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Tianjin SEAM Paving ProjectH2S MEASUREMENTS ON AUGUST 28 AND 31, 2002 Background Sulphur-asphalt paving mixes using SEAM are prepared usually by adding between 2.5 and 3.0% SEAM to a conventional asphalt hot-mix. SEAM mixes must be prepared at temperatures below 150 °C as outlined in the “Safety” attachment in Appendix 1. Temperature Control and Emissions Liquid sulphur and hot asphalt cement react at high temperatures, above 150 °C, releasing hydrogen-sulphide, a toxic gas. This oxidizes to form low concentrations of sulphur-dioxide and sulphur vapor. The latter is an irritant and a nuisance but is not considered harmful. The tolerable limits for pollutants are shown in Table 1. Based on experience, the main hazard associated with sulphur-asphalt paving projects is emission of hydrogen-sulphide emission associated with overheating of the mix. Table 1. Health Standards
The amount of hydrogen-sulphide released depends upon the temperature of the sulphur-asphalt mix and the reaction time. This can be illustrated by laboratory studies where mix batches were prepared at various temperature levels and stored in a closed vessel with the air evacuated at a constant rate. Figure 1 indicates that at processing temperatures below 150 °C the sulphur-asphalt mix gives off very little hydrogen-sulphide. This small amount usually dissipates in open air or in well ventilated areas. At temperatures above 150 °C, the rate of hydrogen-sulphide evolution increases significantly and at very high temperatures emissions may not dissipate readily and are therefore considered hazardous. SEAM Mix Processing SEAM contains additives to reduce mix odor and the amount of hydrogen-sulphide generated by SEAM+asphalt paving mixes. However, it is still very important to process the SEAM mixes at as low a temperature as possible and to minimize SEAM mix storage time. The recommended temperature for SEAM mix processing is 140 °C. This minimizes mix odor and hydrogen-sulphide evolution while at the same time allowing enough time for mix delivery to the paving site and its placement and compaction.
Figure 1. Influence of mix temperature on hydrogen-sulphide gas emission (reaction time=1 h)
Tianjin SEAM Project: Hydrogen-Sulphide and Sulphur-Dioxide Measurements Mixes were produced by adding SEAM to AC 20 and AC 13 specification mixes on August 28 and 31, 2002. H2S and SO2 measurements were taken in all areas where personnel were working both at the batch mixing plant (1 ton size) and at the paving site. Gas emissions were measured using the Industrial Scientific Model TMX 412 meter (H2S and SO2) and the Drager MicroPac, #6408420 (H2S). The observations were as follows. On August 28th the SEAM mix was produced at temperatures generally in the 147 to 150 °C temperature range, measured as the mix was being discharged into the truck at the plant. No hydrogen-sulphide or sulphur-dioxide was detected at the hot-mix plant. At the paving site, H2S was detected near the paver operator: While most of the time the H2S reading was zero, readings of 1 and 2 ppm (parts per million) were detected periodically when the SEAM mix was being discharged into the paver hopper. Similarly, at the rear end of the truck discharging into the paver hopper, where a laborer was pushing the tailgate with a shovel, the H2S measurement was zero most of the time but periodically would register as high as 2 ppm. On August 31st the SEAM mix was produced at a lower temperature than on the first day. It was produced in the 140 to 143 °C temperature range. The H2S readings and mix odor were noticeably lower than on August 28th because of the lower temperature. No H2S was detected near the paver operator. The maximum H2S reading at the rear end of the truck unloading the SEAM mix, near the laborer working in this area, was 1 ppm. The odor from the SEAM mix seemed less strong on the second day of paving, when the mix temperature was lower. Sulphur vapor was observed near the back of the paver and for a distance of approximately 5 meters behind the paver, in an area where the pavement surface temperature was above approximately 110 °C. The sulphur vapor caused eye irritation if a person stood in this area for a longer period of time. This irritation ceased after walking away from the area with sulphur vapor. No personnel were observed working in this area behind the paver for any length of time. No sulphur dioxide was detected with SEAM mix in the temperature ranges between 147 and 150 °C and between 140 and 143 °C. While the odor from the SEAM mix was different than from conventional hot-mix, and personnel were subjected to “nuisance” sulphur vapor, the hydrogen-sulphide readings in all work areas were significantly below the maximum health standard values in Table 1. Appendix 1SEAM USE IN ASPHALT MIXES - SAFETY
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