高频燃烧红外吸收法测定石灰石和白云石中硫

采用GB/T 3286.7—2014中的两种方法分别对石灰石和白云石中硫进行测定,样品不经过预灼烧直接用高频燃烧红外吸收法测定(简称直接法)所得结果会明显低于其经预灼烧法处理后再测定(简称预灼烧法)的结果。考虑到预灼烧法操作较为繁琐,实验对直接法测定结果偏低的原因进行分析,并对直接法的助熔剂条件进行了改进,建立了不用对样品进行处理,直接用高频燃烧红外吸收法测定石灰石和白云石中硫的方法。收集直接法和预灼烧法对白云石标准样品测定时产生的粉尘,采用X射线荧光光谱法(XRF)对其成分进行半定量测定,同时采用高频燃烧红外吸收法对其中硫再次测定。结果表明,直接法所得粉尘中氧化钙的质量分数约为7%、氧化镁的质量分数约为4%,而预灼烧法的粉尘中氧化钙和氧化镁的质量分数均小于0.1%;直接法所得粉尘中硫的质量分数为0.012%,预灼烧法所得粉尘中硫的质量分数仅为0.002%。这说明直接法测定时硫释放率偏低的主要原因可能与样品中高含量的碳酸钙、碳酸镁相关,推测认为:直接法测定时产生的二氧化碳气流将碳酸钙、碳酸镁分解生成的部分碱性氧化物氧化钙、氧化镁细粉带入仪器的低温气路区,造成氧化钙或氧化镁与二氧化硫酸性气体重新反应,最终导致直接法测定硫的结果偏低。实验在国家标准方法(GB/T 3286.7—2014)的助熔剂条件基础上,加入三氧化钼粉酸性氧化物以有效避免样品中的高含量碳酸钙、碳酸镁对测定的影响。改进后的测定条件为:称取0.20 g样品与0.5 g三氧化钼粉在坩埚中混合,再加入0.3 g锡粒、0.5 g纯铁和1.5 g钨粒。实验方法应用于石灰石和白云石实际样品中0.01%～0.27%(质量分数)硫的测定,分析结果与重量法或燃烧-碘酸钾滴定法一致,相对标准偏差(RSD,n=8)为0.8%～2.6%。

Determination of sulfur in limestone and dolomite by high frequency furnace combustion infrared absorption method

When the content of sulfur in limestone and dolomite was determined by two methods in GB/T 3286.7-2014, respectively, the results of direct method (the sample was directly determined by high frequency furnace combustion infrared absorption without pretreatment) were significantly lower than those obtained by pre-ignition method (the sample was determined after treatment by pre-ignition). Since the operation of pre-ignition method was complicated, the reason for the lower determination results of direct method was analyzed, and its flux conditions were improved. Finally, an improved direct determination method of sulfur in limestone and dolomite by high frequency combustion infrared absorption without treatment of sample was established. The dust produced in determination of dolomite certified reference material by direct method and pre-ignition method was collected and semi-quantitatively determined by X-ray fluorescence spectrometry (XRF). Meanwhile, the content of sulfur in it was determined again by high frequency combustion infrared absorption method. The results showed that the mass fractions of calcium oxide and magnesium oxide in collected dust from direct method were about 7% and 4%, respectively, while they were both less than 0.1% from pre-ignition method. The mass fraction of sulfur in collected dust from direct method and pre-ignition method was 0.012% and 0.002%, respectively. It indicated that the low release rate of sulfur determined by direct method was mainly due to the high content of calcium carbonate and magnesium carbonate in sample. It was speculated that the carbon dioxide stream generated in determination by direct method brought some alkaline oxide powders (including calcium oxide and magnesium oxide that were formed by the decomposition of calcium carbonate and magnesium carbonate) into the low temperature gas circuit of instrument, leading to the re-reaction between calcium oxide or magnesium oxide and the acidic gas of sulfur dioxide. As a result, the determination results of sulfur in direct method were lower. Based on the flux conditions in national standard method (GB/T 3286.7-2014), the acidic oxide of molybdenum trioxide powders were added into sample to avoid the influence of high content calcium carbonate and magnesium carbonate on the determination of sulfur. The improved determination conditions were listed as follows: 0.20 g of sample was mixed with 0.5 g of molybdenum trioxide powder in crucible followed by addition of 0.3 g of tin particles, 0.5 g of pure iron and 1.5 g of tungsten particles. The experimental method was applied to the determination of sulfur with mass fraction in range of 0.01%-0.27% in actual samples of limestone and dolomite. The analytical results were consistent with those obtained by gravimetric method or combustion-potassium iodate titration method. The relative standard deviations (RSD, n=8) were between 0.8% and 2.6%.