Arch. Min. Sci., Vol. 60 (2015), No 4, p. 10531070 Electronic version (in color) of this paper is available: http://mining.archives.pl DOI 10.1515/amsc-2015-0070 RADOSLAW POMYKALA*, WALDEMAR KPYS*, ZBIGNIEW PIOTROWSKI* , PAULINA LYKO*, ALEKSANDRA GRZYWA* WPLYW TEMPERATURY NA WLACIWOCI PLYNNYCH ZAWIESIN DROBNOZIARNISTYCH STOSOWANYCH W WYROBISKACH PODZIEMNYCH Underground hard coal exploitation is often done under conditions of spontaneous fire hazard. The primary way to combat this threat is advanced, active or passive fire prevention. An important activity is the isolation of gobs using aqueous suspensions of fly ash as well as mineral binders. Therefore, the fine-grained suspension are often used in conditions of elevated temperature. The paper presents results of research on the effect of temperature (up to 80°C) on the properties of suspensions in a liquid state, including their rheological parameters and setting time. Suspensions prepared using the ashes from the hard coal combustion in fluidized bed boilers, and with the addition of Portland cement CEM I 42.5. During the research it was noted that the increased temperature significantly affect the acceleration of solidification processes of suspensions. In the case of rheological properties, the effect of temperature is ambiguous, among others, due to the phenomenon of sedimentation. However, in most cases, particularly for suspensions of higher solids content a marked increase in shear stress and viscosity of the suspensions with increasing temperature were observed . Keywords: fly ash, suspension, underground fire hazard, rheology Eksploatacja wgla kamiennego w Polsce, odbywa si czsto w warunkach zagroenia poarami endogenicznymi, których ródla naley upatrywa w naturalnych sklonnociach niektórych wgli do samozapalenia. Obecnie podstawowym sposobem walki z tym zagroeniem jest zaawansowana, czynna lub bierna profilaktyka poarowa. Wanym jej kierunkiem jest izolacja zrobów w celu uniemoliwienia lub ograniczenia wymiany gazowej pomidzy gruzowiskiem zawalowym a przestrzeni robocz uzyskiwana za pomoc zawiesin materialów drobnoziarnistych najczciej zawiesin popiolowo-wodnych oraz spoiw mineralnych. Zawiesiny i spoiwa s niejednokrotnie stosowane w warunkach podwyszonej temperatury wynikajcej zarówno z rozpoczcia procesów samozagrzewania wgla jak i z samej temperatury górotworu. * AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF MINING AND GEOENGINEERING, AL. A. MICKIEWICZA 30, 30-059 KRAKOW, POLAND Podwyszanie temperatury zawiesin i spoiw moe mie miejsce ju na etapie transportu rurocigiem (w fazie plynnej), ale przede wszystkim podczas podawania do zrobów, gdzie temperatura jest niemal zawsze wysza ni w czci roboczej cian. W artykule przedstawiono wyniki bada, których celem bylo okrelenie w jaki sposób podwyszona temperatura w zakresie do ok. 80°C, wplywa na wlaciwoci zawiesin w stanie plynnym oraz na czas ich wizania. Do przygotowywania zawiesin wykorzystywano popioly lotne pochodzce ze spalania wgla kamiennego w kotlach fluidalnych a take cement portlandzki CEM I 42,5. Sporzdzanie zawiesin oraz badania prowadzono w oparciu o norm PN-G-11011:1998 ,,Materialy do podsadzki zestalanej i doszczelniania zrobów. Wymagania i badania." Badanie czasu wizania zawiesin prowadzono zgodnie z wymogami normy PN-EN 196-3+A1:2009 ,,Metody badania cementu Cz 3: Oznaczanie czasów wizania i staloci objtoci". Próbki zawiesin umieszczano w komorach klimatycznych, w których panowala zadana temperatura (40, 60 i 80°C) oraz wilgotno (100%). Pomiaru czasu wizania dokonywano w odstpach czasowych od 30' do 6 h. Badania wplywu temperatury na wlaciwoci reologiczne zawiesin realizowano na stanowisku laboratoryjnym skladajcym si z reometru RN 4.1 firmy Rheotest (Rys. 1.) wyposaonym w uklad grzewczy. Badania prowadzono w dwóch schematach pomiarowych: