Non-ferrous metal smelting, arsenic ore mining and processing, fine chemical manufacturing, and pesticide production generate large quantities of highly toxic arsenic-containing industrial waste residue. In such residues, arsenic predominantly exists as trivalent arsenic (As³⁺), pentavalent arsenic (As⁵⁺), free ions, and weakly bound active species, which are highly mobile. When stored in open piles or simple landfills, these residues can easily leach into soil and groundwater under rainfall, causing regional heavy metal pollution. Arsenic compounds are also highly toxic and carcinogenic, posing direct threats to workers and nearby residents. Under current solid waste control standards, arsenic leaching toxicity is a key indicator for the compliant transport, safe landfill, and co-processing of arsenic-containing waste residues. Conventional treatment processes, such as simple iron salt passivation, sulfide precipitation, or lime stabilization, often suffer from low stabilization efficiency, poor long-term leaching resistance, high chemical consumption, and weak adaptability to different operating conditions. These limitations make them unsuitable for the large-scale harmless treatment of high-concentration, complex arsenic-containing residues. By combining the in-situ adsorption and passivation properties of ferrous sulfate with the deep sulfide precipitation properties of sodium sulfide, a dual-agent integrated treatment system has been developed. This system targets and immobilizes active arsenic fractions, forming dense, low-solubility composite mineral phases. It offers low cost, ease of operation, and long-term safety, and has become a mainstream industrial technology for the terminal harmless treatment of arsenic-containing waste residues.
Core Reaction Mechanisms and Synergistic Effects
The stepwise addition of ferrous sulfate and sodium sulfide, with controlled reaction sequencing, is not a simple combination of agents. Instead, it forms a closed-loop synergistic reaction system consisting of “pre-adsorption passivation — deep sulfide arsenic fixation — composite encapsulation locking.” The entire process operates under normal temperature and pressure, requiring no additional energy input for heating or pressurization, and is suitable for full-scale production lines.
1.1 Core Mechanism of Ferrous Sulfate Pre-treatment Passivation
In a weakly acidic to neutral controlled slurry environment, ferrous sulfate dissociates to release ferrous ions (Fe²⁺). These ions rapidly undergo in-situ oxidation and hydrolysis, generating amorphous ferric hydroxide colloids with high specific surface area and strong adsorption activity. Simultaneously, they capture free arsenate (As⁵⁺), arsenite (As³⁺), and colloid-bound arsenic in the slurry, forming low-solubility ferric arsenate as a primary stable precipitate. The ferric hydroxide colloid, with its porous network structure, strongly adsorbs and encapsulates weakly bound active arsenic on the residue surface, while also encapsulating some soluble heavy metal impurities. This prevents initial arsenic leaching and migration. The iron salt pre-treatment also adjusts the slurry’s acid-base buffering capacity, optimizing conditions for subsequent sulfide treatment, inhibiting unwanted side reactions of sodium sulfide hydrolysis, and avoiding chemical waste.
.2 Mechanism of Deep Targeted Arsenic Precipitation by Sodium Sulfide
After iron salt pre-treatment, sodium sulfide is precisely added to the conditioned slurry. The released sulfide ions (S²⁻) have a strong targeted coordination precipitation ability, preferentially reacting with residual free As³⁺ to form arsenic trisulfide (As₂S₃), a highly stable precipitate with extremely low solubility. For residual As⁵⁺, sulfide ions facilitate simultaneous in-situ reduction and coprecipitation, efficiently converting As⁵⁺ into As₂S₃ while generating inert elemental sulfur that enhances the density of the flocs. This process thoroughly eliminates residual soluble arsenic. Compared to using sodium sulfide alone, the pre-treatment with ferrous sulfate increases the utilization efficiency of sodium sulfide by more than 30%, without generating excess free sulfide residues.
1.3 Synergistic Long-Term Arsenic Locking Mechanism of Iron and Sulfur Compounds
The core advantage of this treatment system lies in the in-situ formation of a dense coprecipitate composed of arsenic trisulfide (As₂S₃) and ferrous sulfide (FeS). Ferrous sulfide microcrystals uniformly coat the surface of arsenic trisulfide particles, forming a physical protective barrier that effectively resists natural weathering, rainwater, and pH fluctuations. This resolves the industry pain point of arsenic re-dissolution from single As₂S₃ precipitates under extreme alkaline conditions. The internal iron-arsenic solid skeleton also enhances the mechanical strength of the treated residue. Together, these features significantly improve the long-term leaching resistance and aging resistance of the stabilized waste, ensuring that arsenic remains immobilized for decades, meeting long-term safety requirements for solid waste management.
2. Industrial Treatment Process
The entire process is modular and can be directly integrated with existing plant equipment such as filter presses and stirring tanks, without requiring specialized large-scale devices. It is flexible, easy to operate and maintain, and suitable for both small-scale environmental protection stations and large smelter solid waste workshops.
2.1 Homogenization and Pre-treatment Section
Raw arsenic-containing waste residue is transported to a closed crushing and screening unit, where it is crushed and ground to a fineness of 200 mesh. Stones, plastics, metals, and other inert impurities are removed. The material is homogenized to control moisture content and eliminate reaction dead zones caused by particle size variation or local arsenic enrichment. This ensures uniform contact between reagents and arsenic, laying the foundation for stabilization. The entire process is enclosed to prevent fugitive arsenic dust emissions, complying with environmental standards.
2.2 Quantitative Slurry Preparation and Mixing Section
Pre-treated residue is fed into a corrosion-resistant closed reaction tank. Clean water or process water is added at a liquid-to-solid ratio of 2:1 to 5:1, and the mixture is stirred continuously at room temperature for 15–20 minutes to prepare a uniform, flowable slurry without agglomerates or settling. Samples are taken to measure initial pH and total arsenic concentration, allowing precise calculation of reagent dosages and avoiding excessive chemical use.
2.3 Controlled Ferrous Sulfate Pre-reaction Section
Solid ferrous sulfate or its saturated solution is added in batches at an Fe/As molar ratio of 2–5:1. An online pH control system maintains the slurry in the optimal weakly acidic range of pH 4.0–6.0. Stirring continues at constant temperature for 30–60 minutes. This ensures full oxidation and hydrolysis of ferrous ions, efficient adsorption and passivation of active arsenic, and optimization of the reaction environment to prevent hydrogen sulfide gas release.
2.4 Deep Sodium Sulfide Final Arsenic Fixation Section
After the pre-reaction, industrial-grade sodium sulfide is slowly added at a S/As molar ratio of 1.5–3:1. The pH is adjusted to a neutral to slightly alkaline range of 6.0–8.0. Stirring continues for 60–120 minutes to ensure complete sulfide precipitation of all residual arsenic forms, generating dense iron-sulfur-arsenic composite stable flocs. The entire process is enclosed to prevent gas release, ensuring on-site safety.
Post time: May-18-2026
