Method for removing impurities by gradient oxidation of noble metal alloy

Abstract

The present application belongs to the field of non-ferrous metal smelting, and particularly relates to a rare and precious metal recovery technology, and particularly relates to a noble metal alloy gradient oxidation converting and impurity removal method, comprising the following steps: (1) preparing materials: alloy to be treated, slagging agent, oxidizing agent; (2) removing lead and bismuth; (3) removing copper. The above scheme adopts a scheme of gradient slagging, time period impurity removal and intermediate slagging, controls the adding amount of the slagging agent in the first stage with high lead and bismuth content, and avoids that the slagging agent is consumed by lead and bismuth oxides in large amounts; after most of the lead and bismuth are removed and the slag is completely removed, the slagging agent is used to fully capture cuprous oxide, so that the technical problem that copper is difficult to be separated by slagging due to the fact that lead and bismuth preferentially combine with silicon dioxide in the traditional process and the segmented lead removal process is solved from the root, and efficient and deep removal of copper impurities is realized.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal smelting, and particularly relates to rare and precious metal recovery technology, specifically to a method for removing impurities from precious metal alloys by gradient oxidation blowing. Background Technology

[0002] In the pyrometallurgical process of copper anode slime centered on the Kaldor furnace, the precious metal alloys (commonly known as precious lead or precious bismuth) obtained through lead or bismuth capture contain a large amount of impurities such as lead, bismuth, copper, selenium, and tellurium. These impurities need to be removed through oxidation blowing to obtain high-quality gold-silver alloys (Dol alloys). Traditional oxidation blowing is usually divided into two stages: the first stage adds slag-forming agents such as silicon dioxide (SiO2) to preferentially oxidize impurities such as lead, bismuth, antimony, and arsenic in an oxidizing atmosphere, and removes them by forming silicate slag (such as lead silicate) with SiO2; the second stage removes selenium, tellurium, and some residual copper through further oxygen blowing.

[0003] Traditional one-step or two-step slag-addition blowing methods have significant drawbacks: In the first stage, where lead and bismuth are abundant, the oxides of lead and bismuth (PbO, Bi₂O₃) have a much stronger affinity for the slag-forming agent SiO₂ than for cuprous oxide (Cu₂O). This means that while lead and bismuth are largely oxidized, the limited amount of SiO₂ preferentially reacts with PbO and Bi₂O₃, resulting in Cu₂O remaining largely in the melt as it cannot be effectively slag-formed. Although a secondary blowing process may be performed later, the melt composition becomes more complex, the slag properties change, and the copper removal efficiency is limited. As a result, the final Dow alloy often contains as much as 1-3% copper.

[0004] Excessive copper content in Dow alloys poses a significant threat to subsequent silver electrolytic refining processes. During electrolysis, the standard electrode potential of copper (Cu)... 2+ / Cu: 0.34V) is much lower than that of silver (Ag) + (Ag: 0.80V), copper in the alloy anode will preferentially dissolve into the electrolyte before silver. As electrolysis proceeds, Cu in the electrolyte... 2+ The concentration continuously accumulates and increases, causing the silver electrolyte to become unusable. Furthermore, when Cu... 2+ When the concentration exceeds the critical value, its deposition potential may be close to or even higher than that of silver, causing copper to co-deposit with silver on the cathode, severely contaminating the cathode silver powder, making it difficult for its purity to reach the 4N (99.99%) standard, and reducing current efficiency.

[0005] To solve the problem of copper removal from precious metal alloys, the industry has successively developed a variety of improved processes and related patented technologies, but all of them have limitations to varying degrees, as follows:

[0006] Some existing technologies achieve copper removal by adding chlorides in conjunction with silica slag formation. For example, the Chinese invention patent application entitled "A Method for Refining Copper Removal from Copper Anode Sludge Leaching Residue" (publication number CN114774708A) describes a process where, after producing crude gold-silver alloy in a Kaldor furnace, silver chloride and quartz sand are added twice for refining and copper removal, reducing the copper content of the alloy to below 0.8%. However, this process uses chloride additives, which generate corrosive chlorine-containing fumes during production, leading to significant environmental control challenges. Furthermore, this process does not differentiate the removal order based on impurity characteristics; lead and bismuth still preferentially consume the slag-forming agent, resulting in insufficient deep copper removal capacity and failing to meet the requirements for high-purity silver production. Additionally, the use of silver chloride significantly increases production costs.

