For every metric ton of crude steel produced in the steel industry, the electric arc furnace (EAF) process generates approximately 15–25 kilograms of dust. This dust typically contains 10–35% zinc and is also rich in valuable metals such as iron, lead, and cadmium. Currently, there are two main technical approaches for the wet process of recovering zinc metal from zinc-containing dust: the acid method (sulfuric acid system) and the ammonia method (ammonia-ammonium chloride system).

Acid Process or Ammonia Process? Subject to Rawterial

The acid process is technologically mature and offers high leaching rates (up to 80%–95%), and has long been the mainstream choice for zinc smelting. However, the acid process has a critical weakness—it is highly sensitive to fluoride and chloride ions in the raw materials. In a sulfuric acid system, fluoride and chloride ions severely corrode electrodes and equipment; therefore, the acid process typically requires additional pre-treatment steps—such as alkali washing—to remove fluoride and chloride before leaching. This not only increases capital and operating costs but also makes the process lengthy and complex. The acid process is more suitable for zinc dust with low chloride content and for large-scale production (annual output of 10,000 metric tons or more).

The ammonia process, on the other hand, takes a different approach. In an ammonia-ammonium chloride system, zinc is selectively leached by forming zinc-ammonia complexes with ammonia, while the dissolution rate of impurity elements such as iron and aluminum can be controlled below 5%, with a leaching efficiency of 85%–92%. More importantly, the ammonia process does not require the prior removal of fluoride and chloride; fluoride and chloride ions can be balanced through internal circulation within the system. This characteristic makes the ammonia process particularly suitable for treating zinc dust with high chloride content (up to 10%) and is also well-suited for small- and medium-scale production facilities.

Each process has its own applicable scenarios; the key to selection lies in the characteristics of the raw materials and the scale of the project.

Metalcess Ammonia Zinc Electrolysis Process Description

Metalcess provides a complete ammonia-based electrolytic process route consisting of four core process steps:

Ø  Step 1: Leaching and Solid-Liquid Separation

Crude zinc oxide dust is leached in an ammonia-ammonium chloride solution. Valuable metals such as zinc, copper, cadmium, cobalt, nickel, and silver form complexes and enter the solution, while lead, iron, magnesium, and other elements remain in the leaching residue. The leaching residue can contain up to 50% iron and can be used as raw material for cement plants or iron smelters; if the zinc content exceeds 10%, zinc can be further recovered through volatilization in a rotary kiln. Solid-liquid separation is achieved using a membrane filter press, ensuring highly efficient separation of residue and solution.

Ø  Process Step 2: Purification

Zinc powder is added to the leachate to initiate a displacement reaction, thereby removing impurity metals such as copper, cadmium, lead, and silver. Because the ammonia leaching process is highly selective toward iron, the iron content in the solution is extremely low, resulting in a purification load far smaller than that of acid leaching processes. This characteristic significantly simplifies the purification process and reduces operating costs.

Ø  Process 3: Electrolysis

The purified zinc-ammonia complex solution enters the electrolytic cell, where zinc ions deposit onto aluminum cathode plates to form zinc flakes; graphite plates are used as the anodes. During electrolysis, chlorine gas generated at the anode reacts with the ammonia in the solution to form nitrogen gas, which can be directly discharged after scrubbing. The cell voltage for the electrowinning process is 2.7–2.9 V, with a current efficiency greater than 92%; DC power consumption is approximately 2,800–2,900 kWh per metric ton of zinc. The deposition cycle for the zinc cathode is approximately 1–2 days.

Ø  Process 4: Melting and Casting

The peeled zinc sheets are melted in an induction melting furnace and then cast into zinc ingots using a continuous casting machine. The final product achieves a high purity of 99.96% or higher.

Technical Highlights: Built for Challenges

The Metalcess ammonia-based electrolytic process offers unique advantages in multiple aspects:

Ø  No need for pre-defluorination/dechlorination: This eliminates the need for expensive pre-treatment equipment, such as multi-chamber furnaces, significantly reducing capital costs.

Ø  No iron removal required: Iron is virtually insoluble in ammonia-based solutions, eliminating the complex iron removal steps found in acid-based processes.

Ø  Streamlined process: From ore to zinc ingots, the entire process consists of only four steps—“leaching, purification, electrowinning, and smelting”—making operation and control straightforward.

Ø  Comprehensive Resource Utilization: Leaching residues can be recycled as iron resources, while displacement residues can be recovered for valuable metals such as lead and copper, maximizing resource utilization.

Ø  Suitable for Small and Medium-Sized Operations: Metalcess has developed modular electrowinning systems starting at 5 metric tons per day. These systems are easy to install and are particularly well-suited for small and medium-sized projects.

Core Equipment: A Guarantee of Quality

The design of the core equipment in the zinc system fully accounts for reliability during long-term operation:

Ø  Polymer Concrete Cell: Constructed from a composite of silica sand, FRP reinforcing ribs, and vinyl ester resin, with FRP sealing layers on both the interior and exterior. This material features extremely high impact strength, resistance to high-temperature corrosion, and excellent electrical insulation, resulting in a service life far exceeding that of traditional electrolytic cells.

Ø  Aluminum Cathode Plate: Made from 1070 rolled aluminum sheet, these plates are equipped with extruded conductive rods and aluminum-copper explosive-bonded transition joints. Non-working areas are coated with anti-corrosion insulating material, which not only extends the service life of the plates but also ensures uniform and consistent zinc flake dimensions. The typical service life of aluminum cathode plates is 7–8 months.

Ø  Graphite Anode Plate: These consist of a graphite substrate, a titanium connecting plate, and a titanium-copper conductive rod. The service life of graphite anodes can reach 3–4 years, and the titanium-copper conductive rods can be reused when replacing the graphite plates, thereby reducing long-term operating costs.

Ø  DCS Automation Control System: The entire system employs centralized DCS control and communicates with Siemens PLCs via Ethernet. It enables real-time monitoring of the electrowinning workshop’s operating status and supports both centralized management and manual operation modes, ensuring a stable and controllable production process.