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TMT 15® is a 15 % aqueous solution of Trimercapto-s-triazine, trisodium salt (C3N3S3Na3, CAS-RN 17766-26-6). It is primarily used to precipitate heavy metals. The chemical characteristics of TMT 15® are not comparable to other triazine compounds, e. g. those used as active ingredients in herbicides.
Since 1979, TMT 15® has been exclusively manufactured by Evonik Industries AG. TMT 15® can be ordered directly or through authorised distributors in drums, containers and in bulk.
TMT 15® is used to precipitate single and bivalent heavy metals such as Ag, Cd, Cu, Hg, Ni and Pb from waste water streams. If the effluent, however, contains heavy metal complexes which can not be eliminated as hydroxides due to neutralization, TMT 15® can be used for precipitation.
In particular, TMT 15® has shown high efficiency in the treatment of effluents/waste water from metal working, chemical, photographic or mining industry and especially from thermal processes (municipal-, hazardous waste, sludge/biomass incinerators and power plants). Today, TMT 15® is used in several hundred incineration plants on commercial basis, predominantly for the precipitation of mercury.
Passing a scrubbing system the heavy metals from the flue gas are transfered into the scrubbing water. Usually, the concentration of heavy metals in the scrubbing water exceeds the allowed discharge limit and as a consequence have to be eliminated. For the first step the acid scrubbing water is neutralized with lime [(Ca(OH)2] or caustic soda [NaOH]. During this step most of the heavy metals are precipitated as hydroxides. However, especially mercury [Hg] is only partially removed, as it forms a soluble and very stable mercury-tetrachloro-complex [(Hg(Cl4))2-] in the presence of chlorides in high concentrations. As a consequence other technologies for its removal are required.
Neutralisation combined with TMT 15® is an effective process for separating mercury. Neutralisation followed by TMT 15® dosing yields a precipitate consisting of metal hydroxides and metal-TMT compounds. Most of the heavy metals are removed by the lime or caustic soda, whereas the remaining metal complexes - especially Hg - are precipitated by TMT 15®.
The previously described combination of neutralisation and TMT 15® dosing is the approved standard method to treat flue gas scrubbing water.
The TMT 15® dosage rate to the scrub water is calculated based on the mercury content; under normal operational conditions where the Hg concentration is kept within limits, a TMT 15® addition is not necessary. Whether TMT 15® is required in power plants depends e. g. upon the quality of coal.
For ecological and economic reasons the TMT 15® dosage should be optimised. For example an adjustment of the TMT 15® amount is possible by using an on-line mercury analyzer. This combination is mainly of interest for incinerators where the mercury concentration in the waste water varies considerably.
The leachability of heavy metal-TMT compounds from flue gas scrubbing water has been tested according to the German Standard DIN 38 414, part 4 (at pH 4), the Swiss Standard TVA draft 8/88 and US TCLP standard. The results show that heavy metal-TMT compounds, especially TMT-mercury, are transferred into hardly leachable compounds.
The complete cracking of Mercury-TMT is only possible with aqua regia under heat and pressure (4 h 180 °C / 356 °F). This is not achievable with concentrated nitric or sulphuric acid.
Mercury-TMT precipitates are thermally stable up to 250 °C (480 °F). This allows subsequent drying processes. Due to the high thermal stability of Mercury-TMT TMT 15® is also recommended for the mercury precipitation in flue gas scrubbing water with connected drying processes at the outlet side.
TMT 15® in aqueous solutions can be determined by UV spectroscopy to the single digit ppm range.
The dosage rate of TMT 15® is calculated on the basis of the maximum quantity of Hg which is expected. Based on our experience, about 50 ml of TMT 15® per cubic meter of flue gas scrubbing water can be taken on an average, which means that this quantity can precipitate up to6 g Hg/m3.
Other heavy metals which are usually present in much higher concentrations (in municipal waste incinerators hundreds of grams per cubic meter) are already precipitated as hydroxides during the neutralisation step. Since the solubility of heavy metal-TMT compounds is much lower than the corresponding hydroxides, the excess of TMT 15® reacts with the hydroxides, (the solubility products for Cd(OH)2 and Cd3TMT2 are 3.1x10-14 and approx. 7x10-33 respectively) by forming hardly soluble heavy metal-TMT-compounds.
The conversion of hydroxides to heavy metal-TMT precipitates proceeds until the excess of TMT 15® is used up. This phenomenon has been observed many times under practical applications. A TMT 15® excess in this application is selfregulating.
TMT 15® will be adsorbed on activated sludge.
Besides TMT 15®, sodium sulphide, dithiocarbamates, and sodium trithiocarbonate are commercially available precipitating agents.
TMT 15® has very favourable toxicological and ecological properties compared to all other precipitating agents. Besides the very low acute toxicity (LD50 7878 mg TMT 15®/kg*) TMT 15® is practically not toxic for fish (LC0 13720 mg TMT 15®/l*) and water fleas (Daphnia magna, EC50 4224 mg TMT 15®/l*). TMT 15® is not mutagen and shows no acute toxicity against bacterial population. The toxicity to green algae is low (IC50 273 mg TMT 15®/l*).
TMT 15® has a good storage stability and is a safe product. Should TMT 15® accidentally come into contact with acids only minor amounts of hydrogen sulphide and no carbon disulphide are released and the acidic form of TMT 15® (TMT-H3) would precipitate, which is practically insoluble in water. This reaction is reversible and the TMT sodium salt can be achieved again by the addition of caustic soda.
As TMT 15® is stable in the acidic range, TMT 15® is directly added to the acidic scrubber water for the Hg precipitation in some plants. The acid/base reaction of TMT is used to determine the TMT 15® concentration.
TMT 15® is not a dangerous substance within the meaning of transport regulations.
In addition to the proved application of TMT 15® in scrubbing water from flue gas treatment TMT 15® can also be used directly in the alkaline scrubbers of incineration plants to reduce the emission of mercury in the clean gas. Mercury is precipitated in the scrubbing water. The TMT-mercury-compound is hardly soluble and resistant against temperatures up to 250 °C (480 °F) and as a consequence a reduction to the volatile metallic mercury is inhibited.