褐煤是一种软的、低品位的煤，由压缩的泥煤形成. 又称褐煤, lignite has a low heating value due to its low carbon content and is typically used for the generation of electricity. 也可用于合成天然气的生成, 用于生产肥料和肥料本身, 用于家庭供暖, 以及其他各种菠菜网最稳定正规平台_最全菠菜网大全和工艺.
褐煤也有它的用途, 它通常被认为是建筑菠菜网最稳定正规平台_最全菠菜网大全中的污染物, 特别是混凝土砂. The presence of lignite in concrete sand can cause structural and cosmetic issues with the final product.
混凝土中褐煤的问题
Lignite is lighter than the aggregate that makes up the concrete, 所以当混凝土变硬时，它会上升到表面. This can potentially cause two main problems: staining and popouts.
染色是由褐煤的棕色引起的, 哪些可以在成品混凝土表面涂抹. Staining can be minor or severe, depending on the level of lignite contamination. This isn’t necessarily a deal-breaker in all construction applications using concrete, but ones where the appearance of the concrete surface is important, 褐煤污渍很难看.
弹出比染色更令人担忧, as they can interfere with the structural integrity of the concrete. Lignite particles near the surface can expand, leaving holes or popouts in the concrete.
出于这些原因, many concrete sand specifications allow only a minimal amount of lignite to be present. ASTM国际标准 C-33混凝土砂, which is used in the United States as well as some other countries, 最多允许0个.5% lignite if the surface quality is important and a maximum of 1% if surface quality is unimportant. 其他一些国家使用的标准允许的数量甚至更少.
Several regions have developed a color test as a quick and visual reference of determining if the sand meets the requirements based on how the sand looks. 另一个, more accurate method is a burn test where the material is weighed before and after being subjected to heat. In this test, the lignite is turned to ash and presents a measurable loss of weight.
因为褐煤可以存在于自然形成的砂矿中, it is imperative that concrete sand producers remove as much of this material as possible to meet the specifications required in their region and to ensure a quality product.
沙子样本中的褐煤污染(黑色颗粒)
如何去除混凝土砂中的褐煤
Lignite is typically removed during the wet washing stage of the production using various processes. One of the methods for removing lignite from concrete sand is with hindered settling equipment 如 a 平底分级机.
平底分级机s get their name due to the lack of an underflow discharge cone. Instead, 平底分级机s contain valves that release the material. The flat-bottom style allows for use in areas with height restrictions, 由于植物的进料高度较低. 这也降低了泵送功率和结构要求.
那么它们是如何工作的呢?
平底分级机s feature a plenum chamber near the bottom of the tank where water is injected. The use of a plenum chamber allows for a higher density of teeter nozzle, 哪一种能更均匀地分散上升的水流. When this rising current of water meets the feed coming in at the top of the tank, 形成了一个由接近大小的颗粒组成的致密流化床. Coarse/heavy particles in the feed are able to penetrate the bed of material and fall to the bottom of the tank, where they are removed by valves and discharged to the next stage of the process. Fine/lightweight particles can’t penetrate the bed of material and are carried by the rising current of water to discharge over the weir surrounding the top of the tank.
When used in lignite removal applications, 平底分级机s are able to 分离轻质褐煤 来自较重的沙粒. 褐煤的比重或相对密度约为1.3, compared to the specific gravity of sand, which is generally 2.7. The separation occurs because of this difference in specific gravity.
The amount of rising current inside the 平底分级机 is calculated to send coarse lignite particles into the overflow. Since coarse lignite particles are the highest contributor to popouts in a finished concrete product, 将它们从砂菠菜网最稳定正规平台_最全菠菜网大全中去除是必要的. 然而, because some fine product-sized sand particles may be light enough to overflow with the undesirable lignite (the fine sand being of a similar specific gravity as the coarse lignite), further separation of the overflow is often necessary after the 平底分级机.
通常, a sieve bend is used to separate the coarse lignite from the fine sand particles, which are then recombined with the underflow from the 平底分级机 and fed to dewatering equipment, 如 优质材料螺旋垫圈, 水力旋流器 和/或 脱水屏幕，用于储存.
Sieve bends are vertically curved screens that contain separation media. When the 平底分级机 overflow containing lignite and fine sand is fed onto the sieve bend, 砂粒通过筛媒中的开口. The lignite particles that are too large to pass through these openings instead pass over the screening section and are sent to waste or used for site rehabilitation projects.
Small lignite particles that do pass through the screen media with the fine sand particles can be further removed with 优质材料螺旋垫圈, 水力旋流器或其他平底分类器.
It is important to note that while this process is very efficient for removing lignite, 并不是所有的褐煤颗粒都可以被清除, 而且沙子可能不是完全没有褐煤. 然而, the sand will often meet required specifications with just the removal of the large lignite particles via the 平底分级机.
