Process for recycling precious metals from waste materials at the Brinston plant

Johnson Ma Jie Chemical Co. scrap precious metals recovery plant, located in the northern suburbs of London Brin Youngstown, put into operation in October 1979. The Brinston plant not only recovers precious metals from various waste materials with high efficiency, but also recovers precious metals from primary raw materials (including electrolytic anode mud, etc.). The process of recycling precious metals from various raw materials is shown in Figure 1. Similar to the plant's process, there is a process used by the US Bureau of Mines to recover gold. The operations of these processes include all conventional processes for precious metallurgy, but the control of the production process is advanced.

Figure 1 Process flow of waste precious metal recycling at the Brinston factory

First, the pre-treatment and classification of incoming materials

Although the conventional method of fire-electrolysis is mature in the process of recovering gold and silver from waste raw materials, the sources of waste raw materials are wide, the types are complex, and the grades are very high (from a few tenths to ninety percent). Therefore, the primary problem is to pre-process and classify used raw materials to ensure the accuracy of sampling analysis. Then, according to the type and grade of the raw materials, the most suitable treatment method is selected. Otherwise, the production process will increase and the production cost will increase.

The pretreatment of the raw materials includes incineration, drying, grinding, sieving and mixing (or melting), and then sampling and analysis. These processes are carried out according to the form and type of raw materials, and not all raw materials must be processed through these processes ( See Table 1). Such as wet powder and fine mud, just dried in steam water jacket drying furnace, hot air fluidized layer drying furnace or electric hot plate drying oven. Waste materials containing organic materials (plastics, etc.) and other combustibles (oils, etc.) are usually burned in a muffle furnace. The air is cleaned by static electricity or a bag filter to ensure that the precious metal does not lose with the smoke. Ceramics, crucibles and refractory materials, as well as dried or incinerated materials, are ground by a continuous ball mill that is continuously screened and periodically emptied. Small batches of raw materials are milled with a grinding disc. The main control parameter for grinding is the grinding size. For lean ore, a suitable particle size is -30 mesh. Since the bulk material discharged from the mill is usually a metal block, in the process of pre-treatment of the incoming raw materials, it usually includes grinding the material for sampling and recovering the metal block from the work. That is, the finely divided fine particles are separately sampled and the recovered metal blocks are melted and sampled. In order to improve the environment and eliminate noise, there is a soundproof room in the mill room.

Table 1 Pretreatment and sampling analysis of waste materials

Second, sampling and analysis

Mixing blade iron fines on the concrete floor, then sampled with a cone stack quartering or riffling quarter method. However, since the fine-grained material often contains -30 mesh metal particles, it is easy to cause segregation error of the grade. To this end, the United Kingdom and the United States have designed the following new sampling system.

In the UK, a complete set of automatic sampling systems is used in each standard silo for grinding, sieving, sampling, storage and transportation. The system feeds the -30 mesh fine ore into a large container with inclined rotation. The mixed fine ore is fed into the shuttle feeder through the vibrating feeder, and then fed into the horizontal rotating collection box to ensure that the fine ore is evenly distributed from the sampling system. Flow out. This system has proven to be a versatile sampling system suitable for a variety of fine-grained materials, including poorly flowable powders.

A new sampling system recently designed in the United States is to pass the sieved fine ore through two vertical spiral mixers, then into a saddle-shaped two-way sampler, and take each of the powder mines flowing from both sides. A sample containing 1% of the total. The two comparative samples were respectively ground by a vibrating mill and then analyzed by shrinking in a rotating grid-like sampler to ensure that the samples were truly representative.

The laboratories at the Brinston plant are equipped with equipment for fire test and gravimetric analysis, as well as state-of-the-art instruments such as atomic absorption spectrometers, X-ray fluorescence spectrometers and electronically controlled direct reading emission spectrometers. Product quality is subject to rigorous analysis and testing.

Third, the melting and refining of raw materials

The raw materials which were pretreated and sampled were selected according to the smelting and refining process of Table 2 according to the level of precious metals and other components.

