Türk linyitlerinin ısıl işlemle yüzebilirliklernin iyileştirilmesi

Abstract

Bu çalışmada, ülkemizde çıkarılan bazı linyit kömürlerinin yüzebilme karekterleri, saf ve model reaktif olarak bilinen anyonik, katyonik ve noniyonik kollektörlerle saptanmıştır. Yine linyitlerin flotasyon yeteneğinin iyileştirilmesi amacıyla linyitler ısıl işleme tabi tutularak flotasyon verimlerindeki değişim mikroflotasyon ve Denver hücreleri ile ölçülmüştür. Linyitlerin özellikle ısıl işlem öncesi ve sonrası zeta potansiyelleri ve infrared grafikleri ölçülerek flotasyon ile korrelasyonu yapılmıştır. Deneysel çalışmalarda Manisa, Dodurga (Çorum), İstanbul ve Tekirdağ bölgelerine ait Soma, Alpagut, Yeniköy ve Saray linyit numuneleri kullanılmıştır. Deneylerde hem ısıtılmış (105 °C de 2 saat) hem de ısıtılmamış (25 °C de bekletilmiş) kömürler için gazyağı, fuel oil, anyonik (SDS), katyonik (DAH), noniyonik (Sapogenat T serisi) ve köpürtücü (MIBC) reaktifler kullanılmıştır. Denver ve mikroflotasyon deneyleri ve zeta potansiyel ölçümleri ile dört farklı linyit kömürünün içerdikleri nem ve kül miktarlarına göre yüzebilirlik durumları tesbit edilmiştir. Isıl işlemler ile Yeniköy ve Saray kömürlerinin yüzebilirliklerinin arttığı, Soma ve Alpagut kömürünün yüzebilirliğinde iyileşmenin olmadığı tesbit edilmiştir. Soma ve Saray kömüründe ölçülen zeta potansiyel değerlerinin ısıl işlem sonrası değiştiği tesbit edilmiştir. Soma kömüründe gazyağı kullanılması ters flotasyona sebeb olmuş fakat gazyağı+fuel oil ile doğal pH" da iyi sonuçlar elde edilmiştir. Anyonik kollektör (SDS) ile Soma kömüründe düşük pH larda iyi neticeler elde edilirken, Saray kömüründe düşük seviyede iyileşmeler tesbit edilmiştir. Katyonik kollektör Soma kömüründe düşük pH* larda iyi sonuçlar verirken, Saray kömüründe yanabilir verimi yükseltmiş fakat kül içeriğinde iyileşme sağlanamamıştır. Yalnızca Saray kömüründe kullanılan noniyonik kollektörler ile hiç bir iyileşme sağlanamamıştır. Isıl işlemler sonrası yüzebilirlikteki iyleşmenin, kömürün ısıl işlem sonrası bünyesinde su moleküllerinin ve yüzeyde bulunan oksijen içeren gruplarım kaybetmesinden kaynaklandığı tesbit edilmiştir. Oksijen içeren fonksiyonel gruplarla flotasyon verimleri arasında bir korrelasyon elde etmek amacıyla bir dizi infrared ölçümleri yapılmıştır. Isıl işlem öncesi (25 °C) ve sonrası (105 °C) yapılan infrared ölçümlerinde kömürü hidrofilik yapan hidroksil (-OH) ve karboksil (-COOH) gruplarında azalma, buna mukabil alifatik ve aromatik gruplarda ise artış görülmüştür. Isıl işlem sonrası en fazla iyileşmenin olduğu Saray kömüründe hidrofilik grupların büyük oranda azaldığı ve iyileşmenin gözlendiği, Alpagut linyitinde ise belirgin bir değişimin olmadığı gözlenmiştir. Bu sonuçlar bir kömürün ısıl işlemle yuzebilirliğinin iyileşurilebileceği kömürün yapısındaki hidrofilik ve hidrofobik grupların miktarının net farkına bağlı olduğunu göstermektedir.

