Recycling CRT Monitors and TVs

Many of us still remember those recent times, when CRT monitors were used for visual presentation of information in a PC, and CRT televisions can still be found in almost every home. Nevertheless, the CRT age came to an end, and they were replaced by more advanced liquid crystal and plasma displays. The flip side of this progress was an unusually large number of unnecessary CRT monitors and televisions. According to some estimates, from several thousand to one million monitors and televisions are thrown out in different countries every year, and the total number of obsolete equipment that is still stored in the homes of the owners can amount to millions. It is predicted that the flow of this “e-garbage” will dry out only by 2020–2025. However, the main problem is that kinescopes require special disposal.


To answer this question, let us consider the devices of a device with a CRT and the tube itself, as well as the materials used for its manufacture.
The main components of a computer monitor or TV are a kinescope, a plastic case, printed circuit boards, wires, a deflection system, and protective elements. The kinescope makes up about two thirds of the mass fraction of the entire monitor or TV, as can be seen from the pie chart below.


Fractional composition of a CRT monitor or TV
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In turn, the main structural elements of the kinescope is a CRT, a cone, a screen and an internal magnetic screen with a mask.


Simplified schematic image of a kinescope

Fractional composition of the kinescope in mass percent has the following form:


Fractional composition of the kinescope

The inner surface of the screen is covered with four layers. The first layer is a carbon coating with various surfactant additives. The second layer forms a phosphor coating on which a wax-like layer is applied to level and protect the surface. The aluminum coating forms the fourth layer applied to increase the brightness. In the case of a kinescope cone, its inner side is covered with a layer of iron oxide, and the outer side is covered with graphite. The screen and the cone of the kinescope are interconnected by means of glass cement.

It is widely known that the kinescope is made of glass, the chemical composition of which varies depending on the functions performed by the elements of the kinescope. One of the main functions of glass is protection from X-rays. For this, about 34% by weight PbO is usually introduced into the glass of an electron gun. A slightly smaller amount of lead oxide contains a kinescope cone (22 wt.% PbO). In the case of a kinescope screen, its glass is specially made more thick to absorb dangerous X-rays. In addition, this glass should have good optical properties, so it is made of barium-strontium glass (absorbs X-rays about one and a half times worse than lead glass). Note that in the screens of color TVs produced before 1995, glass containing up to 5 wt.% PbO was used. However, thanks to the efforts of the German Central Association of Electrical and Electronic Industry (ZVEI) to increase the utilization of kinescopes, most manufacturers since 1996 completely switched to the production of screens without the use of lead oxide. This example was not followed by the American producers Corning and Corning Asahi Video (Thompson RCA switched in 1998).

In black and white TVs, the screen and the kinescope cone are made of the same type of glass, which usually contains up to 4 wt.% PbO. This difference in the chemical composition of glasses of different types of TVs is due to more powerful X-rays in color TVs due to an increase in accelerating voltage up to 20-30 kV against 10-20 kV for a black and white TV. The average chemical composition of kinescope glasses is given in the table below (depending on the manufacturer, the glass composition may vary somewhat).



As the reader has probably already guessed, the main environmental hazard is lead oxide, which is part of the kinescope glass. The amount of lead oxide in one kinescope depends on its size and can vary from 0.5 to 2.9 kg with an increase in its measurements from 13 to 32 inches, respectively.


The content of lead oxide (II), depending on the size of the kinescope

A feature of these glasses is that lead ions are relatively easy to leach out of glass and enter the environment. For example, with improper disposal of the kinescope, the leaching of lead ions can occur under the action of organic acids, which are formed at the landfill for household garbage. Of all the lead-containing components of the kinescope, the most easily leached comes from glass cement.
Lead, like its compounds, is a toxicant with a pronounced cumulative effect, causing changes in the nervous system, blood and blood vessels. This circumstance suggests the need for proper utilization of kinescopes by dumping them at special landfills or recycling.

Consider the existing methods of recycling kinescopes.
As a rule, the recycling process begins with the manual dismantling of televisions or computer monitors. This operation dismantles the case, printed circuit boards, speakers, wires, protective metal casing, deflecting system and electronic gun. Also for safety reasons, a vacuum is released from the kinescope during this operation by making a hole at the site of the high-voltage output or through the electron gun throat. Protective iron clamp on top of the connection of the cone of the kinescope with the screen is also cut off. All these components are sent for further processing. As a result, only the kinescope remains, which must be divided into a cone and a screen due to their different chemical composition, which is important for their subsequent disposal.