[0007] Other technologies focus on optimizing smelting conditions and equipment, such as the Chinese invention patent application titled "Kaldor Furnace Oxygen-Enriched Smelting Method" (publication number CN110055429A). This solution enhances the overall oxidation effect by optimizing the spray gun structure and using high-concentration oxygen-enriched air, only increasing the overall smelting rate of various impurities. This technology does not change the slagging agent addition method or reaction system, and cannot solve the core problem of lead and bismuth oxide preferentially combining with silicon dioxide. Ultimately, the copper content of the Dorr alloy remains at the traditional level of 1%-3%, failing to fundamentally solve the problem of copper contamination in downstream silver electrolysis.

[0008] A Chinese invention patent application entitled "A Complex Pyrometallurgical Refining Method for Crude Lead" (publication number CN116411175A) employs a combined process of condensation copper removal, crystallization bismuth separation, and sulfurization for deep copper removal. This process is suitable for ordinary crude lead raw materials, relying on physical condensation and sulfidation reactions to remove copper impurities. However, this route involves cumbersome procedures, frequent site changes, and the condensation and sulfurization processes easily cause precious metal losses. It is not suitable for precious lead or precious bismuth alloys with high gold and silver content, and cannot be adapted to the continuous blowing production mode of the Kaldor furnace.

[0009] A Chinese invention patent application titled "An Efficient Recycling Method for the Co-processing of High-Bismuth Slag and Precious Lead" (publication number CN121780866A) describes a process that primarily uses a two-stage melting and precipitation method for copper removal, supplemented by oxidation and blowing, to achieve impurity removal, emphasizing the comprehensive synergistic recovery of multiple materials. However, because its core copper removal method is melting and precipitation, the copper removal effect is greatly affected by temperature, resulting in incomplete removal of copper impurities. Furthermore, the silica slag-forming strategy has not been optimized, and the problem of lead and bismuth competing for slag-forming agents persists, ultimately leading to a high copper content in the gold-silver alloy.

[0010] A Chinese invention patent application titled "A Method for Treating Copper Anode Sludge Using a Combined Reduction Smelting-Vacuum Distillation-Oxidative Refining Process" (publication number CN119307732A) describes a method that first removes lead and bismuth components through pre-treatment using vacuum distillation equipment, followed by oxidative refining to remove copper. While this approach avoids interference from lead and bismuth in copper removal, it requires additional vacuum distillation equipment, significantly increasing equipment investment, operating energy consumption, and maintenance costs. Furthermore, the process is lengthened, leading to increased mechanical entrainment losses of precious metals during multi-equipment transfer, and limiting the throughput of a single furnace, making it difficult to apply to large-scale industrial production.

[0011] A Chinese invention patent application entitled "A Method for Refining Precious Lead or Crude Silver" (publication number CN101054627A) divides the oxidative refining process into three stages: the first stage involves blast oxidation at 900–1000℃ to remove arsenic and antimony; the second stage involves adding silica in batches at 1000–1200℃, with a total addition amount of 6%–30% of the lead content, to remove lead; and the third stage involves blast oxidation or the addition of an oxidant to remove bismuth and copper. Although this patent adopts a staged slag addition strategy, its core is to prioritize lead removal, with bismuth and copper removal only serving as the final step in the third stage. It fails to recognize the metallurgical mechanism that lead and bismuth oxides have a stronger affinity for silica than cuprous oxide. The total amount and batch allocation of silica are calculated solely based on the lead content, making it impossible to avoid the problem of lead and bismuth oxides preferentially consuming slag-forming agents. The process is carried out continuously without staged slag removal, and the lead slag remaining in the melt in the early stages further interferes with the slag formation and separation of copper. Due to the above-mentioned factors, it is impossible to effectively reduce the copper content in gold-silver alloys.

