SE501043C2 - Methods for chemical destruction of heavy metals in hazardous waste - Google Patents

Abstract

The invention relates to a method of separating and neutralizing heavy metals such as mercury and cadmium and lead of dangerous waste. When a classification of the source has been made, the heavy metals dangerous to the environment are to be destructed separately. At the same time as Hg and Cd are present in the gas phase in a heat treatment oven and lead is in the liquid phase, elementary Se is boiled off. By a following chemical reaction the heavy metals are neutralized when they are transferred into their metal selenides. A further chemical reaction will take place in the oven when a glass melt is formed. Then, the glass melt dissolves the metals which are assimilated by the glass; a vitrification has taken place. In the process the resources of the waste are utilized in such a manner that the composition of the waste will affect the final result, i.e. a glass billet is formed which neutralizes eternally fluorescent lamps, batteries etc. in a safe and simple manner.

Description

501 043 i z Mercury (Hg) is a very toxic heavy metal that can damage the central nervous system.l small amounts it causes headaches and nervous problems.in larger amounts of convulsions and coma conditions, which can lead to death.

Minimized "disease" that turned out to be mercury poisoning. Occurred in Japan in the 1950s.

Some substances used in industrial applications are so environmentally hazardous that they should be completely eliminated from handling. These include both mercury and cadmium which are on the Chemicals Inspectorate's 13 list. Which is a list of hazardous substances that should be banned as soon as replacement material has been produced.

Mercury-containing waste from dental care and collected batteries, etc. is currently stored at SAKAB, but there is no method to neutralize the material there.

When it comes to the neutralization of mercury in fluorescent lamps, there is a plant in Karlskrona which, via a complicated method, condenses out mercury.

This costs society 60 million / year to recycle about 100 kg Hg, (25 mg / fluorescent lamp).

Of spent 15 - 16 mill. fluorescent lamps per year in Sweden, only about 4 million are treated as above.

It is an expensive process, where the fluorescent tubes are collected, for transport to Karlskrona.

If the process were simpler, with destruction devices deployed locally in the country, a larger part of the fluorescent tubes could be taken care of at a lower cost. At present, at least ll mill. fluorescent lamps outside the recycling system, with emissions of about 275 kg Hg / year to the air. to end up in the environment.

Among the harmful substances that are added to our environment, the heavy metals occupy a special position in that they are not broken down. The quantities that have entered production end up in the environment sooner or later.

The reason is mainly a lack of destruction facilities out in the country. The present invention relates to a method of eliminating the heavy metals Hg and Cd contained in waste products, via a chemical bond with selenium (Se), and encapsulation in glass.

The device in which the chemical reaction is induced is an electric heat treatment furnace with a capacity of about 900 - 1000 oC.

Such furnaces usually do not have devices for supply air and flue gas escape, they can be said to be closed.

The hazardous waste is placed in the oven, which must be rendered harmless with regard to the content of heavy metals.

It can consist of light sources containing Hg, but which largely consists of glass. Amalgam residues consist of 50% of Hg. Batteries can contain both Hg and Cd, but have an aluminum casing.

Thermometers contain pure Hg.men with a glass cover. Mercury switches enclose Hg in a housing of g1as. Modern types of power tools include rechargeable batteries with Cd.

The harness is now also supplied with the harness (Se), which can possibly be enclosed in a special ampoule1, made of glass or aluminum.

The molecular weight for Hg is 200.61 kg / kmol and for Cd 112.41 and for Se 78.96. Thus, only a small amount of Se is required to bind the two heavy metals chemically.

When the temperature is allowed to rise in the closed oven, and the material takes temperature. It occurs at 650 oC that the amalgam melts and the Hg content is evaporated by about 50%. The evaporation temperature for Hg is 357 oC, which is why the decoction goes fast.

The batteries have an aluminum housing, melting point 658 oC, and release the mercury at approximately this temperature.

Selenium is the element that must bind the heavy metals in the gas phase at 680 oC, whereby the reaction proceeds rapidly.

The mercury in fluorescent tubes and thermometers, as well as other glass inclusions, is only accessible to the chemical reaction at the melting point of the glass, 800-900 oC. co; 501 043 4 Now that the temperature has risen so high that both Hg and Cd are in the gas phase free in the furnace, both heavy metals are accessible to the selenium vapor, and the metal selenide in the form of crystals falls to the bottom of the furnace.

The prerequisite for the complete binding is that there is a certain excess of selenium in the oven. To bind 10 g Hg, 3.95 g Se is primarily required.

The reaction is accelerated by the high temperature and the reaction goes sharply to the right according to the following: Hg + Se - -> HgSe The mercury selenide consists of grayish disc-shaped crystals and forms a very stable compound which can only be dissolved with royal water.

The chemical reaction is best in oxygen deficiency, because then no oxides can be formed. In the oven, excess oxygen can form selenium dioxide and elemental mercury (HgSe + O 2 - -> SeO + Hg), which would be fatal.

The argon or krypton gas contained in the fluorescent tubes acts as a shielding gas, and facilitates the formation of HgSe.

Should the proportion of fluorescent lamps be small.with a certain destruction.shielding gas can be added from the gas tube, and glass scrap is introduced into the oven.

Since a large part of the mercury has been enclosed in various forms of "glass packaging", there is now a melt at the bottom of the oven, where a large part of the glass is contained. The harmless heavy metals will now be baked into an enclosure in their selenium forms. of glass.along with other metal residues.

Due to the glass content of the waste, the residual material is now well encapsulated, and can now be disposed of in a safe way.

Physical properties of substances included in the process: Hg See Al Cd amalgam. glass t sm oC -38.9 217.4 658.0 321.0 650.0 850.0 t kp oC 357 680 1800 767 - - 501 043 Thus, in order for the process to be effective, the selenium must be overdosed. During the process, the mercury content in the furnace is detected. Through a meter of so-called "sniffer type" takes a smaller flow of about 1-3 l / min which is analyzed continuously. and returned to the oven.

The meter only detects atomic mercury.

During the measurements, the mercury content in the furnace gases is registered every second, and averaging over 6 measuring points, ie 6 seconds, can be obtained.

In this way, information is given about the mercury content in the furnace gas and the process can be interrupted, as there is no longer detectable mercury vapor.

The oven can then be opened, and the ice cream melt with its metal content is shaved out of the oven. The melt is suitably shaped as it falls into a steel mold in the final phase.

Claims (2)

10 15 501 043 PATENT REQUIREMENTS Method for chemically neutralizing heavy metals in solid hazardous waste, which also includes glass, characterized in that the solid selenium is supplied to the waste in an autoclave, that heat energy is supplied for evaporation and / or melting of the heavy metals and for evaporation of the selenium in the autoclave during exclusion of the ambient atmosphere, the amount of selenium being adjusted to achieve molecular equilibrium with the heavy metals to form environmentally neutral metal selenides, and the glass in the hazardous waste being used for permanent encapsulation of the formed selenides. 2. A method according to claim 1, characterized in that shielding gas, for example argon or krypton, is included as part of the waste when it is introduced into the autoclave.