the rating is inferred and SDS-dependent. www.nfpa.org
- UN GHS — Globally Harmonized System of Classification and Labelling of Chemicals (Rev.) — Source for the H-code/pictogram framework; the representative H315/H319/H412 classification reflects residual acidity and metal content and must be confirmed against the operating facility's SDS. unece.org
- HDPE vs Sulfuric Acid — Chemical Resistance — Polyethylene resistance reference: HDPE rated excellent for sulfuric acid in the dilute-to-moderate range (to ~50% at ambient temperature), supporting the Compatible (S) verdict for dilute-acid raffinate. chemicalresistance.org
- Solvent extraction and electrowinning (SX-EW) — overview — Describes the SX-EW process in which raffinate is the copper-depleted aqueous phase recycled to the leach after stripping copper into the organic extractant. en.wikipedia.org
- Hydrometallurgical process for copper-containing materials (US Patent 5,698,170) — Formulation-specific source: states raffinate typically contains under ~0.10 g/L copper and ~5–15 g/L sulfuric acid, with neutralized raffinate targeting pH ~2.5–2.8. image-ppubs.uspto.gov
- Impact of entrained and dissolved organic chemicals associated with copper solvent extraction (ScienceDirect) — Documents that oxime extractant and kerosene-type diluent are carried into raffinate as trace entrained/emulsified organic — the basis for the minor compatibility caveat. www.sciencedirect.com
- Copper Leach / Solvent Extraction / Electrowinning Training (911 Metallurgist) — Industry training reference describing extractant chemistry, raffinate recycle and the dilute sulfuric-acid leach environment that governs material selection. www.911metallurgist.com