地下煤层自燃区岩石磁性增强特征及机理研究——以内蒙古乌达和宁夏汝萁沟煤矿为例

Characteristics and mechanisms of rock magnetic increasing in underground coal spontaneous combustion area——take Wuda coal mine of Inner Mongolia and Ruqigou coal mine in Ningxia as examples

采用室内加温实验和岩矿测试分析手段,对地下煤层自燃区采集的岩石标本,测定其在不同温度条件下岩石磁性、含铁氧化物含量及其结构的变化情况,分析其特征和变化规律,探讨温度变化引起岩石磁性变化的机理.实验研究表明,温度变化可引起岩石磁性变化,通常在加温过程中多数岩石样品在低于400℃时其磁性较弱且没有明显的变化,继续升温达到含铁氧化物居里点温度前磁性增加,高于居里点温度后磁性消失;再由居里点温度或以上降温过程中磁性显著增强,并在常温下获得较岩石加温前更强磁性,但磁性变化较大.磁性变化主要与岩石中的黄铁矿、赤铁矿、菱铁矿、褐铁矿等含铁氧化物含量及其结构变化有关,岩石样品加热至700℃再冷却到常温以后,其铁磁性矿物含量明显增加;矿物结构有所变化,加温前一般为不规则微粒状、短脉状,加温后多为规则微粒状,且颗粒大小比加温前略有减小,一般为10～150μm,大多属于多畴(MD)结构,部分膺单畴(PSD)结构.磁性增强的主要原因是含有少量含铁矿物的岩石在温度升高时产生了新的铁磁性矿物(如磁铁矿、磁黄铁矿、磁铁矿等).上述研究成果对解释煤层自燃区磁异常的成因具有重要意义.

The temperature experiment and lithologic analysis were conducted on the samples obtained in the underground coal spontaneous combustion area to study the mechanisms of how rock magnetic value changes with the temperature. The magnetic value, iron-containing oxides percentage and structure changes of the samples in different temperatures were tested to analyze the characteristics and changing rules of the samples. The results are as follows, temperature changing causes the changes of magnetic value; the magnetic value changes very small below 400 ℃, andincreases with temperature before it reaching the Curie temperature; magnetism of the samples vanishes when the temperature beyond the Curie temperature; magnetic values of samples increase obviously when they are cooled below the Curie temperature, and it is higher than being heated before. The changes of magnetic value are dominated by mineral content and structure in Iron-containing oxides, such as Pyrite, hematite, siderite, limonite etc. The content of ferromagnetic mineral in rock samples increase obviously by heating samples to 700 ℃ and cooling it to normal temperature. The structure of mineral is changing in the experiment and the size of particle is smaller than the before as well. Generally, the particle size is 10-150 μm which belong to Multi- domain structure （MD）, with some of them belong to Single-domain structure. The main reason for magnetic value increase is that new ferromagnetic mineral （pyrite, hematite, siderite, limonite etc. ） was created from few Iron-contained mineral when the temperature is increased. The research above is significant to explain the magnetic abnormally in coal spontaneous combustion area.