Current technologies for recycling of waste printed circuit boards mainly target towards energy and precious metals and this leads to adverse environmental impact due to the presence of brominated flame retardants, heavy metals, polychlorinated biphenyls etc. Mechanical processing is considered to be most environmental friendly approach. Pyro and hydrometallurgical processing routes are also being followed.

Gold recovery from e waste has gained attention because of its concentration in the e waste which is around 100 times more than that is found in gold ore.Several leachants like cyanide,thiourea and thiosulphate etc are used for recovery of gold.Cyanide process tops the list because of simplicity of operation and better economics.but its high toxicity raises a need for development of its substitute.Thiourea can be the best substitute for cyanide.Leaching of gold by thiourea is considered due to rapid reaction.

Regarding this question i have one alternative to discuss i.e. the application of native microorganisms through bioleaching technique.

The construction of a metabolically-engineered strain of Chromobacterium violaceum that produces more (70%) cyanide lixiviant and recovers more than twice as much gold from electronic waste compared to wild-type bacteria. Comparative proteome analyses suggested the possibility of further enhancement in cyanogenesis through subsequent metabolic engineering.

http://www.nature.com/srep/2013/130719/srep02236/full/srep02236.html

The disposal of waste electrical and electronic equipment (WEEE) or "electronic waste" is an important issue all over the world., Environmental risks were seldom considered fully by routine methods. Microwave-induced pyrolysis, which is collaboration of microwave heating and tip discharge, was proposed as a novel way to dispose of WEEE. Due to the rapid heating by microwave irradiation, dioxins and furans could be avoided effectively. After pyrolysis, the gas and liquid products could be recovered as fuel gas and chemical feedstock respectively, the abundant metals, such as Fe, Cu, etc., would be more convenient to be separated and recovered from the solid products. Thus, by means of microwave-induced pyrolysis, it is potential to realize metals recycle, products reuse as well as pollution avoidance.

http://10.1109/APPEEC.2010.5449100

One of the key challenges is the collection of ewaste products. It is not just a technical matter, but also related to get precious metal bearing fractions, either from shredding or dismantling, to facilities with a high recovery level. See here for a good paper on what it takes for developing and emerging economies: http://www.sciencedirect.com/science/article/pii/S0956053X12001407

In addition, UNU recenty released an e-waste worldmap with both first rough quantity estimates per country and what policies are there: http://step-initiative.org/index.php/WorldMap.html. This attracted a lot of media attention last week.

I am not sure about the precious metals recovery, but in generally people use the technique called "Laser Induced Break down Spectroscopy (LIBS)" to Identify the type of metallic species in the scrap (qualitatively). Once you are identified the type of metals those are embedded in the scrap, then the job will become easy to sort out them.

The unstoppable demand for electronics is resulting in shortages of critical raw materials including some rare earth metals. A 2010 report from the UN Environment Programme listed 14 metals that are used in the manufacture of high tech product that are in critical supply including cobalt, gallium, indium and magnesium. In a recent statement the UN Commission for Sustainable Development outlined that only 1% of “specialty metals” used in electronics manufacturing are currently recycled compared with 50% of common metals such as steel. Ironically, the green energy sector consumes proportionately more rare earth than most other sectors. For example a hybrid or electric car contains 10 -20 kg of rare earths as opposed to a normal petrol or diesel car. However, researchers at the University of Southern Australia are developing a process for better recovery of metals from e-waste. The process that they are calling “Urban Mining” involves grinding e-waste into a powder which can then be put into flotation plant similar to those used in the mining industry to separate materials. Air is pumped in to agitate the concentrate separating the precious metals into layers. The process is known as froth flotation.

https://www.2degreesnetwork.com/groups/waste-management/resources/positive-or-negative-many-faces-e-waste/

Printed circuit boards (PCBs) are the most essential components of all electrical and electronic equipments, which contain noteworthy quantity of metals, some of which are toxic to life and all of which are valuable resources. To optimize the parameters required for recovery of tin from waste PCBs, initially the bench scale studies were carried out using fresh solder (containing 52.6% Sn and 47.3% Pb) varying the acid concentration, temperature, mixing time and pulp density. The experimental data indicate that 95.79% of tin was leached out from solder material using 5.5M HCl at fixed pulp density 50 g/L and temperature 90°C in mixing time 165 min. Waste Manag. 2012 Oct;32(10)

The ammonium thiosulphate based leaching of gold and silver

from PCBs waste, using crushing as a pretreatment, was investigated. It was possible to achieve 98 % gold and 93 % silver recovery within

48 hours of ammonium thiosulphate leaching. This type of leaching is a better leaching procedure for recovery of gold and silver from PCB

waste than the classical toxic cyanide leaching. 84 % Cu, 82 % Fe, 77 % Al, 76 % Zn, 70 % Ni, 90 % Pd, 88 % Pb and 83 % Sn recovery of

the accompanying metals was achieved, using sulphuric acid with hydrogen peroxide, sodium chloride and aqua regia. A four steps leaching

process gave a very satisfactory yield and a more rapid kinetics for all observed metals solubilization than other technologies.

http://actamont.tuke.sk/pdf/2011/n2/2ficeriova.pdf

The financial viability of a sound recycling program depends on the avoidance or limitation of external costs that are related to collecting, sorting and processing materials, coupled with the financial success that depends on the revenues of selling the recycled materials. A comparison between costs, benefits and risks should always be accounted in each particular case. However, in all regions, compliance with legislations and their actual enforcement will probably be the biggest challenge of all.

The e-waste disposal industry has the potential to be profitable. Disposal of e-waste is also an economic activity but this needs to be regulated and the workers and the environment protected.

Electronic waste (e-waste) is one of the fastest-growing pollution problems worldwide given the presence if a variety of toxic substances which can contaminate the environment and threaten human health, if disposal protocols are not meticulously managed.Several tools including Life Cycle Assessment (LCA), Material Flow Analysis (MFA), Multi Criteria Analysis (MCA) and Extended Producer Responsibility (EPR) have been developed to manage e-wastes especially in developed countries.

http://www.sciencedirect.com/science/article/pii/S0956053X13000147

According to data collected by the StEP Initiative, the annual world volume of end-of-life electronics is expected to jump one-third to 65.4 million tons by 2017. That's a 33 percent jump, in five years, of the refrigerators, TVs, mobile phones, computers, e-toys, and other electronics people around the world have used and discardedhttp://www.theverge.com/2013/12/15/5208626/step-initiative-global-ewaste-map.

The entropy problem haunts all recycling efforts. We go from concentrated ore and oil bodies to widely distributed consumer products. The energy involved in recycling is almost prohibitive without a lot of energy supplied to the process to gather the distributed products in a central recycling facility