oznaczonych jako ,,test standardowy" oraz ,,test dlugi". W pierwszym, pomiaru dokonywano podczas zmiany prdkoci obrotowej rotora od 0 do 1000 obr/min oraz od 1000 obr/min do 0, co odpowiadalo szybkoci cinania w zakresie 0÷435 [1/s]. Kady etap trwal 60s. W ten sposób uzyskiwano moliwo wykrelenia krzywych plynicia przy prdkoci rosncej oraz malejcej, co dobrze oddaje cechy tego typu mieszanin transportowanych grawitacyjnie, rurocigami. W schemacie drugim podstawowym celem bylo okrelenie wplywu czynnika czasu na wlaciwoci reologiczne zawiesin, w nawizaniu do etapu transportu zawiesin rurocigami oraz wyplywu i penetracji zrobów. W tym celu prowadzono pomiary napre stycznych podczas ruchu obrotowego ze stal prdkoci obrotow 500 obr/min (prdko cinania ok. 220 [1/s]), przez okres 10 minut, a nastpnie z prdkoci 100 obr/min (ok. 44 [1/s]) przez 5 minut. Sklady sporzdzonych zawiesin oraz podstawowe ich wlaciwoci (gsto, rozlewno oraz ilo wody nadosadowej) zestawiono w tabeli 1 oraz na rysunkach 2÷3. Sporzdzone zawiesiny charakteryzowaly si gstoci w zakresie od 1,288 do 1,458 g/cm3, w zalenoci od zawartoci popiolu w zawiesinie oraz dodatku cementu. Jednym z podstawowych bada kontrolnych konsystencji zawiesin jest okrelenie ich rozlewnoci. Zwikszenie iloci popiolu wplywalo na zmniejszanie si rozlewnoci zawiesin, a zastpowanie czci popiolu cementem powodowalo sytuacj odwrotn, wynikajc z rónic w wododnoci tych skladników. Z tego powodu wraz ze zwikszaniem si gstoci zawiesin ze wzrostem iloci cementu (o takim samych stosunku czci stalych do wody s/w), ich rozlewno zwikszala si. Np. zawarto cementu w iloci 20% w zawiesinach spowodowal wzrost rozlewnoci w stosunku do zawiesin bez cementu od 35 mm do 90 mm. Ilo wody nadosadowej zawiesin ulegala zmniejszeniu wraz ze wzrostem udzialu popiolu w stosunku do wody. Natomiast obecno cementu powodowala jej. Sezonowanie zawiesinie w temperaturach w zakresie od 40 do 80°C wplywalo na ograniczenie iloci wody nadosadowej. Czas wizania dla zawiesin o takim samym udziale czci stalych do wody wyranie zmienial si w zalenoci do temperatury, w której sezonowano próbki (Rys. 4÷7). We wszystkich przypadkach wraz ze wzrostem temperatury nastpowalo przyspieszenie pocztku jak i skrócenie czasu wizania zawiesin. Zawarto cementu jeszcze bardziej potgowala to zjawisko 3/4 czas wizania zawiesin w temperaturze 60 czy 80°C skracal si nawet poniej 10 godzin od sporzdzenia zawiesiny. Wyniki bada wlaciwoci reologicznych omówiono na przykladzie zawiesin sporzdzonych na bazie popiolów fluidalnych bez dodatku cementu. Wyniki bada ,,testów standardowych" przedstawiono na rysunkach 8÷13, w postaci krzywych plynicia i krzywych lepkoci, a dla pomiarów czasowych w postaci krzywych obrazujcych zmian napre stycznych w czasie, na rysunkach 14÷18. Podczas ogrzewania zawiesin, szczególnie do temperatury ok. 40°C, a take w krótkim okresie czasu, obserwowano zmniejszanie si napre stycznych, co w przypadku transportu hydraulicznego rurocigami przeklada si moe na obnienie oporów przeplywu. W wyszych temperaturach sytuacja byla bardziej zrónicowana i zalena zarówno od pierwotnej konsystencji zawiesin jak i od rodzaju popiolu. Dla ogrzewanych do temperatury 60°C zawiesin o niskiej zawartoci czci stalych, oraz przy niszych prdkociach obrotowych, obserwowano stopniowe zwikszanie wartoci napre stycznych wraz z uplywem czasu pomiaru. Rozpatrujc uzyskane wyniki w kontekcie zastosowania w praktyce, mona przyj, e niewielkie podniesienie temperatury zawiesin podczas ich transportu nie powinno znaczco wplywa na parametry przeplywu rurocigami. Jednak w przypadku transportu zawiesin o dobrych wlaciwociach wicych, na dlugich trasach przez wyrobiska o wyszej temperaturze skal moe dochodzi do przyspieszania procesów hydratacji oraz osadzania si zawiesin na ciankach rurocigów. Zawiesiny deponowane w zrobach, w których