Lead refining is a critical process in both primary metallurgy and secondary recycling. Whether extracted from ore or recovered from used lead-acid batteries, crude lead contains impurities such as copper, silver, antimony, arsenic, tin, and bismuth that must be removed before the metal can be used in batteries, radiation shielding, or other industrial applications. Understanding the steps involved in lead refining helps operators choose the right equipment and achieve the purity levels required by modern industry.
Overview of Lead Refining Methods
There are two principal approaches to refining lead: pyrometallurgical refining and electrolytic refining.
Pyrometallurgical refining uses heat and chemical reactions to separate impurities. It is the most widely used method, especially in facilities that process large volumes of lead from recycled batteries. This route typically involves several sequential steps in specialized furnaces and kettles.
Electrolytic refining uses an electrical current to deposit pure lead onto cathodes while impurities collect as anode slime. This method can achieve purities above 99.99% and is often selected when very high purity is required or when significant silver and gold content makes recovery of these precious metals economically attractive.
Step 1: Drossing and Initial Melting
The first step in pyrometallurgical refining is melting the crude lead and removing the easily oxidized impurities. When molten lead is heated and stirred, oxygen reacts with metals such as zinc, tin, and aluminum to form oxides known as dross. This dross floats to the surface and is skimmed off. Copper also crystallizes as the melt cools slightly below copper's freezing point, allowing it to be removed by skimming.
For recycling operations, used lead-acid batteries must first be broken and separated into their component parts. A lead acid battery breaking and separation system crushes the batteries and classifies the acid, lead paste, lead grids, and plastic. This prepares the lead paste for smelting in a furnace.
Step 2: Softening
After drossing, the lead still contains antimony, arsenic, and tin, which harden the metal. In the softening step, the molten lead is transferred to a reverberatory furnace where the temperature is raised and a blast of air is introduced. The air oxidizes antimony and arsenic, which are then skimmed from the surface. This process is called softening because it removes the elements that make lead hard, leaving a softer, more malleable product.
Step 3: Desilvering
Silver is a common impurity in lead, particularly in metal derived from certain ores. The Parkes process is the standard method for removing silver. Zinc is added to the molten lead at temperatures around 450°C. Silver has a much higher affinity for zinc than for lead, so it dissolves into the zinc phase. Because zinc-lead alloys are less dense than pure lead, the zinc-silver mixture rises to the surface and is skimmed off. The removed zinc-silver crust is then processed separately to recover both metals.
Step 4: Dezincing
After desilvering, the lead contains dissolved zinc. This is removed by vacuum dezincing or by allowing the melt to cool so that zinc-rich crystals form and can be skimmed. Alternatively, chlorine or sulfur can be used to react with and remove the remaining zinc. Effective dezincing is important because residual zinc can cause defects in battery grids and other cast lead products.
Step 5: Bismuth Removal
Bismuth is one of the most difficult impurities to remove from lead because their chemical properties are similar. Two main methods exist. The Kroll-Betterton process uses calcium and magnesium to form intermetallic compounds with bismuth, which are then skimmed off. The other approach is electrolytic refining, which effectively separates bismuth from lead by electrodeposition. For battery-grade lead, where bismuth content must be very low, electrolytic refining or careful pyrometallurgical treatment is essential.
Step 6: Final Refining in Kettle Furnaces
For many applications, particularly the production of battery-grade lead, the final refining step takes place in a lead refinery furnace or kettle. These kettles heat the lead to precise temperatures and allow for the removal of any last traces of impurities. Modern kettle furnaces with electric near-infrared heating can offer energy savings of 30–50% compared to traditional fuel-fired designs. San Lan Technologies supplies lead refinery kettle furnaces capable of refining crude lead to 99.999% purity, with options for natural gas, diesel, or electric heating.
Electrolytic Refining as an Alternative
In electrolytic refining, impure lead slabs serve as anodes while thin sheets of pure lead serve as cathodes. The electrolyte is typically a lead fluorosilicate solution with fluosilicic acid. When direct current passes through the cell, pure lead dissolves from the anode and deposits onto the cathode. Impurities either remain in solution or fall to the bottom as anode slime, which often contains valuable amounts of silver and gold. This method can produce lead with purity exceeding 99.99% and is especially suitable when precious metal recovery is a priority.
Recycling Used Lead-Acid Batteries: A Complete Flow
Because recycled lead now accounts for the majority of global lead production, the refining steps within battery recycling plants deserve special attention. Facilities that invest in integrated lead acid battery recycling equipment can achieve high recovery rates while meeting strict environmental standards. The typical flow includes:
Battery cutting and acid drainage. A used lead battery cutter slices each battery into sections and empties the sulfuric acid. Cutting speed is typically around 45 seconds per piece.
Breaking and separation. The battery pieces are crushed and separated into lead paste, lead grids, plastic, and hard rubber.
De-sulfurization. The lead paste contains lead sulfate, which has a high melting point and would generate sulfur dioxide if smelted directly. A de-sulfurization unit treats the paste to remove sulfur, lower the melting temperature, and reduce emissions.
Smelting. The de-sulfurized paste is smelted in a rotary furnace or blast furnace. Rotary furnaces are particularly effective for paste reduction, with batch capacities ranging from 2 to 20 metric tons and lead recovery rates higher than those of traditional blast furnaces.
Refining. The crude lead from smelting is transferred to a lead refinery kettle furnace for final purification to battery-grade quality.
Throughout this process, air pollution control systems capture and neutralize gases to ensure environmental compliance.
Conclusion
Lead refining involves a series of well-defined steps designed to remove specific impurities and achieve the purity required for end-use applications. From initial drossing through softening, desilvering, and final kettle refining, each stage targets particular contaminants. For secondary lead producers, integrating breaking, separation, de-sulfurization, smelting, and refining into a continuous flow maximizes recovery rates and minimizes environmental impact. Selecting reliable lead acid battery recycling equipment and refining machinery is essential for any operation aiming to produce high-purity lead efficiently and sustainably.
