Assessing and Recycling Precious Metals

There seems to be a continually growing demand all over the world for precious metals, whether for use in industrial applications or for jewelry. While silver and gold have been used for thousands of years in jewelry, very recently precious metals are performing specialist and useful functions in chemical processes, electronics manufacturing, automotive and aerospace systems, for example, catalytic converters. CARBOLITE equipment is used all over the world both for recycling these comparatively rare commodities and also for evaluating their purity. Two key processes in the precious metals market are cupellation and smelting.

Smelting furnace SCF 4

Smelting furnace SCF 4

Smelting

Smelting refers to the separation of precious metal from non-metallic impurities. When bulk ores are involved, extremely large installations are needed, using heat and chemical reducing agents in order to decompose the ore, drive off other elements such as slag or gases and leave only the metal behind. When the process involves pre-used materials – for instance, catalytic converters or jewelry – the equipment is smaller in scale but deals with the same principles in order to guarantee a high-quality end material.

Precious metals recovery is considered to be a typical use of smelting. In a catalytic converter, exhaust gases pass through a ceramic honeycomb which is coated with a fine layer of palladium group metals (PGMs). This comprises of platinum ore, typically two to three orders of magnitude richer than the mined ore. The honeycomb ceramic is crushed into a powder during the recycling process and then fed into the furnace. Up to 98% of PGMs may be recovered.

The raw material to be assayed is typically ground into powder form for recycling purposes, before being placed in crucibles where it is melted with lead oxide and a suitable flux at 1200 ºC. The lead, in its molten state, merges with the precious metal and the impurities produce a slag, which can be effortlessly removed when the sample cools. A cupellation furnace is then employed for removing the lead, leading to further treatment in order to ascertain the precious metal content.

CARBOLITE’s SCF furnaces are specially designed for providing the temperatures needed for smelting, while surviving the gases given off by the process. High-performance silicon carbide elements are attached, which allow non-stop operation up to 1350 ºC. Furthermore, furnace chambers are lined with silicon carbide tiles capable of protecting the elements and the insulation, with hard-wearing insulating brick offering a solid hearth.

Material is processed in ceramic crucibles of different sizes, and the SCF range includes five models to suit varied customers’ requirements. The smallest model is a single chamber unit, while the other four are made up of twin chambers. The hinged chamber lids keep the hot face insulation away from the operator and thus incorporate chimneys in order to vent fumes. Furnace temperature is regulated by a three-term microprocessor-based controller, and a 24-hour seven-day time switch allows the equipment to be switched on automatically without the need for an operator.

Smelting

Assaying

Accurate analysis is regularly performed in order to determine the precious metal content in an extensive range of different materials. This is mainly important for fineness determination for the hallmarking of precious metals for jewelry use. The precious metal concentration in different materials can differ widely from amounts measured in parts per million (ppm) for car catalysts or powdered ore samples to almost 100% in jewelry alloys or bullion bars.

Determining the precious metal content of silver, gold and platinum is basic to the jewelry industry and is mainly vital for those valuing, selling or buying second-hand material or scrap gold. While X-ray fluorescence (XRF) spectrometry is more accurate than in the past and is also capable of providing reliable results, there are a number of disadvantages including the initial purchasing expense. For silver and gold, the most accurate and preferred method still continues to be the traditional cupellation process.

The cupellation of gold is a refining process in which all other elements are eradicated, leaving just pure gold. For assaying gold by cupellation, small samples are precisely weighed and then wrapped in lead with a pre-determined quantity of silver in order to assist with the collection of gold and the removal of base metals. The wrapped samples are then placed on porous blocks, called cupels, and heated to 1100 ºC in a furnace, as a result of which the cupels absorb the lead and any base metals as oxides.

A pure gold and silver bead is left behind, which is boiled in nitric acid, dissolving the silver and leaving behind a residue of fine gold. When the weight of the residue is compared with the original sample, it is possible to calculate the gold content by weight.

The assay of gold alloys by cupellation is the standard method employed by the UK Assay Offices and a reference method laid down by the International Hallmarking Convention in accordance with ISO 11426 : 1997 and ASTM E1335 – 08. The CARBOLITE furnaces designed for this process are known to have a reputation for close temperature uniformity, maximum fume control and low heat loss.

Features

Smelting Furnaces

  • Maximum operating temperature of 1400 °C
  • Each chamber lid is vented by a 50 mm diameter chimney
  • Silicon carbide heating elements protected by silicon carbide tiles
  • Specifically designed for smelting of precious metals
  • Four models available including multiple and single crucible options

Cupellation Furnaces

  • Refractory brick insulation and silicon carbide tiles resist the corrosive lead fumes produced by the process and protect the heating elements
  • Removable brick spy hole plug allows simple viewing of samples during heating
  • Three models are available with a continuous operating temperature of 1200 ºC
  • Silicon carbide elements below and above the chamber provide even long-life and heating
  • Controlled variable air-flow is pre-heated by passing over the lower elements ensuring good temperature uniformity
  • Fumes are extracted via an insulated exhaust duct, with lead condensates being safely collected in a removable container

Carbolite Gero Ltd.

This information has been sourced, reviewed and adapted from materials provided by CARBOLITE GERO Ltd.

For more information on this source, please visit CARBOLITE GERO.

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