In practice, the separation of the cone and the screen is most often performed using a diamond saw, a heated nichrome wire or a laser. After that, an internal magnetic screen with a mask is removed from the cut-out kinescope, and the screen itself is sent to a chamber in which a phosphor is collected with a vacuum cleaner (buried in a special test site). Thus, two types of glass are obtained at the output - lead and barium-strontium.

This process is presented in the video below.


There is also a slightly different way of separating lead and barium-strontium glass. This method consists of the following technological operations: crushing of kinescopes, extraction of magnetic fraction, mechanical removal of coatings, washing the glass with water, drying, and, finally, separation into lead, barium-strontium and mixed glass using special analyzers (X-ray fluorescent or ultraviolet) and air guns . Note that in this technology, water is used in a closed cycle, and the amount of waste is 0.5% (glass dust, phosphor, coatings). This glass separation method is used by Swissglas AG (Switzerland), RTG GmbH (Germany), SIMS (United Kingdom).

We now turn to the most important issue - the disposal of lead and barium-strontium glass. Until recently, these glasses were mainly sent to factories for the manufacture of new kinescopes. However, with the advent of liquid crystal and plasma displays, the production of kinescopes ceased, which made this method of processing practically irrelevant. Nevertheless, in China there are three enterprises (Shaanxi IRICO Electronic Glass, Henan AnCai Hi-Tech and Henan AnFei Electronic Glass), which can use up to 100 thousand tons of glass per year, which is only a small part of the total (5.2 million tons according to a Qinghua University report).

It should be noted that barium-strontium glass has found application in the production of building materials due to the low leachability of barium and strontium ions, the concentration of which does not exceed the permissible limits. Therefore, the following discussion will deal only with the disposal of lead glass.

At present, the only and most widely used method of processing lead glass is its use as recyclable lead. For this, metallurgical melting furnaces for lead are used, in which the flux is partially replaced by lead glass. However, the number of furnaces that use lead glass in their technological process is rather small for the whole world. For example, Doe Run (USA), Xstrata and Teck Cominco (Canada), Boliden Rönnskär Smelter (Sweden), Metallo-Chimique (Belgium).

Due to the small number of furnaces and the high cost of transporting recycled materials to them, this led to the fact that it was easier to send lead glass to the landfill. However, some companies involved in the disposal of "electronic waste", have chosen a different path.
For example, to solve this problem, the company SWEEEP Kuusakoski Ltd. (United Kingdom), in collaboration with Nulife Glass, Sheffield University and Aalto University, developed and launched a furnace for glass lead production on November 30, 2012. The furnace is heated by electricity, and the raw material is pre-crushed and lead glass mixed with a reducing agent (chips up to 3 mm in size). After the reduction process at 1200 ^o C, lead granules and glass are obtained at the outlet. This oven can process up to 10 tons of glass or up to 2 thousand large TVs per day.

Report from the opening ceremony


Alternative methods for recycling lead glass have also been proposed. In general, they all boil down to the idea of ​​using glass for the manufacture of building materials (foam glass, for example) or as an additive to building materials such as brick, concrete, cement, decorative tiles, etc. Building materials with a high lead glass content can be used to protect against x-ray radiation. It has also been proposed to use lead glass in the ceramic industry to create glazes that are resistant to leaching.

The main disadvantage of building materials with lead glass additives is the reduction of their mechanical properties. In addition, the results of the tests for leachability showed that the concentration of lead ions in most cases exceeds the permissible limits (according to American standards, the concentration of lead ions should not exceed 5 mg / l). Also note that in many countries the use of toxic substances in building materials is prohibited by law.

The above mentioned problem can be solved by special chemical treatment of glass, the essence of which lies in the preliminary leaching of lead. In this process, leaching is generally carried out with nitric acid for one hour, followed by washing and drying the crushed glass. Next, the leaching products are sent to a chemical plant for further processing, and the resulting glass chips can be used in building materials. This lead glass recycling method is used in Hong Kong.

In conclusion, it should be said that the problem of disposing of old TVs and CRT monitors will be relevant for at least the next decade. The situation with the solution of this problem may differ significantly in different countries of the world, which is primarily due to the lack or availability of technologies and processing plants, government support, recycling culture. In the CIS countries, as well as in Ukraine, the state of affairs in this regard can be said to be depressing. Only not in many cases kinescopes turn out to be on special testing grounds, and one has only to dream about their processing.