ICP-OES Protocol for Antimony Ore
designed for routine Sb-ore grading at 0.5 – 60 % Sb and trace-metal control
ICP-OES stands for “Inductively Coupled Plasma – Optical Emission Spectrometry.”
Inductively coupled plasma (ICP): a very hot (≈ 8,000 °C) argon plasma that atomizes and excites the elements in a digested sample solution.
Optical emission spectrometry (OES): the instrument measures the characteristic light (emission lines) each excited element gives off and converts that intensity into a concentration.
Together, ICP-OES is a multi-element technique widely used for rapid, parts-per-million (or lower) chemical analysis of metals, ores, waters, and many other materials.
1 Purpose of the method
Quantify major antimony (Sb) content to ±2 % relative accuracy.
Screen for associated penalty or credit elements (As, Pb, Zn, Fe, Au, Ag) in one run.
Provide data suitable for payability formulas, process metallurgy, and environmental compliance.
2 Recommended instrumentation
| Item | Minimum specification |
|---|---|
| ICP-OES | Radial or dual-view plasma, 1.2 kW RF, argon flow 15 L min⁻¹ |
| Nebuliser | Concentric glass (0.8 mL min⁻¹) or HF-resistant Mira Mist |
| Wavelengths | Sb 206.834 nm (primary), 217.582 nm (confirmation); As 188.979 nm, Pb 220.353 nm, Zn 213.857 nm, Fe 238.204 nm |
| Internal standard | Yttrium 371.030 nm (matrix-blank in ore) |
3 Sample preparation
Crush & pulverise – to <75 µm (200-mesh) in a tungsten-carbide or agate mill.
Homogenise – riffle-split to obtain a 0.5 g analytical charge.
Dry – 105 °C, 1 h; cool in desiccator.
4 Digestion options
| Route | When to use | Acid recipe (per 0.5 g) | Heating |
|---|---|---|---|
| Aqua regia (AR) | Oxide stibnite ores, minimal silica | 6 mL HCl (37 %) + 2 mL HNO₃ (70 %) | 95 °C block, 1 h |
| HF-assisted AR | Refractory quartz-rich or roasted feed | 5 mL HCl + 2 mL HNO₃ + 1 mL HF (48 %) | Closed-vessel microwave, 200 °C, 30 min |
| Four-acid “total” | Assay-lab referee work | 3 mL HCl + 2 mL HNO₃ + 2 mL HF + 1 mL HClO₄ | Hot plate to fumes off HClO₄, cool, re-take to 100 mL with 2 % HCl |
Neutralise HF digests with saturated H₃BO₃ before final dilution to protect glassware.
Final solution: 100 mL in 2 % v/v HCl carrier matrix (Sb stable for ≥24 h).
5 Calibration and quality control
| Step | Details |
|---|---|
| External standards | 0, 1, 5, 10, 25, 50 mg L⁻¹ Sb in 2 % HCl; include matrix-match (2 % HCl + 0.5 % HF if HF route used). |
| Internal standard spike | 5 mg L⁻¹ Y added to all standards, blanks, samples. |
| Certified reference material (CRM) | NCS DC73314 (antimony concentrate, 56.2 % Sb) or equivalent; analyse every 15 unknowns—accept if within ±3 % of certificate. |
| Method blank | Full acid suite without sample; Sb < 0.02 mg L⁻¹ requirement. |
| Duplicate pulp | 1 in 20; RPD ≤ 5 %. |
6 Instrument run parameters (typical)
RF power 1250 W
Plasma gas 15 L/min Ar
Aux gas 1.5 L/min Ar
Nebuliser gas 0.65 L/min Ar
Pump rate 1.0 mL/min
Read delay 30 s
Replicates 3 (2 s integration each)
Background 0.1 nm off-peak correction both sides
7 Detection limits & linearity
| Element | DL (mg kg⁻¹ in ore) | Working range (solution) | Notes |
|---|---|---|---|
| Sb | 5 | 0.5 – 300 mg L⁻¹ | Linear to 300 mg L⁻¹ (≈30 % Sb in ore at 100× dilution) |
| As | 2 | 0.2 – 50 mg L⁻¹ | Hydride elements—verify no over-reduction |
| Pb | 1 | 0.1 – 40 mg L⁻¹ | |
| Zn | 0.5 | 0.05 – 30 mg L⁻¹ | |
| Fe | 1 | 0.1 – 100 mg L⁻¹ |
If Sb peak exceeds calibration, auto-dilute 10× and re-run.
8 Reporting format (example)
Antimony (Sb) 48.3 % ± 0.9 (ICP-OES, HF-AR digestion)
Arsenic (As) 0.12 %
Lead (Pb) 0.04 %
Zinc (Zn) 0.03 %
Iron (Fe) 0.61 %
Reporting basis: dry weight, 105 °C.
DL: Sb 0.005 %, others see table.
9 Common pitfalls & fixes
| Symptom | Likely cause | Remedy |
|---|---|---|
| Low Sb recovery in CRM | Incomplete digestion of stibnite needles | Add HF or repeat four-acid route; extend microwave hold. |
| Drift high on Sb | Cone build-up (Sb₂O₃) | Flush with 5 % HCl every 30 samples; clean torch daily. |
| Matrix suppression | High Fe/Si in silica-rich ore | Use Y internal standard and matrix-matched calibration; switch to axial view if available. |
10 Key take-aways
ICP-OES delivers simultaneous Sb and impurity metals with sub-ppm detection when digestion is complete.
HF-assisted aqua regia is the best balance between safety and completeness for quartz-bearing antimony ores.
Always document digestion route, wavelengths, DLs and QC performance so payability disputes can be settled unambiguously.
With this protocol your lab can turn around robust Sb assays—traceable, reproducible and ready for contract negotiations or metallurgical modelling.