panuje wysoka temperatura w krótkim czasie ulegn zestaleniu ograniczajc m.in. moliwo przeplywu powietrza oraz innych gazów. Jednoczenie jednak lokowanie zawiesin w zrobach, w których panuje wyranie wysza temperatura, np. w przypadku zaawansowanych procesów samozagrzewania, moe przyczynia si do zmniejszania zdolnoci penetracji gruzowiska oraz powodowa szybsze zatykanie wylotu rurocigu. Slowa kluczowe: popiól lotny, zagroenie poarowe, reologia 1. Introduction The underground coal exploitation in Poland is often connected with the hazard of endogenous fire, which appear due to natural properties of some kinds of coal to self ignition. The deeper the excavation is carried out the bigger the number of longwalls where this process can occur. The fire hazard is constantly present in Polish coal mines (Trenczek, 2008), and the places where it is really intense are the gob (caving) of longwall system. The basic tool to fight with this phenomenon is advanced, passive or active fire prevention. The important thing is to insulate the workings to prevent or limit the gas exchange between gob and the working space, what is also important in the case of methane danger. The ash-water suspension and mineral binders are in Polish coal mines an important means in fire prevention. They allow for external (binders) and internal (suspension) insulation of gob. The main aim of the suspension use is to fill and seal the rubble caving.. The suspensions properties and problems with their application for caving insulation have been widely described in many publications and the monographic works (Dong et al., 2013; Dziurzyski & Pomykala 2006; Frczek 2010; Krause et al., 2009; Lisowski 1997; Mazurkiewicz et al., 1997; Myslek 2001; Plewa & Sobota 2002; Piotrowski & Lukowicz 2007; Piotrowski et al., 2009, Pomykala 2005; Strumiski 1996). The continual researches, analysis and simulations are conducted for concerning their effectiveness, location and the application process itself (Dziurzyski et al., 2014; Piotrowski 2011). Mineral binders, as well as ash-water suspension as means on fire prevention are mainly used to isolation of caving from the currently excavated places. The effectiveness such actions is proved by many examples (Jarczyk & Kania 2001; Madaj et al., 2012; Zimoczyk et al., 2006). Fine-grained suspension is often used in the presence of increased temperature occurring mainly in abandoned caving but also in current excavations. The increase in suspension temperature may appear during the pipeline transportation and after its placing in the gob. In the first case the main source of the heat can be rock mass or the chemical processes within the suspension, especially when the ash used in it contains products of flue gas desulphurization. The practice in this field reveals that in case of some ashes in the result of long mixing their water suspensions, what may be treated as pipeline flow simulation, even temperatures above 45°C can be obtained (Pomykala, 2006). The temperature raise in the suspension in liquid phase occurs most often during its application to the caving. The local temperature there is usually higher than in the longwall working space. It may result from the temperature of the rock mass or because the process of the coal self heating has just started. In the first case the temperature may be over 50°C, in the latter may reach significantly higher rates. The properties of ash-water suspension and mineral binders used in coal mines are of significant difference. It is due to the properties of the materials used, mainly flying ashes obtained from different types of installations. It influences the properties of the suspension and binders made from them. This refers also to fire prevention, as the consistency of the suspension affects the ability to caving penetration, the content of