Color Generattion of Colloidal Gold

The intense red to purple color seen for the Purple of Cassius pigment is achieved by precipitating a colloidal gold solution onto a solid oxide substrate, typically tin oxide. To maximize color intensity,the gold particles need to be unagglomerated and less than 40 nm in size. The color effect is caused by a narrow adsorption band at 520 nm, referred to as the surface plasmon resonance band. If particles agglomerate together and increase above 40 nm, a blue shift occurs and light scattering starts to dominate, resulting in muddy color hues. In 1902, Gustav Mie [112], using classical electromagnetic theory, calculated from bulk properties of metallic gold the absorbance of colloid gold particles as a function of the particle size. His theoretical work also predicted that the wavelength of the plasmon band depends on the shape, surface composition, and dielectric environment of the gold particles.

These concepts are used in modern-day production and research and development to predict and control the color effects associated with Purple of Cassius. For example, most gold colloid solutions seen in the literature are as dispersions in water with fairly consistent dielectric constants. When considering glass enamels, the glass media in which the colloid is dispersed may consist of many different components, all with vastly different dielectric constants. This can lead to massive differences in hue and color strength for the final gold enamels. A practical demonstration illustrating the effect of dielectric constant of the surrounding metal oxide on color shift. Gold nanoparticles of approximately the same size (before and after heat treatment) have been deposited onto three oxides having differing dielectric values. It can clearly be seen that the oxides with low dielectric constants such as silica have red shifts whereas high dielectric materials such as titania shift the color to blue shades which is in accordance with predictions made by Mie. Another strategy for controlling the color is to alloy silver with gold. According to Mie, additions of silver will shift the plasmon band to a shorter wavelength and hence make the particles appear redder. Theory predicts that a single plasmon band with alloys having up to 25% silver will exist, but above this level two peaks appear, one caused by silver and the other gold. In practice, an excess of silver is used to redden off the gold.

Printed Circuit Boards PC Contain 100 g of Gold per tonne

Some circuit boards (such as power supply boards and electronic boards found in monitors) contain on average less than 100 g of gold per tonne. They are «low grade» boards. However, some boards (e.g. graphic boards, audio boards and network boards) contain a lot more precious metal. «high grade» boards contain between 400 and 500 g/tonne, and are found in laptops and mobile phones. «Very high grade» boards, containing more than 500 g/tonne, come from large mainframe computers or phone centres.

Once at the smelter, the different metals (gold, copper, silver, selenium,tellurium, lead, palladium, etc.) are recovered through complex processes. Due to the complexity of the technologies used and given that recovery practices can be highly polluting, the entrepreneur must sell the printed circuits to appropriate industries that can conduct recovery operations in an environmentally sound manner.

Business in Computer Recycling

Local business interested in computer recycling and reuse can play an important role in local and national authorities’ efforts to manage used and end-of-use PCs and related equipment in an environmentally sound manner. These businesses may protect against possible leaching of harmful materials and prevent unsafe practices common to the informal market. Businesses that repair, refurbish and upgrade PCs and related equipment for reuse provide the market with good products at affordable prices, thus bridging the digital divide. Finally, businesses can take advantage of valuable raw materials contained in PCs by extracting them in an environmentally sound manner, or if not possible, by selling certain materials to facilities that can do so properly. Governments can regulate formal businesses better under this process and ensure that movements of waste are properly monitored and controlled.

As local communities continue to access technology, more recyclable material will become available locally. Opportunities exist, and will grow over time, for local businesses to take advantage of the value contained in used and end-of-use PCs and related equipment. This guide provides a suggested blueprint for creating such a business, and for doing so in a sustainable and environmentally sound manner in conformity with national and international laws and regulations.

The second-Hand Market

The second-Hand Market

In developing countries, refurbished computers from the second-hand market provide opportunities for people who cannot afford to buy new equipment. Growing demand for refurbished equipment in these countries is matched by the need for spare parts for maintenance and computer repair. Recycling businesses can provide repaired and refurbished computers to this rapidly growing market at affordable prices.

The growth in the price of raw material

A local recycling business may also profit from the sale of components and recovered materials to facilities equipped to provide environmentally sound recycling. The rise of raw material prices has made recycling used computer components an economically viable enterprise for environmentally sound facilities,creating a market for properly disassembled components and scrap parts. This can be a profitable alternative for entrepreneurs, while preventing harmful practices such as dumping, open burning and incineration.

Economic Opportunities with the Recovery

The potential economic opportunities associated with the recovery of materials are also responsible for the advent of informal markets in developing countries not equipped to process used and end-of-use computers efficiently and safely for workers’ health and the environment. 

From an economic perspective, informal material recovery processors lose significant amounts of metals that could be more effectively recovered through the use of existing state-of-the-art technologies. More importantly, these informal markets employ recycling and materials recovery techniques that expose workers and the environment to potentially harmful pollutants.