Crude lead bullion coming out of a smelting furnace is almost never pure enough to sell directly. Whether your plant runs a blast furnace, a rotary furnace for paste reduction, or a complete lead acid battery recycling line, the metal leaving the furnace still carries copper, antimony, arsenic, tin, silver and bismuth. Among these, antimony is usually the most stubborn — and the most important to remove before the lead can be used in quality battery production. This article looks at how lead refining removes antimony and other impurities step by step, and what that means for a recycling business.
Why antimony matters more than most impurities
Antimony in lead comes mainly from the grid alloy inside scrap batteries. When scrap battery plates are smelted, the antimony that was deliberately added to strengthen the grid stays in the metal and generally shows up at levels around 0.5% to 2.5% in crude bullion. A little antimony hardens lead, which is useful for deep-cycle applications, but it ruins the properties of pure battery lead. Battery-grade specifications commonly demand antimony below 10 ppm for pure lead grades, and manufacturers also watch copper, arsenic, tin, bismuth and zinc tightly. In short, refining is what turns a low-value crude metal into a premium product.
The equipment: where refining actually happens
Most secondary lead refining is carried out in a lead refinery kettle furnace, a refractory-lined vessel that holds molten lead while different reagents are stirred in and impurity-rich dross is skimmed off the surface. The kettle is the workhorse of the whole process because it combines heating, stirring and skimming in one controlled environment. Modern lead refinery machines are available in natural gas, diesel or electric heated versions, and electrically heated kettles using near-infrared heating can save 30-50% on energy while giving tight temperature control — an important point because every refining step depends on holding the melt in a specific temperature window.
The step-by-step refining sequence
Impurities cannot all be removed with one treatment. Each one needs a particular chemistry, so typical fire refining runs through a sequence of steps. For crude lead containing around 0.8% antimony and 0.15% copper, the full cycle looks like this:
- Copper removal (drossing). The melt is held at 400-450°C in an oxidizing atmosphere. Copper forms a solid copper oxide that floats to the surface and is skimmed off as dross, bringing copper from about 0.15% down to below 0.005%.
- Antimony removal (softening). The temperature rises to 750-850°C and elemental sulfur or sodium sulfide is added. The antimony reacts to form antimony sulfide dross that is skimmed away, cutting antimony from around 0.8% to below 0.01%.
- Arsenic and tin removal. Air or oxygen is blown through the melt at 650-750°C. Both metals oxidize and are removed as dross, usually down to below 0.005% each.
- Final polishing. Caustic soda is added at 550-650°C to take out residual sulfur and trace metals and to improve the flow of the metal for casting.
A full batch normally takes between 6 and 10 hours, depending on how dirty the feedstock is and how high the final purity needs to be.
Looking closer at antimony: the softening reaction
Antimony removal is sometimes called softening, because taking antimony out makes the lead softer. The key reaction is simple: 2Sb + 3S → Sb₂S₃. Elemental sulfur reacts with dissolved antimony to form antimony sulfide, which rises to the surface as dross and is skimmed off. In practice a single softening cycle typically removes 95-98% of the antimony, and a melt holding 2.0% antimony needs about 8-16 kg of sulfur per tonne of lead over 3-4 hours to reach below 0.01%. The antimony-rich dross recovered this way is not wasted — it can hold 40-60% antimony and is sold on to antimony refiners, which is a nice revenue bonus on top of the cleaner lead.
Temperature control is everything here. Around 700°C the sulfur-antimony reaction is too slow to be economical, while above 850-900°C lead itself starts to vaporize and is lost. That is why a stable lead refinery furnace with reliable heating and good insulation matters: a kettle that swings in temperature wastes either reagent or metal.
Removing the other impurities
Beyond antimony, a complete refining line has to deal with:
- Copper — taken out first by oxidative drossing, because if it stays in the melt it interferes with every later step.
- Arsenic, tin and tellurium — typically removed together using the Harris process, where fused sodium hydroxide (NaOH) and a controlled dose of sodium nitrate react to pull these elements into the dross.
- Silver — recovered with the classic Parkes process, in which zinc is added and the silver rises into a silver-zinc crust that is skimmed off.
- Bismuth — handled with additions of calcium and magnesium when it has to be kept very low.
- Zinc and magnesium — if present, these can be removed by vacuum refining, which evaporates the volatile metals off the lead under low pressure.
What this means for a recycling plant
Good refining turns otherwise saleable-at-a-discount crude lead into battery-grade metal that commands a much better price, and it unlocks a secondary income from antimony and silver-rich dross. The economics make sense at almost any scale: even a plant handling ten thousand tonnes a year can add substantial value simply by upgrading its bullion. If your project is a new lead acid battery recycling line or an expansion of an existing smelting operation, choosing the right lead refining kettle and planning the refining sequence around your actual feedstock is the fastest way to raise output value and meet battery manufacturer specifications.
