EFFECT OF STEAM ACTIVATION TEMPERATURE ON THE PORE STRUCTURE OF SHARGUN COAL (SURKHANDARYA, UZBEKISTAN): COMPARISON OF THE 600 AND 850 °C REGIMES

Uzbekistan imports almost all of the activated carbon used in industrial water treatment, and coal from the Shargun deposit is an obvious candidate feedstock. An earlier stage of this work established that the raw coal has no micropore network accessible to nitrogen and a surface area of only 12.142 m2 /g, so its value as an adsorbent depends entirely on activation. Here the governing parameter of that step — temperature — is examined by comparing two regimes, 600 and 850 °C, each consisting of one hour of pyrolysis followed by one hour of steam activation in the same laboratory rig. Pore structure was determined by nitrogen physisorption at 77.35 K and morphology and composition by SEM–EDS. At 600 °C no micropore network formed: the surface area did not exceed the feedstock value and the modal half-width moved from 2.59 to 7.66 nm, that is, existing pores merely widened. At 850 °C a microporous structure appeared: the surface area reached 33.704 m2 /g, the modal half-width fell to 1.66 nm, and uptake at a relative pressure of 0.053 was 7.94 cm3 /g against 0.167 for the raw coal, a factor of 47.5. Over the common width range 0.74–5.53 nm the activated sample exceeds the feedstock by 1.67 times in pore volume and by 6.7 times in surface area. The BET equation failed for both samples but for opposite reasons: C = 1.799 for the raw coal and C = −36.725 for AC-850. The working regime should therefore be built around 850 °C, and the reported surface area is a lower bound, since the micropore filling region was not sampled.