[0012] In recent years, there have been studies on the step-by-step impurity removal of precious lead in oxygen-enriched refining of silver furnaces (Experimental Study on Step-by-Step Impurity Removal of Precious Lead in Oxygen-Enriched Refining of Silver Furnaces. Mining and Metallurgy, 2023, 32(1):78-83.). Although the studies distinguished between the arsenic and antimony removal stage, the bismuth removal stage, and the copper removal stage, and removed impurities by adjusting the oxygen concentration step-by-step, the improvement direction focused on the control of the oxidizing atmosphere. The addition of silica was not distributed in stages and in a non-uniform proportion. In particular, the gradient addition scheme of slag-forming agent was not clearly defined. Therefore, the improvement on the copper removal efficiency was limited.

[0013] In summary, current mainstream precious metal alloy oxidation blowing processes and various improved processes either focus on lead removal and efficiency improvement while neglecting deep copper removal, or rely on corrosive additives and new large-scale equipment to achieve limited copper reduction. These processes generally suffer from insufficient copper removal depth, significant environmental pressure, high modification costs, and complex processes. Even some patents employing staged or batch-based slag-forming agent addition or oxygen concentration alteration blowing modes do not address the gradient slag-forming and operating condition control schemes designed to address the slag-forming competition mechanism between lead, bismuth, and copper. Currently, there is still a lack of a process method that can leverage existing Kaldor furnaces and refining furnaces without large-scale modifications, optimizing slag-forming agent addition strategies and blowing stages to achieve efficient and deep copper impurity removal while simultaneously removing lead and bismuth. Therefore, developing a method that can achieve efficient and deep copper impurity removal during the oxidation blowing stage is crucial for improving the final silver product quality and overall process economy. Summary of the Invention

[0014] This invention addresses the problem in existing precious metal alloy oxidation blowing processes where lead and bismuth preferentially consume slag-forming agents, leading to incomplete copper removal. It provides a method for removing impurities in precious metal alloy gradient oxidation blowing.

[0015] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for removing impurities from noble metal alloys by gradient oxidation blowing, comprising the following steps:

[0016] (1) Material preparation:

[0017] Alloy to be processed: Alloys or alloy melts containing precious metals, lead, bismuth, and copper;

[0018] Slagging agent: Silicon-containing slagging agent; Total addition amount: The ratio of the total mass of lead and bismuth in the alloy to be treated to the total mass of slagging agent added is 2.5 to 7.5;

[0019] Oxidizing agent; air with an oxygen volume content of 15% to 40% or oxygen-enriched air;

[0020] (2) Lead and bismuth removal: Oxidize and blow the alloy melt to be treated, add slag-forming agent, the amount of which is 40% to 80% of the total amount of slag-forming agent added, and continue blowing until the total content of lead and bismuth in the melt drops to 30% to 50% of the initial content, then stop blowing and remove the slag.

[0021] (3) Copper removal: Continue oxidation blowing and add the remaining slag-forming agent to remove the slag and obtain a low-copper gold-silver alloy.

[0022] The above scheme adopts a gradient slag addition, time-stage impurity removal and intermediate slag removal method. In the first stage with high lead and bismuth content, the amount of slag-forming agent added is controlled to avoid the slag-forming agent being consumed in large quantities by lead and bismuth oxides. After most of the lead and bismuth are removed and the slag is completely removed, the slag-forming agent is used to fully capture cuprous oxide. This fundamentally solves the technical problem in traditional processes and segmented lead removal processes where lead and bismuth preferentially combine with silicon dioxide, making it difficult for copper to be slag-formed and separated. This achieves efficient and deep removal of copper impurities. Detailed Implementation

[0023] The technical solution of the present invention will be further described in detail below with reference to embodiments and comparative examples.

[0024] A method for removing impurities from a precious metal alloy by gradient oxidation blowing includes the following steps:

[0025] (1) Material preparation:

[0026] The alloy to be treated is an alloy or alloy melt containing precious metals, lead, bismuth, and copper; the alloy to be treated is a precious lead alloy or a precious bismuth alloy obtained by treating copper anode mud by lead trapping or bismuth trapping.