supernatant water, water demand, setting time. One of the factors, which may influence the fine-grain suspension properties, is temperature. The aim of the researches was to determine the influence of raised temperatures (to 80°C) on the properties of the suspension in the liquid phase and on its setting time. 2. Materials and the research methods To prepare the tested suspension there have been used flying ashes obtained from the coal combustion in fluidal bed boilers (`fluidal ashes') in two different installations (ashes PK and PJ), and the Portland cement CEM I 42,5. The preparations of suspension and the tests have been conducted according to the Polish standard PN-G-11011:1998 "Materials for hydraulic solidified backfilling and sealing of caving. The requirements and tests." The tests on the suspension setting time have been conducted according to the standard PN-EN 196-3:2009 Methods of cement testing Part 3: The determination of setting time and volume stability. For the testing purposes there have been prepared fine-grained suspension, differ in composition and content of the solid parts. The suspensions have been prepared observing the limits of fluidity 150 to 300 mm. It forced the use of different suspensions recipes (different shares of solids) for each ashes. If the cement has been used it always replaced the referring amount of ash. Having been mixed, the suspension samples were placed in container, with the determined temperature (40, 60, and 80°C) and humidity (100%). The measurements were taken every 30 minutes up to 6h depending on the properties of suspension and the temperature of seasoning. The tests on the influence of the temperature on the rheological properties of suspensions were conducted at the laboratory stand consisting of viscometer (reomether) RN 4.1 by Rheotest (Fig. 1) containing the system allowing to set the temperature during the tests. The unit consists Fig. 1. Viscometer Rheotest RN4.1 with the equipment of annular cylinder filled with water connected with circulation system water bath, heat exchanger and circulation pomp. For the tests there were used the cylinder having the diameter of 40 mm and the rotor diameter 28 mm. Such a set allows to obtain the shear rate 435[1/s] at the rotation speed 1000 rot/min. The researches were conducted within two measurement schemes: "standard test ST" and "long test LT". During the first one the measurement was taken during the changes of rotation speed from 0 to 1000 rot/min (c.a. 435[1/s]) and from 1000 rot/min to 0. Each stage lasted for 60 sec. This let draw the flow curves at the increasing and decreasing speed what well reflects the properties of these types mixtures transported by gravity, in pipelines (Stryczek et al., 2009. Stryczek & Winiowski, 2001; Pomykala et al., 2012). The second method aimed to determine the influence of the other factor time. To set this there were conducted measurements while the rotation movement with the constant speed. To make the test closer to reality the speed was 500rot/min what refers to shear rate c.a. 220 [1/s] lasting for 10 minutes, and later the movement with the speed 100 rot/min (44 [1/s]) lasting for 5 minutes. This scheme was thought to be related to suspension transportation in the pipelines as well as outflow and caving penetration. 