Research On The Pressurized Leaching Process For Efficient Recovery Of Platinum And Rhodium From Waste Fiberglass Refractory Bricks

Fiberglass is a high-performance inorganic non-metallic material available in a wide variety of forms; it is extensively used in applications such as electrical insulation, thermal insulation, circuit board substrates, and fiber-reinforced materials. my country currently leads the world in fiberglass production, accounting for more than half of the global output. Platinum-group metals-specifically platinum (Pt) and rhodium (Rh)-are essential components in fiberglass manufacturing, utilized in items such as fiber-drawing bushings, crucibles, bubblers for pool furnaces, wrapping materials for barrier bricks, and thermocouples. During high-temperature production, platinum and rhodium migrate into the surrounding refractory bricks or waste glass cullet through processes such as volatilization-deposition or diffusion. The content of these metals typically ranges from 0.2 to 1.2 kg/t-far exceeding the grades found in natural ores-and with stockpiles increasing annually, the reserves represent a significant resource. Current recovery methods include mineral processing, iron enrichment, aqua regia dissolution, alkali fusion, microwave-assisted alkali fusion, and combined beneficiation-metallurgy processes; however, issues such as low processing efficiency, severe environmental pollution, and suboptimal leaching rates hinder the development of platinum-group metal recovery from these refractory bricks. This study investigates a novel, high-efficiency process for recovering platinum and rhodium from fiberglass refractory bricks using a mixed hydrochloric-nitric acid system combined with pressurized leaching technology.

The raw materials used in the experiment were obtained from a fiberglass plant; the contents of the major elements are shown in Table 1. The analysis results indicate that the primary elements in the refractory brick include silicon, aluminum, and iron. Specifically, the Pt content is 0.1778% and the Rh content is 0.0165%, with Rh accounting for 8.49% of the platinum-rhodium alloy.

Leaching experiments were conducted in a pressurized autoclave. A specific amount of raw material was placed in the autoclave liner, and hydrochloric acid, nitric acid, and water were added at a predetermined liquid-to-solid ratio. The autoclave was heated to the target temperature, and leaching was carried out under mechanical stirring. Upon completion of the leaching process, the mixture was filtered and washed; samples of the filtrate and the filter residue (dried at 80°C) were then collected for analysis.

This study primarily investigates the effects of factors such as the volumetric concentration of the mixed acid, the acid-to-ore ratio, the amount of nitric acid, the reaction time, and the reaction temperature on the leaching rates of platinum and rhodium.

Given the highly corrosive nature of the hydrochloric acid–nitric acid system toward equipment, it is desirable to minimize the system's acidity while ensuring effective leaching of platinum and rhodium. The initial acidity of the mixed acid can be adjusted by varying its volumetric concentration; therefore, the effect of this concentration was investigated. Experiments were conducted using 100 g of raw material, 75 mL of hydrochloric acid, and 25 mL of nitric acid, with a reaction time of 4 hours and a temperature of 150°C; the mixed acid volumetric concentrations tested were 10%, 15%, 20%, and 25%. The results are shown in Figure 1.

Figure 1 Effect of mixed acid volume concentration on the leaching of platinum and rhodium.

The experimental results indicate that, at a fixed dosage of mixed acid, increasing the volumetric concentration of the mixed acid raises the initial acidity of the system and leads to a gradual increase in the leaching rates of both platinum and rhodium. When the volumetric concentration reaches 20%, the leaching rates for platinum and rhodium reach 97.55% and 86.86%, respectively. Beyond a concentration of 20%, the improvement in leaching rates is marginal; therefore, a volumetric concentration of 20% is recommended to minimize the corrosivity of the system.

To minimize acid consumption, enhance acid utilization efficiency, and facilitate downstream processing, experiments were conducted by fixing the mixed acid volume while varying the amount of refractory brick to investigate the leaching rates of platinum and rhodium under different acid-to-ore ratios. Tests were performed using 500 mL of mixed acid (at a volume concentration of 20%) at a reaction temperature of 150°C for 4 hours, with raw material masses of 83 g, 100 g, and 125 g, corresponding to acid-to-ore ratios (ratio of mixed acid volume to refractory brick mass) of 4, 5, and 6, respectively; the results are shown in Figure 2.

Figure 2 Effect of acid-to-ore ratio on the leaching of platinum and rhodium.

The experimental results indicate that the leaching rates of both platinum and rhodium increase as the acid-to-ore ratio rises. The difference in leaching rates between the two ratios of 5 and 6 is minimal; therefore, an acid-to-ore ratio of 5 is selected to minimize acid consumption.

In the hydrochloric acid-nitric acid leaching system, nitric acid primarily acts as an oxidizing agent to oxidize platinum and rhodium. Experiments were conducted to investigate the effect of nitric acid dosage while maintaining the acidity of the leaching solution. Using 100 g of raw material, 400 mL of water, a reaction time of 4 hours, and a temperature of 150°C, the nitric acid dosage was varied (15 mL, 20 mL, 25 mL, and 30 mL), with corresponding adjustments made to the hydrochloric acid dosage (85 mL, 80 mL, 75 mL, and 70 mL, respectively); the results are shown in Figure 3.

The results indicate that the nitric acid dosage influences the leaching efficiency of platinum and rhodium; as the dosage increases, the oxidizing power of the reaction system intensifies, leading to a gradual rise in the leaching rates of platinum and rhodium. However, beyond a dosage of 20 mL, the improvement in leaching rates becomes marginal, whereas higher dosages prolong the time required for the subsequent nitric acid removal process. Therefore, to optimize the process while ensuring effective leaching, it is advisable to minimize nitric acid usage; a dosage of 20 mL is recommended.

Figure 3 Effect of nitric acid dosage on the leaching of platinum and rhodium.

This study determined the optimal process conditions for the pressure leaching of fiberglass refractory bricks through a series of experiments. Under the conditions of 100 g of refractory bricks, a liquid-to-solid ratio of 5:1, 80 mL of hydrochloric acid, 20 mL of nitric acid, a reaction temperature of 200°C, a pressure of 1.6 MPa, and a reaction time of 4 hours, the leaching rates reached 99.05% for platinum and 98.21% for rhodium, while the platinum and rhodium contents in the residue were reduced to 0.0016% and 0.0003%, respectively. Conventional recovery processes for fiberglass refractory bricks suffer from various limitations; in contrast, the pressure leaching process achieves exceptionally high leaching rates and processing efficiency, demonstrating both practicality and feasibility.