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Method of low temperature pyrolysis as an answer for PCBs utilization.

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Courtesy of Technokomplex

When people buy new computers, TV-sets or trendy mobile phones they do not think much about the future of substituted models. In a consumer society where devices are changed due to their obsolescence rather than break down the problem of pollution by e-wastes becomes more relevant.

Disposal of p
rinted circuit board (PCB) is an issue that has various solutions. Firstly, it is so due to the fact the boards contain components and parts that can be reused without any special treatment. Secondly, methods of utilization can vary greatly depending on the desired output. Finally, some of the methods do more harm to nature than nontreated boards.

Method of low temperature pyrolysis
is an answer for PCBs utilization. It causes no harm to the environment and allows gaining profit.

Unfortunately, about 85% of used printed boards are disposed in landfills today, which means not only the loss of limited material resources – precious metals, but also pollution of nature by the hazardous wastes that interact with the environment and form toxins in soil and groundwater.

Approaches to PCBs processing differs fundamentally depending on
the main derived products: 1) components and radio parts applicable to reuse 2) precious metals 3) liquid fuel, pyrolysis carbon and gas (pyrolysis products).
Content of precious metals in printed boards is
10 times more than in mineral ore.

Taking in consideration the above there are several approaches to the boards utilization. Today there is no confirmed data on precise materials ratio, but the magazine “Components and technology” (10'2008) provides information as follows:
Glass polymer - 70%, Cu - 16%, Solder - 4%, Fe – 3%, Ni – 2%, Ag – 0.05%, Au – 0.03%, Pd – 0.01%, other – less than 0.01%.

In general, all the methods and approaches have a right to exist and are more or less economically feasible in comparison to PCBs disposal in landfills.

As a rule, at the first stage of PCB utilization some components are dismantled for reuse. Traditionally it is done manually, but many foreign enterprises today use robot technology. The rapid depreciation of components, however, devaluate such feedstock.

There are two basic methods of printed circuit board processing that can be: a) applied simultaneously in complex lines for complete boards utilization; b) applied separately in equipment lines used for partial board processing.

Mechanical treatment allows winning 92-95% of metals (mainly copper) from already “empty” PCBs. The mechanical treatment includes:
- PCBs shredding;
- magnetic separation of ferrous materials from the total mass of shredded wastes;
- further grinding PCBs into powder in a ball mill;
- sieving of the powder;
- particle recirculation by static electricity;
- secondary grinding and separation of wastes that cannot be processed.

The method is more suitable for treating empty PCBs. Wastes that cannot be further processed are pyrolized to obtain both precious metals and pyrolysis products.

Hydrometallurgical method, namely refine extracting (or affinage) is a process of precious metals separation from impurities to obtain high quality final product. Refining is done at the special refining facilities. It is applied for precious metals leaching from e-wastes by 60% nitric acid 3- and 60% sulfuric acid. The price for remelting precious metals extracted from PCBs is about 92-98% of the total price for metals themselves.

The method is good for utilization of nontreated PCBs, but has several weak points:
- high reagent consumption for ballast metals dissolution;
- need to process excessively large scrap mass (2/3 of which are polymers);
- small amount of precious metals with a high impurities content in sediments that undergo
refining (affinage);
-
toxins emitted into the air by chemical degradation of polymers by robust solutions at elevated temperatures.

It is evident the mechanical method with further pyrolysis is more feasible and less harmful.

Pyrolysis of printed circuit boards is of the special value. Since the composition of PCBs is extremely heterogenous all the mentioned above methods based on materials fractionation become inefficient due to incredibly small amount of precious metals presented in layers and coupled with copper, solders, ferrous and non-ferrous metal alloys.

Accordingly, a complex and expensive multisystem equipment is required for commercially viable implementation of these methods. It is available for a limited group of investors who also have a right to carry out precious metals refining according to the Decree of the Russian Government of August 17, 1998 № 972.

In general, pyrolysis, being a chemical method, becomes the best solution for recycling and utilization of PCBs, even in terms of logic, as the main component of the boards is a synthetic polymer (including polymers with fiberglass).

Pyrolysis of PCBs like pyrolysis of other polymers provides liquid fuel and gas. Ceramics and other solid components are separated. Fuel and gas can be reused. Charred metal alloy called “black metal” contains a high percentage of copper, small amount of iron, calcium, nickel, zinc and aluminium which are subject to chemical reduction.

According to the tests performed in the Pirotex pyrolysis plant printed boards are the source of 25% liquid fuel from the total mass of feedstock. Taking into account liquid fuel is the most commercially valuable product among derived ones, the result is good.

When recycling PCBs in the Pirotex pyrolysis plant the owner of equipment gains profit by:
- receiving printed boards for utilization (in Russia the figures vary between 4 and 50 roubles for 1 kilo of wastes, in Europe the figures are much higher);
- selling derived pyrolysis products, namely liquid fuel;
- selling “black metal”.

Therefore, utilization of PCBs in the Pirotex pyrolysis plant is an ecologically friendly and economically beneficial business.

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