3. The course of the measurement: The "standard test" was conducted as the first one for each of the suspensions at the temperature of 25°C. Next, the suspension in the measuring vessel was changed to eliminate the "memory effect". Than it was placed in the heating cylinder, then two standard tests with the suspension heating were conducted (the results are shown in the charts marked as two values of temperature initial and the final one). After the each test the temperature was controlled. In the next step the suspension was replaced by a new one and the "long test" started (10 minutes at the speed of 500 rot/min and 5 minutes at the speed of 100 rot/min. This stage was followed by two standard tests (marked as S1) in target temperature. Their aim was to determine the changes the properties of the suspension which had been heated for a few minutes. The suspension markings used in the charts contains the following information: the type of ash (PK or PJ), the ratio of solids to water w/s (e.g. 067:1 abbreviated 0.67) the content of cement (mass participation in solids originated from replacing the given mass of ash), the seasoning temperatures were marked on the charts for rheological properties. If not marked in the other way, the single value marks the measurement taken in the given temperature; two values (e.g. 25-40°C) mean the measurement rheological properties during the suspension heating. 4. The basic properties of suspension For the research purposes there have been prepared suspensions with the ash PK, of three proportion of solids to water 0.67:1, 0.77:1 and 0.90: 1 (marked as PK0.67; PK0.77; PK0.9), and also ash PJ for which s/w = 1.0 and 1.11. Content of cement was various from 0 to 20% (example marking: _c20% means 20% cement content of suspension solids). The basic properties of suspensions are showed in table 1 and figures 2 and 3. TABLE 1 Basic properties of suspension The The amount of supernatant water [%], The ratio proportion Suspension at temperature: of solids Fluidity of cement density to water [mm] in solids [g/cm3] 40°C 60°C 80°C 25°C s/w [%] Suspension symbol PK_0,9_c0% PK_0,9_c10% PK_0,9_c20% PK_0,77_c0% PK_0,77_c10% PK_0,77_c20% PK_0,67_c0% PK_0,67_c10% PK_0,67_c20% PJ_1,11_c0% PJ_1,11_c10% PJ_1,11_c20% PJ_1,0_c0% PJ_1,0_c10% PJ_1,0_c20% 0,90 0,90 0,90 0,77 0,77 0,77 0,67 0,67 0,67 1,11 1,11 1,11 1,00 1,00 1,00 1,407 1,415 1,417 1,350 1,355 1,358 1,288 1,290 1,308 1,458 1,446 1,447 1,381 1,413 1,418 5,00 6,25 7,50 6,25 6,25 10,00 11,25 12,50 15,00 8,75 10,00 15,00 13,75 15,00 20,00 6,25 6,50 7,50 5,25 6,25 8,75 11,25 12,50 13,75 10,00 10,00 15,00 11,25 15,00 17,50 3,75 5,00 6,25 5,25 5,55 8,00 9,75 11,25 13,25 6,25 7,50 11,25 7,50 8,75 11,25 1,25 2,50 3,75 2,50 4,25 4,75 6,25 7,25 7,50 5,00 5,25 5,50 6,75 6,75 7,50 The discussed suspensions were made of density from 1.288 to 1.458 g/cm3. This is connected with the content of ash in relation to water in the suspension and the presence of cement. The differences were noticed in the case of suspension fluidity testing. The increase in the amount of ash PK in the suspension caused the decrease in the flow from 230 mm to 155 mm. Replacing of the part of ash for cement caused the reverse situation resulting from the water demand of solids in suspension. Despite the increase of suspension density according to the increase of the amount of cement on suspension showing the same s/w, their fluidity increased as well. For example, the 20% content of cement in suspensions resulted in the increase of flow in relation to no-cement suspensions by: 35 mm (for PK 0.9); 50 mm (for PK 0.77 and 90 mm (for PK 0.67). The amount of supernatant water went down according to the increase of ash share in relation to water. However, the presence of cement in the suspension caused the increase in the amount of supernatant water. The biggest amount of supernatant water was observed in the suspension PJ_1,0_c20% at the temperature 25°C what showed the significant exceeding of the PN-G11011:1998 standard. Seasoning temperature in the range 40°C÷80°C caused the limitation in the amount of supernatant water. This was connected with the speed of water bonding processes within the suspension components, depending on seasoning conditions (Neville, 2000). 