In this study, the floatability of some Turkish lignites have been determined using pure and model surfactants (anionic, canonic and nonionic). The lignites subjected to thermal treatment as function of temperature and treatment time have shown varying improvements depending on the rank of coal. Changes in the floatability of lignites have been determined with either a microflotation or Denver cell. The zeta potential and infrared behaviour of the samples have been measured before and after the thermal treatment and was correlated with the flotation response. The coal samples referred as, Izmir, Alpagut, Yenikoy and Saray were respectively received in their original form from Manisa, Dodurga (Coram), Istanbul and Tekirdağ Regions. The size of the coals was reduced to -0.210 mm by a combination of jaw-roll crusher followed by a pulverizer. The minus 0.038 mm material was removed to avoid slimes coating for microflotation experiments. The minus 0.212 mm materials were used for experiments in Denver cell. The minus 0.150 mm material was used for the measurement of zeta potentials. This fraction was further ground in an agate mortar to a size suitable for zeta potential measurements. The minus 0.212 and -0.212+0.038 mm fractions were analyzed for its ash and moisture contents and used in all the flotation experiments. The moisture contents of the above samples are 11.2, 14.42, 25.50 and 18.29 %, respectively. The corresponding ash contents for the above coals respectively are 30.00, 53.07, 26.10 and 21.005%. Two types of oil and three types of surfactants were used as collectors in flotation studies: kerosene, fuel oü and mixture of these oils, anionic sodium dodecyl sulfate (SDS), cationic dodecylamine hydrochloride (DAH) and nonionic (a set of Sapogenat T). MIBC (methylisobutylcarbinol) at a dosage of 500-700 g/t for Denver flotation and 50 g/lt for microflotation was used as a frother in the experiments. Oil type collectors were used only in Denver cell. The pH was adjusted with HC1 and NaOH. All experiments were conducted at ambient temperature (22-25 °C). Tap water was used for Denver flotation and distilled water was used for microflotation and zeta potential measurements. VTÎ Denver flotation experiments were done in a 2 It Denver cell at a speed of 1200 rpm. Microflotation tests were carried out in a 150 ml column cell (25x220 mm) using an electronically controlled apparatus. A sample of 100 grams was used in the Denver cell and was conditioned for 10 minutes either in the absence or presence of the collectors. In the microflotation tests, two grams of sample were conditioned in a beaker for 10 minutes either in the absence or presence of the collectors and then introduced into the flotation system. The material was floated for one minute at a nitrogen flowrate of 50 cm^/min. The float and unfloat products were dried, weighed and analyzed for ash in order to calculate the flotation recoveries. Zeta potential measurements were conducted by a microprocessor controlled Zeta Meter 3.0. The measurements were made on samples conditioned for 10 min in the absence or presence of collectors. Each data point represents the average of 10 measurements. Two size fractions of Soma lignite (-1+0.212 and -0.212 mm) were used in the Denver flotation with MTBC at natural pH. The size of -0.212 mm was found to float better than the -1+0.212 mm size fraction. Thus the -0.212 mm size of fraction was used in the subsequent flotation studies. Kerosene as a collectors and 500-700 g/t MIBC as a frother were used in the flotation of Soma lignites at natural pH (7.5+0.2). As the amount of kerosene increased, the ash content decreased with an increase in flotation recoveries. The maximum recoveries were obtained at 10 kg/t of kerosene addition. Because the ash content obtained was higher than the run-of-mine coal, these results were not sufficient to achieve acceptable flotation recoveries. A total of 5 kg/t kerosene and fuel oil mixtures with varying ratios was used in the flotation of Soma lignite with MIBC as a frother. The best result was obtained at a mixing ratio of 60 % kerosene+40 % fuel oil and a natural pH. At this ratio 25.83 % ash content and with a flotation recoveries of 59.74 % was obtained from a run-of-mine coal assaying 30 % ash. This ratio of mixture was used in the subsequent experiments. The effect of pH on the floatability of Soma lignite was tested with the same mixture of 60 % kerosene+40 % fuel oil and 500-700 g/t MIBC. The maximum recoveries were obtained at natural pH. The zeta potential for Soma lignite exhibited a decrease as a function of pH. The effect of zeta potential on the original and thermally treated Soma lignite as a function of pH was