Lead, a heavy and ancient metal, has played a complex role in human civilization. It is the core
of the batteries that keep electricity safe, a reliable barrier for soundproofing buildings, and a
formidable and highly toxic substance. Lead's utility is closely intertwined with its dangers, and
dissolving lead - converting solid lead or its compounds into ions in solution - is the central key
to unlocking the recycling of lead resources and realizing safe disposal and regeneration of value.
This is not only a key step in the metallurgical process, but also a sophisticated technological
game to fight pollution and manage danger.
Lead dissolution: the key leap from solid to ionic
The dissolution of lead is essentially the use of a specific chemical medium to break the metallic bond
or compound lattice structure of lead, so that it enters the solution in an ionic form (e.g. Pb²⁺). This
process serves two core objectives:
Dissolution of metallic lead (the core of recycled lead): Mainly used to recycle used lead-acid batteries,
lead alloys, lead materials, etc. It is the lifeblood of the recycled lead industry. Solid metallic lead needs
to be converted into solution for deep purification.
Dissolution of lead-containing materials/wastes (environmental protection and resource utilization):
Handling of smelting soot, lead-containing sludge, contaminated soil, electronic wastes, etc., with the
aim of safely separating lead and achieving harmless disposal or valuable metal recovery.
Dissolution of metallic lead: the cornerstone of recycled lead
For solid metallic lead (e.g. battery grids, lead lumps), the main routes are electrolytic refining pre-treatment
or wet direct recovery:
Dissolution prelude to pyro-refining (molten lead removal):
Waste lead material is first melted at high temperature in a reflector furnace, short kiln or rotary kiln.
This process is not a “dissolution” per se, but is an important basis for subsequent wet dissolution.
In the molten state, by adding flux (such as caustic soda, sodium nitrate, coke) for oxidation slagging or
reduction, to remove most of the tin, antimony, arsenic, copper and other impurities, to get a higher purity
of crude lead.
This crude lead can be cast into anode plates and enter the electrolytic refining process. In the electrolyte
composed of lead silicofluoride (PbSiF6) and free silicofluoride (H2SiF6), the lead on the anode plate undergoes
dissolution (Pb -> Pb²⁺ + 2e-), which is the key dissolution process of metallic lead in the electrolytic system,
and the dissolved lead ions are deposited in the cathode to obtain high-purity refined lead.
Wet direct dissolution and recovery (emerging trend):
Fluoroboric acid system (HBF4): Lead fluoborate solution (Pb(BF4)2) has good conductivity and stability, can
directly dissolve metallic lead (Pb + 4HBF4 -> H2PbF6 + H2↑?). Need to be corrected, the actual reaction is
Pb + 2H+ -> Pb²⁺ + H2↑, in fluoroborate medium). The dissolved solution can be used directly for electrodeposition
of fine lead. The system is efficient, but there are environmental challenges with fluoride handling.
Alkaline dissolution (minor limitations): Strong bases (e.g., NaOH) can dissolve lead under certain conditions
to form leadites (e.g., Na2PbO2), but the reaction is slow, inefficient, and impurities are difficult to isolate,
limiting applications.
Dissolution of lead-containing materials/wastes: the core battlefield
of environmental protection attack
Treatment of complex lead-containing wastes (non-pure metals), the goal is to leach lead efficiently and selectively,
separate it from other impurities, and facilitate subsequent recovery or stabilization:
Acid leaching (mainstream and highly efficient)
Reaction: PbO + 2CH3COOH -> (CH3COO)2Pb + H2O.
Advantages: relatively mild, good selectivity (especially for calcium-containing wastes), easy crystallization and
recovery of lead acetate.
Challenges: slow reaction, high cost, more suitable for specific scenarios (e.g. leaded glass, ceramic lead removal).
Reaction: 3Pb + 8HNO3 (dilute) -> 3Pb(NO3)2 + 2NO↑ + 4H2O; PbO + 2HNO3 -> Pb(NO3)2 + H2O.
Advantages: high solubility, lead nitrate [Pb(NO3)2] is extremely soluble and not easy to block.
Challenges: high cost, produces nitrogen oxides (NOx) requiring strict treatment, nitrate wastewater is difficult
to treat, easy to introduce impurities to dissolve.
Reaction: PbO + 2HCl -> PbCl2 + H2O; PbCO3 + 2HCl -> PbCl2 + CO2↑ + H2O; PbSO4 + 2NaCl -> PbCl2 + Na2SO4
(need to be promoted by chlorine ion).
Advantage: fast dissolution, good effect on oxidized lead (PbO, PbCO3, PbSO4), relatively simple equipment.
Challenges: Limited solubility of lead chloride (PbCl2) (especially at low temperatures, easy to crystallize and clog),
and the introduction of chloride ions increases the difficulty of subsequent wastewater treatment. Need to control
the acid concentration, temperature, redox potential (such as to avoid generating PbCl2 colloid).
Hydrochloric acid (HCl) system:
Nitric acid (HNO3) system:
Acetic acid (CH3COOH) system:
Alkaline immersion method (selective dissolution):
Principle: Dissolution of amphoteric oxides (PbO) to form lead acid salts (Na2PbO2) or lead-ammonia
complexes ([Pb(NH3)4]²⁺).
Advantages: less dissolution of impurities such as iron, calcium and silicon, better selectivity, less equipment corrosion.
Challenges: low dissolution efficiency relative to acid method, poor dissolution of PbSO4, PbS, ammonia volatility
needs to be controlled.