5. Suspension and binder setting time in the condition of elevated temperature The results of the researches on the setting time of suspension samples seasoned in temperatures 40, 60 and 80°C are shown in the figures 4÷6. The amount of supernantant water [%] PK_0,9_c0% PK_0,9_c10% PK_0,9_c20% PK_0,77_c0% PK_0,77_c10% PK_0,77_c20% The amount ofsupernatant waterj [%] 40 60 Seasoning tempereture [oC] The amoun of supernatant water [%] PK_0,67_c0% PK_0,67_c10% PK_0,67_c20% 40 60 80 Seasonoing temperature [oC] Fig. 2. The amount of supernatant water of suspension of s/w respectively 0.9; 0.77; 0.67, made on the basis of flying ash from fluidal boiler (PK) and Portland cement in the amount of 0%, 10% and 20% of solids 20 The amount of supernatant water [%] 15 10 5 0 25 The amount of supernatant water [%] PJ_1,11_c0% PJ_1,11_c10% PJ_1,11_c20% PJ_1,0_c0% PJ_1,0_c10% PJ_1,0_c20% 40 60 80 Seasoning temperature [oC] 40 60 80 Seasoning temperature [oC] Fig. 3. The amount of supernatant water of suspension of s/w respectively 1.0 and 1.11, made on the basis of flying ash from fluidal boiler (PJ) and Portland cement in the amount of 0%, 10%and 20% of solids The setting time for the suspension of the same s/w changed significantly according to the seasoning temperature. In all cases, along with the temperature increase the setting started faster and the setting time was shortened. The presence of cement in the suspension even make this phenomenon more intense, causing that the setting time at temperatures 60°C or 80°C lasted less than 10 hours. This time shortening is connected with the faster occurrence of hydration products, what was described also in other publications (Fattuhi, 1997; Kurdowski, 1991; Lothenbach et al., 2007; Wenhua et al., 2012). 120 100 Time [h] 80 60 40 20 0 25 120 100 Time[h] 80 60 40 20 0 25 80 PK_0,9_c0% 80 PK_0,9_c0% Time [h] 60 50 40 30 20 10 0 25 40 60 Seasonong temperature [oC] 80 PK_0,9_c10% Fig. 4. The setting time of suspension of s/w = 0.9 :1 based on fluidal flying ash PK and Portland cement in amount of 0, 10 and 20% of solids 6. The rheological properties The results on rheological properties researches have been described with the example of suspension made with fluidal bed ash PK and PJ without cement. The results of "standard tests" are shown as flow and viscosity curves, and for the timing measurements as the curves showing the changes of shear stresses in time. For the test purposes there were prepared suspensions of same relations ash to water like previously (PK0.67, PK0.77, PK0.9 and PJ1.1, PJ1.0). The figure 7 shows the flow curves of (marked S1) for the suspensions at the temperature 25°C. The flow curve for the suspension PK0.9 seems to be of a great interest. The consistency of this suspension differs greatly from the others, looks rather like paste. With the increase of the rotation speed, the flow curve shows the changeable slope largely similar to plastic liquid. During the second phase of the test, when the rotor reduces its speed, the curve is more uniform and characteristic for rheological Bingham model. Such differences indicate on significant thixotropic of the suspension. The other suspensions show similar flow curves in the comparable rate. The next step was made to compare the flow curves of suspension which were heated during the test to standard curves at the temperature of 25°C. While heating two tests were made immediately so the initial temperature in test starts from 25°C this means that the suspension was poured into the vessel just before the test. In other cases the second test is described (initial temperature of second test is the same as final temp. of the first one). The results are shown in figures 8÷12. 