determined with Zeta-Meter 3.0 apparatus. While the zeta potentials reach values as high as -40 mV at pH 11 for the untreated coal and decreases down to about -12 mV between 3 and 7.5 pH, it exhibits zeta potential values of -35 to -10 mV in the respective pH region with the treated coal. This reveals that a significant portion of the oxygen containing functional groups are removed on heating the Soma lignite. VTTT Flotation experiments were done only thermally treated Soma coal at different temperatures (60, 105, 150 and 200 °C) and thermal treatment times using the mixture of 60 % kerosene and 40 % fuel oil and 500-700 g/t MBC. At all temperatures, increasing the heating time decreased the flotation recoveries while the ash contents increased. Similarly, at all heating times, an increase in temperature caused a decrease in flotation recoveries with a corresponding increase in ash contents. Microflotation experiments were conducted to find out the effect of SDS addition on the floatability of Soma lignite in terms of combustible recoveries and ash contents at natural pH (8.5+0.2). The floatability of heat treated and untreated lignite appears to be similar with anionic surfactant in terms of both ash content and combustible recoveries. There was virtually no differences in the zeta potential values between the two sets of Soma lignite in the presence of SDS. This also supports the flotation results given above. The maximum recoveries were obtained at pH 11 with the corresponding maximum ash contents occurring at the same pH. However, the ash reduction is not enough to warrant a detailed explanation. Minimum ash contents were obtained at pH 3 but flotation recoveries were low. The effect of SDS on the zeta potential of treated and untreated Soma lignite as a function of pH show that although there is a remarkable decrease in the zeta potentials of thermal treated lignite, no significant ash reduction or improvement in flotation recoveries is achieved in the presence of SDS. Microflotation experiments were carried out to reveal the effect of D AH addition on the floatability of Soma lignite in terms of the combustible recoveries and ash contents at natural pH (8.5+0.2). While the recoveries increased up to a DAH concentrations of 5x1 0"4 M, ash contents showed a marginal reduction. Flotation recoveries and ash contents were very close for both treated and untreated lignites. The zeta potential data supported the above finding in that the potentials also become less negative at a rapid rate above 10" 4 M DAH concentration. These experiments were conducted at the natural pH in distilled water. Combustible recoveries in the presence of 3x10-4 y[ decreases upon increasing pH and reaches a minimum value at about pH 11. The ash contents at this pH increased marginally. Microflotation experiments were carried out to understand the effect of heat treatment on the combustible recovery and ash content of three different lignites at varying temperatures. While Alpagut and Yenikoy lignites were treated at 105 °C and 150 °C, Saray lignite was treated at 60, 105 and 150 °C. In all of the microflotation experiments, in the absence of so called collector surfactants, MTBC was used as a frother at a constant concentration of 50 mg/1. Therefore, the recovery achieved with MIBC without any heat treatment was regarded as the reference state for comparison. The floatability of Alpagut lignite at natural pH of 9 ±0.2 indicates no improvement upon heat treatment, if at all, a marginal decrease in its floatability TX and a corresponding increase in the ash content is observed. This reveals that Alpagut coal contains inappreciable amounts of oxygen containing functional groups in its structure. In particular, the large amount of mineral matter (53.07 % ash) in the Alpagut lignite leaves very little room for improvement in the flotation recoveries on thermal treatment. The effect of thermal treatment on the combustible recoveries and ash contents of Yenikoy lignite, evidently, there is a consistent increase in combustible recovery from about 45 % up to 60 % and a corresponding increase in the ash content from about 18 to 20 %. The feed coal contains 26 % ash. These results indicate that at natural pH (7+0.1) there appears to be no significant reduction in ash content upon heat treatment. Since this lignite contains ash less than Alpagut lignite, thermal treatment seems to increase the recoveries at the expense of ash contents. The amount of functional groups in Yenikoy lignite is expected to be higher than the Alpagut lignite. The combustible recoveries increase with increasing the heating time and heating temperature for Saray coal. The combustible recoveries attain a maximum