Table 2 Smelting and refining processes of raw materials

The Brinston factory uses a 5t disc ball machine. The disc has a diameter of 3.4 m, a peripheral height of 0.3 m, and a tilt angle of about 50°. The disc is rotated clockwise to adjust the speed. The fine ore and lime, iron oxide are compounded in a proportion of semi-silicate slag of CaO:FeO:SiO ₂ = ₁ :1.3:1.6, and the hopper is charged into a 5t screw mixer. It takes about 1 hour to charge and mix. The mixed material is supplied to the disc ball machine by a screw feeder, and the water sprayed onto the disc is adjusted to control the adhesion of the material. The produced ore balls are sent to a blast furnace or a reverberatory furnace for melting according to the grade of the fine ore.

(2) Smelting of the ore ball

The low-grade powder ore is sent to the blast furnace for smelting. The blast furnace designed by Johnson Majestic is highly automated. The charge of the ore, coke , lead oxide and pyrite is supplied by 14 charging funnels. Each funnel is equipped with a vibrating feeder, which is controlled by a computer with precise micro-information, which not only ensures uniform feeding, but also ensures normal reaction in the furnace. The vibrating feeder feeds the material to the belt conveyor and is automatically loaded into the furnace by the top of the blast furnace. A water jacket is provided in the tuyere area, from which oxygen is enriched into the furnace. The flue gas is discharged by the electrostatic precipitator and the bag filter. After the charge is melted, it flows into a front bed heated with petroleum , where it is separated by precipitation. The upper slag is cast into blocks for storage. The middle layer of matte copper contains a large amount of precious metals, which are sent to crushing, grinding, leaching and electrolysis to produce electrolytic copper. The lower layer of precious lead is sent to the ash.

The high grade ore ball is smelted by a reverberatory furnace and then sent to the ash.

(three) ash blowing

The precious metal products smelted in the Blastingham furnace and the reverberatory furnace are directly sent to the ash. The gold produced by ash blowing contains 98% silver and 0.5% to 1.5% gold, and is cast into an anode for silver electrolysis. The flue gas is purified by a high-efficiency bag filter or an electric dust collector to ensure that any valuable metal is included in the soot. The main component of the soot is lead oxide and returned to the blast furnace ingredients. In the ash blowing operation, the most important thing is to determine the lead content of the air in the workshop according to the design requirements. To this end, it is necessary to ensure the pumping capacity of the chimney and the ventilation of the workshop, and regular routine medical examinations and treatments are carried out for the production workers.

(4) Silver electrolysis

The alloy anode plate produced by ash blowing is electrolyzed in the Mübius electrolysis cell to produce electrolytic silver with a purity of 99.99%. Gold anode mud enriched to 40%, and containing a platinum group metal.

(5) Chlorination smelting

Silver electrolytic anode mud and materials and alloys containing 20% ​​to 50% gold are liquefied and concentrated by electric furnace chlorination. That is, after the raw material is melted in the refractory clay crucible, the refractory clay tube is inserted to pass chlorine gas into the bottom of the melt, and smelting causes the chlorides such as zinc , copper, nickel and silver to float. After the scum is removed, a batch of crude gold is added to continue the smelting. At the end of the smelting, sodium chloride is added to clean the surface of the melt to further chlorinate the impurities. The chlorination operation is usually carried out in a small crucible, and the gold produced is placed in a large inclined furnace where the ingot is as pure as 99.5% to 99.6%. The flue gas from the chlorination process passes through a special dust collection system to ensure that the air around the furnace is clean and the valuable metals are recovered. Although chlorination can produce high-purity gold, it takes a long time to chlorinate, which will increase the loss of gold in the flue gas.

(6), electric furnace smelting

The Brinston factory smelts high-grade raw materials and old currency, which are also smelted in electric furnaces. There are 9 electric furnaces from 4kg to 500kg in the melting workshop. The specifications are:

500kg tilting furnace 200kW 1kHz 2 sets

250kg tilting furnace 200kW 1kHz 1 set

135kg sealing furnace 200kW 1kHz 1 set

75kg sealed furnace 75kW 3kHz 2 sets

35kg sealing furnace 75kW 3kHz 1 set

7kg lift type furnace 30kW 3kHz 1 set

4kg lifting furnace 30kW 3kHz 1 set

(7) Gold electrolysis

The gold electrolysis uses the Walvier method under the following conditions: a liquid temperature of 60 ° C and a cell voltage of 1.5 to 2 V. The cell is stirred by mechanical or air. The anode cycle was 24h, and the cathode gold purity was 99.99%.

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