120 100 Time [h] 80 60 40 20 0 25 40 60 80 Seasonning temperature [oC] 60 50 Time [h] 40 30 20 10 0 25 40 60 80 Seasoninig temperature [oC] PK_0,77_c20% PK_0,77_c0% Time [h] 60 50 40 30 20 10 0 25 80 PK_0,77_c10% Fig. 3. The setting time of suspension of s/w = 0.77:1 based on PK ash and Portland cement in amount of 0, 10 and 20% of solids 140 120 100 Time[h] 80 60 40 20 0 25 60 50 Time[h] 40 30 20 10 0 25 80 Time[h] PK_0,67_c0% 60 50 40 30 20 10 0 25 80 PK_0,67_c10% PK_0,67_c20% Fig. 4. The setting time of suspension of s/w =0.67:1 based on PK ash and Portland cement in amount of 0, 10 and 20% of solids 80 70 60 50 40 30 20 10 0 25 60 PJ_1,1_c0% Time [h] 50 40 30 20 10 0 60 50 Time [h] 40 30 20 10 0 25 80 PJ_1,1_c20% 80 25 80 PJ_1,1_c10% Time [h] Fig. 5. The setting time of suspension of s/w = 1.1:1 based on PJ ash and Portland cement in amount of 0, 10 and 20% of solids 140 120 100 80 60 40 20 0 25 60 50 Time [h] 40 30 20 10 0 25 80 Time [h] Time [h] PJ_1,0_c0% 60 50 40 30 20 10 0 25 80 PJ_1,0_c10% PJ_1,0_c20% Fig. 6. The setting time of suspension of s/w =1.1:1 based on PJ ash and Portland cement in amount of 0, 10 and 20% of solids 120 PJ_1,1_S1_25oC PJ_1,0_S1_25oC PK_0,77_S1_25oC PK_0,9_S1_25oC PK_0,67_S1_25oC 100 Shear stresses [Pa] 0 0 50 100 150 200 250 Shear rate [1/s] 300 350 400 450 Fig. 7. Flow curves for the tested suspensions 25 Shear stresses [Pa] 15 PK_0,67_25-34oC PK_0,67_34-38oC PK_0,67_25-47,5oC 5 PK_0,67_47,5-53oC PK_0,67_S1_25oC 0 0 50 100 150 200 250 Shear rate [1/s] 300 350 400 450 Fig. 8. The flow curves obtained during heating the suspension PK0.67 in comparison with standard (PK_0.67_S1) 50 45 40 Shear stresses [Pa] 35 30 25 20 15 10 5 0 0 50 100 150 200 250 Shear rate [1/s] 300 350 400 450 PK_0,77_25-34oC PK_0,77_34-38oC PK_0,77_25-48oC PK_0,77_48-53oC PK_0,77_S1_25oC Fig. 9. The flow curves obtained during heating the suspension PK0.77 in comparison with standard (PK_0.77_S1) 140 120 100 80 60 40 20 0 0 50 100 150 200 250 Shear rate [1/s] 300 350 400 450 PK_,9_25-35oC PK_0,9_35-40oC PK_0,9_25-46oC PK_0,9_46-55oC PK_0,9_S1_25oC Shear stresses [Pa] Fig. 10. The flow curves obtained during heating the suspension PK0.9 in comparison with standard (PK_0.9_S1) 35 30 25 20 15 10 5 0 0 50 100 150 200 250 Shear rate [1/s] 300 350 400 450 PJ_1,0_25-38oC PJ_1,0_38-40oC PJ_1,0_25-41,5oC PJ_1,0_41,5-55oC PJ_1,0_S1_25oC Shear stresses [Pa] Fig. 11. The flow curves obtained during heating the suspension PJ1.0 in comparison with standard (PJ_1.0_S1) 50 Shear stresses[Pa] 30 PJ_1,1_25-38oC 20 PJ_1,1_38-40oC PJ_1,1_25-50oC 10 PJ_1,1_50-58oC PJ_1,1_S1_25oC 0 0 50 100 150 200 250 Shear rate [1/s] 300 350 400 450 Fig. 12. The flow curves obtained during heating the suspension PJ1.1 in comparison with standard (PJ_1.1_S1) In all cases, lower values of shear stresses were obtained during the first stage of heating in comparison with the standard flow curve, whereas during the second stage of heating shear stresses were getting higher in some cases beyond the limits described by the standard curves. During the next step the values of shear stresses during the `long test' (10 minutes at the speed of 500 rot/min and 5 minutes at the speed of 100 rot/min.) were defined. The results are shown in figures 13÷17. 20 18 16 Shear stresses [Pa] 14 12 10 8 6 4 2 0 0 100 200 300 400 500 Time [s] 600 700 800 900 PK_0,67_test 10'+5'_25 PK_0,67_test 10'+5'_40 PK_0,67_test 10'+5'_60 Fig. 13. The measurement of shear stresses during the "long test" on suspension PK_0.67 40 35 30 Shear stresses [Pa] 25 20 15 10 5 0 0 100 200 300 400 500 Time [s] 600 700 800 900 PK_0,77_test 10'+5'_25 PK_0,77_test 10'+5'_40 PK_0,77_test 10'+5'_60 Fig. 14. The measurement of shear stresses during the "long test" on suspension PK_0.77 160 140 120 Shear stresses [Pa] 100 80 60 40 20 0 0 100 200 300 400 500 Time [s] 600 700 800 900 PK_0,9_test 10'+5'_25 PK_0,9_test 10'+5'_40 PK_0,9 test 10'+5'_60 Fig. 15. The measurement of shear stresses during the "long test" on suspension PK_0.9 50 45 40 Shear stesses [Pa] 35 30 25 20 15 