🔥 Caking power of hard coal — the Roga method (the Roga index)

Petrochemistry PN-G-04508

Overview

Caking power (caking capacity) is a characteristic property of solid fossil fuels which makes it possible to distinguish coking coals from non-coking ones quickly. The Roga method consists in examining the mechanical strength (by drum treatment) of the coke obtained by degassing at 850 °C a blend of the coal with a reference anthracite under strictly standardised laboratory conditions. The present description is based on the laboratory instruction of the Wrocław University of Science and Technology, carried out according to the earlier standard PN-81/G-04518 (a different number from the present PN-G-04508 — the correspondence of the two designations has to be checked); the current edition PN-G-04508:2020-05 was not available and before implementation it has to be checked.

Coking coals (types 33–37 of the Polish classification: gas coal, gas-coking coal, orthocoking coal, metacoking coal, semi-coking coal) pass into the plastic state during heating with no access of air in the range 350–500 °C; the other types of hard coal, brown coal and anthracite give a dusty coke, not caked or of low strength. Caking power, besides the volatile matter content, dilatation, the swelling index and the gross calorific value, is a parameter of the classification of hard coals.

The theoretical values of RI run from 0 to 100; in practice the best caking coals have an RI of about 85. The division: non-caking coals — 0, very weakly caking — up to 20, weakly caking — 20–30, moderately — 30–40, well — 40–60, very well caking — above 60. The Roga method according to PN-G-04508:2020-05 is in the scope of accreditation of GIG (range 0–90).

Method principle

A blend of 1.00 ± 0.01 g of coal (analysis sample, particle size below 0.2 mm) with 5.00 ± 0.01 g of reference anthracite is carbonised under the constant load of a steel block in a porcelain crucible of standardised dimensions at 850 ± 15 °C for 15 min. The coke button obtained is weighed and sieved through a 1 mm sieve; the residue on the sieve is weighed and subjected to three five-minute drum treatments at a speed of 50 rpm in the Roga drum, the sieving and weighing being repeated after each cycle. The Roga index: RI = 100/(3Q) × [(a + d)/2 + b + c], where Q is the mass of the sample after coking, a the mass on the sieve before the first drum treatment and b, c, d the masses on the sieve after the first, the second and the third drum treatment.

Applications

• Assessment of the suitability of a hard coal for the industrial production of coke
• Classification of hard coals (a parameter beside Vdaf, dilatation, SI and the gross calorific value)
• Selection of the constituents of coking blends and of inert additives
• Rapid distinction of coking coals from non-coking ones

Key parameters

Standard

Standard number

PN-G-04508:2020-05

Title (PL)

Węgiel kamienny — Oznaczanie zdolności spiekania metodą Rogi

Title (EN)

Hard coal — Determination of caking power by the Roga method (tłumaczenie robocze)

Step-by-step procedure

1. Preparation of the blend

   Put 1.00 ± 0.01 g of coal (analysis sample, 0.2 mm) and 5.00 ± 0.01 g of reference anthracite into the weighed crucible; mix thoroughly for 2 min with the stirring rod, level the surface with the tip of the rod and, without shaking, place the steel weight with tweezers. Put it for 30 s into the pressing device (the weight does not touch the walls).

   ⏱ Time: 2 min + 30 s

2. Carbonisation

   Cover the crucible with the blend and the weight with the steel lid and put it into the furnace at 850 ± 15 °C; the furnace is to return to that temperature within ≤ 5 min. 15 min after it was put in, take the crucible out and let it cool (about 45 min).

   ⏱ Time: 15 min • 🌡 Temperature: 850 ± 15 °C

3. Weighing of the coke button

   Take off the lid and the weight with tweezers, knocking the grains of coke adhering to the underside of the weight back into the crucible. Weigh the crucible with its contents (Q).

4. First sieving

   Pour the contents onto the 1 mm sieve; transfer the larger pieces back into the crucible with tweezers and sieve the rest by hand without rubbing the coke intensively against the sieve. Transfer the oversize into the crucible and weigh it (a).

5. Drum treatment

   Pour the oversize onto the bottom of the Roga drum, close the lid and run the drum for 5 min (50 rpm); transfer to the sieve, sieve and weigh the oversize (b). Repeat the drum treatment and the sieving twice more (c, d).

   ⏱ Time: 3 × 5 min

6. Calculation

   RI = 100/(3Q)·[(a + d)/2 + b + c]. Take as the result the mean of two parallel tests on the same coal, rounded to a whole number; give the masses from all the weighings in the test report.

7. Cleaning up

   Discard the pieces of coke and the undersize below 1 mm, ignite the empty crucibles at 850 °C and clean the sieves and the inside of the drum of dust with a dry paper towel.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• A furnace at 850 °C and a hot crucible with the weight — tongs and heat-resistant gloves, cool for about 45 min before dismantling
• During the carbonisation flammable gases and tar are evolved — ventilation, do not open the furnace during the 15 min
• Coke and anthracite dust during the sieving and the drum treatment — goggles, a dust mask

Frequently asked questions