Electrode paste cylinders (Söderberg paste)

01

Choose the paste form and match its baking behaviour

Elkem describes Söderberg electrode paste as a carbon material made in cylinders or briquettes, and offers its ELSEP paste as cylinders in a range of diameters, rectangular blocks and briquettes, with formulations for slag-forming processes (pig iron, ferrochrome, manganese alloys, ferronickel, platinum matte), slag-free processes (ferrosilicon, silicon, calcium carbide) and high-purity ferrosilicon. The paste is filled into a vertical cylindrical steel casing, which Elkem gives as 1,200–2,000 mm in diameter. Above 500 °C the binder cokes and the paste bakes into a solid carbon electrode, reaching full strength at approximately 900 °C; the electrode is consumed at the tip and slipped down, with new paste added at the top. [1]

A North-West University dissertation on Söderberg paste raw materials states that calcined anthracite is crushed and screened to sizes from under 1 mm to approximately 15 mm, mixed with tar-pitch binder and cast into moulds, and that paste cylinders of 0.4 m to 1 m diameter are the main product, with briquettes and blocks also made. It defines plasticity as the percentage increase in diameter of a small paste cylinder heated under standardised conditions: a binder content of 30–40% gives a high-plasticity paste that flows easily, while 15–25% gives a dry, low-plasticity paste. It cites the paste softening and filling the casing at about 50 to 100 °C and a baking zone of about 400 to 500 °C, and warns that a baking zone that drops below the contact clamps can cause a soft (green) break. [2]

Rongxing, a Chinese paste maker, recommends cylindrical paste because it transfers its full weight to the bottom of the casing, unlike small blocks or egg-shaped paste, giving higher density and strength after baking and avoiding paste hanging in DC furnaces. It lists egg briquettes of 13 × 8 × 5 cm and trapezoid briquettes 7.5 cm high weighing 0.7 kg, with cylinders made to order. [4] [3]

Editorial calculation: one metre of baked electrode in a 1,200 mm casing holds π × 0.6² ≈ 1.13 m³ of carbon, or about 1.63–1.74 t at a baked apparent density of 1.44–1.54 kg/dm³; in a 2,000 mm casing, π × 1.0² ≈ 3.14 m³, or about 4.52–4.84 t per metre. Use your slipping rate and density data to size paste stock and deliveries. [1] [3]

Give the supplier the furnace type and product (alloy), AC or DC, open or closed furnace, electrode diameter and casing design, current and slipping rate, paste form and cylinder diameter, charging method, plasticity range, the paste and baked-property limits with test methods, packaging, climate during transport and storage, and monthly quantity.

02

Set limits for ash, volatile matter and baked properties

Rongxing's electrode paste page lists ash content of 2–10%, electrical resistivity of <55–90 uΩm, volatile matter of 10.0–15.0, density above 1.46 g/cm³, compressive strength above 18 MPa and ductility of 6–20, made from electrically calcined coal, calcined coke, graphite scraps, asphalt and coal tar. [3]

The same page reproduces a customer specification with Rongxing's test values for cylinders and briquettes: green apparent density 1.572–1.666 kg/dm³ (ISO 12985-1); baking loss 10–12% and baking shrinkage 3.9–6.5%; after baking at 1000 °C, apparent density 1.44–1.54 kg/dm³, total porosity 18.4–23.1%, specific electrical resistance 58–72 μΩm (ISO 11713), flexural strength 4–8 MPa (ISO 12986-1), compressive strength 18–35 MPa (ISO 18515), coefficient of thermal expansion 3–4 × 10⁻⁶/K (ISO 14420), thermal conductivity 3–6 W/mK (ISO 12987), real density 1.84–2.04 kg/dm³ (ISO 9088) and ash 0–5% (ISO 8005). Rongxing reports 66 and 68 μΩm, 20.0 and 25 MPa, 1.53 and 1.56 kg/dm³ and 2.1 and 2.3% ash for its two forms. [3]

Rongxing notes that lower resistance can cost strength, that volatile matter that is too high can cause under-baking and too low over-baking, and that parameters must be matched to furnace conditions. [4] The Roos dissertation adds that producers measure similar paste properties by differing methods, so a specification should name the test method for each value. [2]

In China, YB/T 5215-2015 Electrode paste, issued 2015-07-14 and effective 2016-01-01, replaces YB/T 5215-1996 and covers classification, technical requirements, test methods, inspection, packaging, labelling, storage, transport and quality certificates for Söderberg paste used in closed, semi-closed and open submerged arc furnaces; its grade limits were not available in the reviewed summary. [5]

03

Data sheets, lot certificates and furnace references from paste makers

When requesting quotations, ask each paste maker for its grade data sheet with test methods, lot test certificates, paste form and cylinder sizes, raw-material sources (calcined anthracite, pitch), plasticity results, packaging and storage advice, and references on furnaces of your type.

04

Trace the paste and electrode

05

Ash, volatiles, resistivity and baked strength with their ISO methods

Rongxing's reproduced customer specification supplies the ISO methods named here; cylinder length is the one field no reviewed source gives.

06

Send the furnace, electrode and paste limits together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Furnace: [alloy; AC or DC; open or closed]
Electrode: [diameter; casing; current; slipping rate]
Paste form: [cylinder diameter and length; block; briquette]
Paste limits: [ash; volatile matter; plasticity]
Baked limits: [resistivity; compressive strength; density; methods]
Supply: [packaging; storage climate; tonnes per month]

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