10 5 0 0 100 200 300 400 500 Time [s] 600 PJ_1,0_test 10'+5'_25 PJ_1,0_test 10'+5'_40 PJ_1,0_test 10'+5'_60 Fig. 16. The measurement of shear stresses during the "long test" on suspension PJ_1.0 70 60 Shear stresses [Pa] 50 40 30 20 10 0 0 100 200 300 400 500 Time [s] 600 700 800 900 PJ_1,1_test 10'+5'_25 PJ_1,1_test 10'+5'_40 PJ_11_test 10'+5'_60 Fig. 17. The measurement of shear stresses during the "long test" on suspension PJ_1.1 The results of `long tests' reveal great differences among the suspensions characteristics depending on the kind of ash used and the share of solids. For the suspensions PK0.77, PK0.9 and PJ1.1 the values of shear stresses are smaller than the ones obtained at the temperature of 25°C. An example of this is a temporary shear stress decrease in the first minutes of the PK0.77 test. However, along with time and due to the raised temperature the gradual increase in shear stresses can be observed (also for suspensions with smaller solid content: PK0.67 and PJ1.0). The growth rate of shear stress at 60°C is much higher than at lower temperatures. Moreover, points out different shape of curves for the suspension PK0,9, as was the case for the flow curves. 7. Conclusions The main objective of the researches was to determine the influence of temperature on the properties of liquid and solidifying suspensions and mineral binders applied to the insulation of underground workings and gob. The conducted researches proved that the raised temperature causes the acceleration of suspension solidifying processes. This refers not only to the binders or suspensions with share of cement but also to suspensions based only on fly ash. Similar trend was observed also for supernatant water. The influence of temperature on the rheological properties is not so obvious and the differentiated courses of the curves or the sedimentation, especially among the suspension with the bigger content of solids, make the result interpretation more complicated. During the suspension heating within the short time, the decrease of the shear stresses was noticed. This may reduce the flow resistance during the hydraulic transportation. In other cases the situation was more differentiated on dependent both on initial consistence of suspensions and the type of ash. But generally, for the suspensions heated to the temperature of 60°C the value of shear stresses was growing steadily with time. Analyzing the obtained results for the practical purposes it can be assumed that a slight raise of suspension temperature should not affect the parameters of the pipeline transportation. However, it should be taken under consideration that the transportation through the excavations of the higher temperature of rocks may cause some problems, especially in the case of suspensions of good binding properties (contained desulphurization products). On these occasions the hydration processes can be fastened and the suspension can embed on the inner surface of the pipeline. The suspensions applied for gob insulation at elevated temperature may solidified faster, limiting this way the opportunity of air and other gases flow. However, the high temperature of the rock (eg. as a result of a late start preventive operation and/or advanced self-heating processes) may adversely affect the ability of caving penetration by the suspensions, leading to rapid clogging of the outlet pipe, thereby reducing the effectiveness of these actions. The results of the researches shown in the article have been developed within the realization of the research project of the Ministry of Science and Education no N N524 338240 "The temperature influence on the properties of suspensions and binders used in fire prevention in the coal mines" and statutory research no 18.104.22.1682.
Archives of Mining Sciences – de Gruyter
Published: Dec 1, 2015