Core steps: electrolytic refining

Raw material preparation - anode plate:

The raw material is usually crude copper (with a purity of about 98.5-99.5%) obtained through preliminary smelting (such as smelting, blowing, and pyrometallurgical refining).

These coarse copper are cast into thick anode plates. The anode plate usually contains precious metals such as gold, silver, platinum, palladium, as well as impurities such as selenium, tellurium, arsenic, antimony, bismuth, lead, nickel, iron, sulfur, oxygen, etc.

Electrolytic cell configuration:

Electrolyte: The main components are copper sulfate solution and sulfuric acid. Sulfuric acid provides conductivity and prevents hydrolysis of copper salts.

Cathode: A mother plate made of very thin pure copper sheet (known as the "starting electrode sheet") or inert materials such as stainless steel and titanium is used as the cathode substrate. High purity copper will deposit on the cathode.

Anode: Hanging a coarse copper anode plate to be refined.

Multiple anodes and cathodes are alternately arranged in the electrolytic cell, immersed in the electrolyte.

Electrolysis process:

Apply direct current between the anode and cathode.

Anodic reaction: Copper atoms in the crude copper anode plate lose electrons, oxidize and dissolve into copper ions that enter the solution. Meanwhile, impurities that are more inert than copper, such as gold, silver, platinum group metals, selenium, and tellurium, do not dissolve but instead detach from the anode in the form of solid particles and sink to the bottom of the tank to form anode mud. Impurities that are more reactive than copper, such as iron, zinc, nickel, arsenic, and antimony, will dissolve into the electrolyte.

Cathodic reaction: Copper ions in the solution acquire electrons at the cathode and are reduced to deposit high-purity metallic copper. Under ideal conditions, only copper ions can preferentially deposit at the cathode because copper has a higher standard electrode potential. If very trace amounts of precious metal ions enter the solution, they usually do not precipitate at the cathode due to their extremely low concentration and deposition potential different from copper.

Control conditions: It is crucial to strictly control parameters such as electrolyte composition (copper ion concentration, acidity, additive concentration), temperature, current density, and electrode spacing. Adding additives such as gelatin, thiourea, or chloride ions can help to deposit cathode copper more smoothly and densely, reduce dendritic crystals and impurity inclusions, and improve purity and physical quality.

Cathode copper harvesting:

The electrolysis process lasts for several days to over ten days. When the copper deposition on the cathode reaches a sufficient thickness (a few millimeters to centimeters), remove the cathode.

If using a motherboard, peel off the deposited pure copper foil (cathode copper).

These stripped copper sheets are high-purity copper obtained by electrolytic refining (usually with a purity of up to 99.95% -99.99%).

Further purification after electrolytic refining (optional, for higher purity or specific requirements)

Although electrolytic refining itself can usually reach 99.99%, for certain applications (such as semiconductor target materials, superconducting materials) or to ensure higher stability and consistency, subsequent processing may sometimes be necessary:

Regional melting:

Slowly pass the electrolytic copper rod through a narrow high-temperature melting zone.

The solubility of impurities in solid and liquid phases is different (segregation effect). When the melting zone moves, impurities will be "swept" to one end of the copper rod.

Repeat this process multiple times, and finally remove the end rich in impurities to obtain copper with extremely high purity (up to 5N, 6N, or even higher). This method is particularly effective in removing oxygen, sulfur, and certain metal impurities dissolved in copper.

Vacuum melting/vacuum induction melting:

Smelting electrolytic copper in a vacuum environment.

Low pressure environment helps to remove dissolved gases (such as hydrogen, oxygen, nitrogen) and volatile metal impurities (such as zinc, cadmium, lead, magnesium, etc.). It can further reduce gas content and certain trace impurities.

Other methods:

Electrolytic refining secondary refining: The cathode copper obtained from the first electrolysis is used as the anode for another electrolytic refining to further remove residual trace impurities.

Electron beam melting: Copper is melted by electron beam bombardment in ultra-high vacuum, effectively removing volatile impurities and gases. The cost is relatively high.

Key points for impurity removal

Precious metals and selenium tellurium: mainly enriched in anode mud, can be recovered from it, and are important by-products.

Reactive metal impurities: Dissolve in the electrolyte and gradually accumulate. It is necessary to regularly extract a portion of the electrolyte for purification (such as electrolytic copper removal, crystallization copper removal, extraction, etc.), remove these impurities, replenish fresh electrolyte, and maintain electrolyte purity.

Oxygen, sulfur, and gases are effectively removed during the pyrometallurgical refining stage and vacuum melting/zone melting stage.

Other metal impurities: Selective deposition through electrolysis process (impurities do not deposit on the cathode) and subsequent area melting/vacuum melting removal.

Summarize the main process of extracting 99.99% pure copper

Raw material preparation: Make anode plates from crude copper (~99%).

Core purification: electrolytic refining - crude copper anode dissolution, high-purity copper deposition at the cathode (purity up to 99.95-99.99%), precious metals entering the anode mud, and active metals entering the electrolyte.

Electrolyte management: Continuously purify the electrolyte and remove accumulated impurities.

(Optional) Deep purification: Performing regional melting and/or vacuum melting on electrolytic cathode copper to remove trace gases and metal impurities, achieving or exceeding 99.99% purity, or meeting specific low gas content requirements.

Therefore, electrolytic refining is the most important and cost-effective method for industrial large-scale production of 99.99% pure copper. Regional melting and vacuum melting are supplementary methods used to achieve higher purity or specific application requirements.

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