[0027] Slagging agent: Silicon-based slagging agent, specifically one or more combinations of quartz sand, quartz, silica, feldspar, and diatomaceous earth; Total addition amount: The ratio of the total mass of lead and bismuth in the alloy to be treated to the total mass of the slagging agent added is 2.5 to 7.5 (added in two stages);

[0028] Oxidizing agent; air with an oxygen volume content of 15% to 40% or oxygen-enriched air;

[0029] (2) Lead and bismuth removal: The alloy melt to be treated is subjected to oxidative blowing in a Kaldor furnace. A slag-forming agent is added. In this stage, the slag-forming agent is added in a short amount, which is 40% to 80% of the total amount of slag-forming agent added, preferably 50% to 60%. The blowing is continued at a temperature of 1000-1150℃ for 0.5 to 2 hours until the total content of lead and bismuth in the melt drops to 30% to 50% of the initial content. The blowing is then stopped and the slag is removed.

[0030] (3) Copper removal: Continue oxidation blowing and add the remaining slag-forming agent. The blowing temperature is raised to 1100-1250℃ and the blowing time is 0.5-2h. After the hammering is completed, remove the slag to obtain a low copper gold-silver alloy.

[0031] Compared with the prior art, the present invention has the following outstanding advantages:

[0032] 1. Achieving efficient and deep removal of copper impurities. This invention proposes a gradient slag addition and staged impurity removal approach. Combined with an intermediate slag removal process, the amount of slag-forming agent added is controlled in the first stage when lead and bismuth content is high, preventing excessive consumption of the slag-forming agent by lead and bismuth oxides. After most of the lead and bismuth have been removed and slag has been completely removed, the slag-forming agent is then used to fully capture cuprous oxide. This fundamentally solves the technical problem in traditional processes and staged lead removal processes where lead and bismuth preferentially combine with silicon dioxide, making copper difficult to separate through slag formation. This process can stably control the copper content in Dow alloys below 0.5%, and under optimal conditions, it can be reduced to below 0.2%, with a copper removal effect far superior to conventional blowing processes.

[0033] 2. Ensures stable production of high-purity silver products and enhances product value. The significantly reduced copper content in the Dow alloy fundamentally alleviates the Cu content issues during subsequent silver electrolytic refining. 2+ The accumulation in the electrolyte and the risk of cathode co-deposition provide key guarantees for the stable production of high-purity (above 4N) cathode silver, thereby improving product added value and market competitiveness.

[0034] 3. Optimize slag phase state and reduce precious metal loss. Staged slag formation results in a relatively simple slag phase composition and stable melting point at each stage, improving slag fluidity, facilitating slag-gold separation, and reducing the mechanical inclusion loss of precious metals in the slag. Simultaneously, the intermediate slag removal step avoids interference from earlier slag on later reactions, further enhancing the copper removal effect.

[0035] 4. Strong process adaptability and good equipment compatibility. The principle of this method is clear, and the entire process can be implemented using existing Kaldor furnaces and various conventional precious metal refining furnaces without the need to modify the main equipment. Only the slagging agent addition method, blowing system and process nodes need to be optimized. The modification cost is low and the threshold for implementation is small, which has broad prospects for industrial application and promotion.

[0036] Example 1

[0037] Raw material: Precious lead alloy (from copper anode mud processed in the Kaldor furnace), composition: Au 0.52%, Ag 31.20%, Pb 49.80%, Bi 10.50%, Cu 3.80%, the remainder being impurities. Total lead and bismuth content = 60.30%.

[0038] Slagging agent: Quartz sand (SiO2≥98%). Total mass of lead and bismuth: Total mass of slagging agent = 5.0.

[0039] First stage: Add the first batch of slag-forming agent, accounting for 60% of the total slag-forming agent. Temperature 1050℃, introduce oxygen-enriched air with an oxygen content of 25%, and smelt for 1.0 hour until the total lead and bismuth content drops to 40% of the initial value. Remove all slag.

[0040] Second stage: Add the remaining slagging agent (40% of the total). Raise the temperature to 1180℃, introduce oxygen-enriched air with an oxygen content of 35%, and smelt for 1.5 hours. After completion, remove the slag to obtain a low-copper gold-silver alloy.

[0041] Results: The final gold-silver alloy contained 0.23% copper, with a silver direct recovery rate of 98.6% and a gold direct recovery rate of 99.2%.