Sodium hydroxide (NaOH) or ammonia (NH3-H2O) system:
Complex leaching (enhance efficiency and selectivity):
Add complexing agent (such as EDTA, citrate, tartrate, etc.) in acid or alkali leaching solution to form stable soluble
complexes with lead ions.
Function: Significantly improve the solubility and dissolution rate of insoluble lead compounds (e.g. PbSO4, PbCrO4);
enhance the selectivity of lead, and reduce the co-solubilization of impurities.
Application: Especially suitable for the remediation of low concentration, difficult to treat lead-containing contaminated
soil or sludge.
Key: complexing agent cost, recycling and harmless decomposition of the final complex are challenges.
The Toxic Shadow Follows: Hazard Control of Dissolution Processes
Lead dissolution process, especially acid leaching, is a high-risk link for toxicity release, and safety protection and pollution
control are the lifeline:
Confinement and Negative Pressure: The leaching reactor and solution conveying pipeline are strictly confined, and
negative pressure is maintained in the operation area to prevent lead dust, acid mist and lead vapor from escaping.
Efficient filtration and washing: The solid-liquid separation after leaching (press filtration, centrifugation) should be
efficient, the residue should be washed sufficiently to reduce entrainment, and the washing water should be recycled.
Exhaust gas treatment: Acid mist (HCl, HNO3) generated by acid leaching is absorbed by lye (NaOH) spraying; special
adsorption/filtration devices are required for possible volatile lead compounds.
Wastewater “zero discharge” closed loop:
Neutralization and precipitation: Adding alkali (lime, NaOH) to convert dissolved lead into low solubility hydroxide
(Pb(OH)2) or carbonate (PbCO3) precipitation to remove, is the core means.
In-depth treatment: Combination of sulfidation (generating extremely insoluble PbS), ion exchange, adsorption (activated
carbon, special adsorbents), membrane technology (RO, ED) to ensure compliance with standards for reuse or discharge.
Sludge stabilization: Lead-containing precipitated sludge should be stabilized/cured (cement curing, pharmaceutical
stabilization) to meet the standards for safe landfill or resource utilization.
Personnel protection: Strictly wear protective clothing, gas masks (against lead dust/acid mist), gloves; regular blood lead
monitoring; mandatory workshop ventilation.
After Dissolution: The Road to Rebirth of Lead Ions
Dissolution is only the first step, the subsequent treatment of lead ions determines the final value and safety:
Precipitation Recovery:
Neutralization precipitation: Generation of Pb(OH)2 or PbCO3 is the most common method, and the product can
be sent to pyrometallurgical smelting.
Sulfidation precipitation: adding Na2S and other sulfiding agents, generating extremely insoluble PbS (Pb²⁺ + S²- -> PbS↓),
precipitation purity is high, can be used directly as lead concentrate or sent to smelting. It is especially suitable for treating
low concentration of lead wastewater.
Crystallization method: Suitable for high concentration system (such as lead nitrate, lead acetate solution), through cooling,
concentration and crystallization to recover salt products.
Electrowinning recovery (high value):
The purified lead ion solution (e.g. lead silicofluoride, lead fluoborate or purified leach solution) can be electrolytically
deposited at the cathode to directly obtain high purity metallic lead (Pb²⁺ + 2e- -> Pb). This is a key step in the wet
process of regenerated lead to obtain the final product.
Resource utilization:
Recovered lead compounds (e.g. PbO, PbSO4, PbCO3) can be used directly as chemical raw materials (pigments,
stabilizers, glass ceramics).
Leaded waste residues (e.g., solidified bodies) after treatment to standards can be used for building materials
(alternative aggregates) or road base materials under strict supervision.
Cutting Edge of Innovation: Cleaner, Smarter Dissolution
Lead dissolution technology continues to evolve to meet the challenges of both environmental protection and
efficiency:
Green leaching agent development: Finding efficient, low-toxicity, easily degradable bio-acids or organic acids to
replace strong acids and bases.
Enhanced leaching technology: ultrasonic and microwave-assisted leaching to improve efficiency; electrochemical
dissolution (anodic oxidation) to improve selectivity.
Complexing agent regeneration and degradation: research and development of recyclable or easily biodegradable
complexing agent.
Online monitoring and intelligent control: real-time monitoring of solution pH, potential, lead ion concentration, key
impurities, automatic adjustment of dosage and process parameters, optimization of dissolution efficiency and
impurity suppression, reducing pharmaceutical consumption and the risk of secondary pollution.
Integration of short-flow process: Develop “leaching-purification-electrowinning” integrated high-efficiency short-flow
process, reduce intermediate links, reduce risks and costs.
Conclusion
Dissolving lead is not a simple chemical reaction, but a precise systematic project integrating chemical engineering,
environmental science and safety management. It is the necessary way to transform hazardous waste into valuable
resources, the solid starting point of the recycling chain of the renewable lead industry, and the key technical fortress
to control lead pollution and guard the ecological environment. From the exploration of highly efficient selective
leaching agent, to the precise capture and purification of lead ions in the dissolution solution, to the strict defense
of the “toxic” factors in the whole process, every step of the process is cohesive with the wisdom of technology and
responsibility. To harness the power of lead dissolution is to harness its two-sided nature - to release its industrial
value while firmly locking its environmental toxicity, so that this ancient metal can find a safe and long-lasting foothold
in the sustainable development of modern civilization. The road to lead's rebirth begins with subtle dissolution
and ends with the reshaping of its value and the zeroing of its risks.