value at about 2 hours of heating time at all temperatures studied. At heating times of over 5 hours, the combustible recoveries appear to reach a constant value, increasing the temperature, compared to the feed ash, marginally reduces the ash content. The results generally reveal that although the recoveries increase considerably, no significant improvement in ash reduction is achieved. There appears to be a direct correlation between the feed ash and the improvement achieved upon thermal treatment. Similarly, the amount of oxygen containing groups is also related to its original capillary moisture and in turn to the reduction achieved by thermal treatment. The foregoing results indicated that Saray coal responds best to thermal treatment among the coals studied and this particular coal was thus selected for further testing. The treated samples yield higher recoveries at all pH values but particularly at low pH's, i.e. pH values below natural pH of 4.5. Interestingly, the ash contents are also the lowest in the respective pH range, i.e. 16.50 % ash at pH 2.5 as opposed to 22.51 % ash at pH 10 for the treated coal. While the zeta potentials reach values as high as -100 mV at pH 10 for the untreated coal and decreases down to about -40 mV at pH 3, it exhibits zeta potential values of -30 to -10 in the respective pH region with the treated coal. The floatabilhy of heat treated and untreated Saray lignite appears to be similar with anionic surfactant (SDS) in terms of both ash content and combustible recoveries. There was virtually no differences in the zeta potential values between the two sets of data. This also supports the flotation results. The maximum recoveries are obtained at pH 6 with the corresponding minimum ash contents occurring at the same pH. However, the ash reduction was not enough to warrant a detailed explanation. Although there was a remarkable decrease in the zeta potentials of thermal treated lignite, no significant ash reduction or improvement in recoveries is achieved in the presence of SDS. With the canonic surfactant (DAH), while the recoveries remain low up to DAH concentrations of 10~4 M, they increase almost exponentially above this concentration for both treated and untreated lignites. However, the ash contents show a trend as if the system undergoes a reverse flotation process, i.e., the float product contains more ash than the unfloated gangue. The zeta potential data supported the above finding in that the potentials also became less negative at a rapid rate above 10~4 M DAH concentration. These experiments were conducted at the natural pH of 4.5. Cationic surfactants are normally employed in the alkaline pH. As shown in Figure 1 1, the combustible recoveries in the presence of 2x1 0~4 M decreases upon increasing pH and reaches a rninimum value at about pH 9. The ash contents at pH 9, however, remain similar. The low recoveries, however, can be improved by conducting flotation at concentrations higher than 2x1 0~4 M. Microflotation experiments were done to find out the effect of nonionic surfactants (Sapogenat T-040, T-l 10 and T-300) addition on the floatability of Saray lignite. The floatability of heat treated and untreated lignite appears to be similar with nonionic surfactants at all sets in terms of both ash contents and combustible recoveries. With nonionic surfactants, increasing the amount of surfactant increases combustible recoveries but also ash content remain constant. Although there is decrease in the zeta potentials of thermal treated lignite, no significant as reduction was achieved in the presence of nonionic surfactant. The FTIR (Fourier Transform Infrared) measurements show that there is a direct correlation between the flotation recoveries and the peaks obtained with FTIR. Coal contains both hydrophilic and hydrophobic groups in its structure. While hydroxyl (-OH) and, carboxyl (-COOH) groups are hydrophilic, the aliphatic (R-H) and aromatic (Ar-H) groups are considered as hydrophobic. It is the net effect of these two groups that determines the state of floatability or wettability. The results reveal that the subbituminous Soma coal is adversely affected by heat treatment because there is a decrease in the hydrophobic groups. Conversely, there is a significant reduction in the hydrophilic groups with Saray and Yenikoy coals. Similarly, these coals are more amenable to improved flotation recoveries. Alpagut coal, on the other hand, shows no change in flotation recoveries upon heat treatment and also there is a marginal reduction in the hydroxyl and carboxyl groups. It appears that the amount of hydrophobic and hydrophilic groups in coal dictates whether a particular coal is amenable to thermal treatment.