[0042] Example 2

[0043] Raw material: Precious bismuth alloy, composition: Au 0.78%, Ag 24.60%, Pb 35.20%, Bi 12.30%, Cu 3.10%, and other impurities. Total lead and bismuth content = 47.50%.

[0044] Slagging agent: silica (SiO2≥95%). Total mass of lead and bismuth: Total mass of slagging agent = 2.5.

[0045] First stage: Add 40% of the total slagging agent. Temperature 1000℃, introduce air with 15% oxygen content, smelt for 2.0 hours, until lead and bismuth drop to 30% of the initial level, then remove the slag.

[0046] Second stage: Add the remaining slagging agent (60%). Temperature 1100℃, oxygen-enriched air with 20% oxygen content is introduced, blowing time 2.0h. Slag removal.

[0047] Results: The final copper content was 0.44%, the direct silver recovery rate was 97.1%, and the direct gold recovery rate was 98.4%.

[0048] Example 3

[0049] Raw material: Precious lead alloy, composition: Au 0.61%, Ag 34.80%, Pb 46.30%, Bi 8.20%, Cu 4.90%, and other impurities. Total lead and bismuth content = 54.50%.

[0050] Slagging agent: Feldspar + quartz sand mixture. Total mass of lead and bismuth: Total mass of slagging agent = 7.5.

[0051] First stage: Add 80% of the total slagging agent. Temperature 1150℃, introduce oxygen-enriched air with 40% oxygen content, and smelt for 0.5 hours until the lead-bismuth content drops to 50% of the initial value (i.e., to 27.25%). Remove slag.

[0052] Second stage: Add the remaining slagging agent (20%). Temperature 1250℃, oxygen-enriched air with 40% oxygen content is introduced, blowing time 0.5h. Slag removal.

[0053] Results: The final copper content was 0.47%, the direct silver recovery rate was 98.7%, and the direct gold recovery rate was 99.3%.

[0054] Example 4

[0055] Raw material: Precious lead alloy, composition: Au 0.68%, Ag 28.30%, Pb 48.20%, Bi 9.40%, Cu 3.60%, and other impurities. Total lead and bismuth content = 57.60%.

[0056] Slag-forming agent: Diatomaceous earth (approximately 75% SiO2, calculated based on effective components). Mass ratio = 4.0.

[0057] First stage: Add 50% of the total slagging agent. Temperature 1080℃, oxygen content 30%, blow for 1.0 h, until lead and bismuth decrease to 35% of the initial level (approximately 20.16%). Remove slag.

[0058] Second stage: Add the remaining 50%. Temperature 1200℃, oxygen content 30%, blowing for 1.0 h.

[0059] Results: Copper content 0.19%, silver direct recovery rate 98.9%, gold direct recovery rate 99.2%.

[0060] Example 5

[0061] Raw material: Precious bismuth alloy, composition: Au 0.53%, Ag 37.60%, Pb 30.50%, Bi 14.20%, Cu 2.40%, and other impurities. Total lead and bismuth content = 44.70%.

[0062] Slag-forming agent: Quartz stone, mass ratio = 6.0.

[0063] First stage: Add 70% of the total slagging agent. Temperature 1100℃, oxygen content 25%, blow for 1.0 h, until lead and bismuth drop to 45% of the initial level (approximately 20.12%). Remove slag.

[0064] Second stage: Add the remaining 30%. Temperature 1150℃, oxygen content 30%, blowing for 1.2 hours.

[0065] Results: Copper content 0.17%, silver direct recovery rate 99.1%, gold direct recovery rate 99.4%.

[0066] Comparative Example 1: The traditional one-step method does not satisfy the requirement of phased gradient slag addition.

[0067] Raw materials: Same as in Example 1.

[0068] Method: Without dividing into sections, add all the slag-forming agent at once (total amount is the same as in Example 1, mass ratio 5.0), and blow with 30% oxygen-enriched air at 1150℃ for 2.5 hours without removing slag in the middle.

[0069] Results: The final gold-silver alloy contained 1.72% copper, 97.2% silver, and 98.1% gold.

[0070] Comparative Example 2: Mass ratio exceeds the upper limit of 8.0.

[0071] Raw materials: Same as in Example 1.

[0072] Method: Slag was added in stages with a gradient (60% in the first part and 40% in the second part), but the amount of slag-forming agent added was reduced so that the total mass of lead and bismuth: total mass of slag-forming agent = 8.0 (>7.5). The other conditions were the same as in Example 1.

[0073] Results: The final copper content was 1.25%, the direct silver recovery rate was 95.9%, and the direct gold recovery rate was 96.7%.

[0074] Comparative Example 3: Mass ratio below the lower limit of 2.0

[0075] Raw materials: Same as in Example 1.

[0076] Method: Slag was added in stages with a gradient, but the amount of slag-forming agent was increased to make the mass ratio = 2.2 (< 2.5). The other conditions were the same as in Example 1.

[0077] Results: The final copper content was 0.33%, the direct silver recovery rate was 90.9%, and the direct gold recovery rate was 92.7%.

[0078] Comparative Example 4: The amount added in the first stage was insufficient, but the residue was not skimmed off after the first stage.

[0079] Raw materials: Same as in Example 1.

[0080] Method: The slag addition ratio is the same as in Example 1, but the slag is not removed after the first stage, and the second stage is directly started.

[0081] Results: The final copper content was 1.1%, the direct silver recovery rate was 97.9%, and the direct gold recovery rate was 98.7%.

[0082] Comparative Example 5: The mass ratio was within the range, but excessive slag was added in the first stage, exceeding 80% of the total slag agent.

[0083] Raw materials: Same as in Example 1.

[0084] Method: Slag is added in stages, but the first slag-forming agent accounts for 90% of the total (exceeding the range of 40% to 80%), and the second slag-forming agent accounts for 10%. The remaining conditions are the same as in Example 1.

[0085] Results: The final copper content was 0.89%, the direct silver recovery rate was 98.9%, and the direct gold recovery rate was 98.7%.

Claims

1. A method for removing impurities from a precious metal alloy through gradient oxidation blowing, comprising the following steps: (1) Material preparation: Alloy to be processed: Alloys or alloy melts containing precious metals, lead, bismuth, and copper; Slagging agent: Silicon-containing slagging agent; Total addition amount: The ratio of the total mass of lead and bismuth in the alloy to be treated to the total mass of slagging agent added is 2.5 to 7.5; Oxidizing agent; air with an oxygen volume content of 15% to 40% or oxygen-enriched air; (2) Lead and bismuth removal: Oxidize and blow the alloy melt to be treated, add slag-forming agent, the amount of which is 40% to 80% of the total amount of slag-forming agent added, and continue blowing until the total content of lead and bismuth in the melt drops to 30% to 50% of the initial content, then stop blowing and remove the slag. (3) Copper removal: Continue oxidation blowing and add the remaining slag-forming agent to remove the slag and obtain a low-copper gold-silver alloy.

2. The method for removing impurities from precious metal alloys by gradient oxidation blowing according to claim 1, characterized in that: The silicon-containing slag-forming agent is any one or more combinations of quartz sand, quartz stone, silica, feldspar, and diatomaceous earth.

3. The method for removing impurities from precious metal alloys by gradient oxidation blowing according to claim 1, characterized in that: In step (2), the slag-forming agent is added in a short amount, and its addition amount accounts for 50% to 60% of the total mass of the slag-forming agent.

4. The method for removing impurities from precious metal alloys by gradient oxidation blowing according to claim 1, characterized in that: The blowing temperature in step (2) is 1000-1150℃, and the blowing time is 0.5-2h.

5. The method for removing impurities from precious metal alloys by gradient oxidation blowing according to claim 1, characterized in that: The blowing temperature in step (3) is 1100℃ to 1250℃, and the blowing time is 0.5 to 2 hours.

6. The method for removing impurities from precious metal alloys by gradient oxidation blowing according to claim 1, characterized in that: The alloy to be treated is a precious lead alloy or a precious bismuth alloy obtained by treating copper anode mud through lead trapping or bismuth trapping.

7. The method for removing impurities from precious metal alloys by gradient oxidation blowing according to claim 1, characterized in that: The oxidation blowing is carried out in a Kaldo furnace.