Cov pas nrig hlau nplaum feem ntau yog siv los lim hlau pins hauv cov ntaub ntawv raw;Lim txhua yam zoo hmoov thiab kua, hlau impurities hauv semi kua thiab lwm yam khoom sib nqus.Tam sim no, nws tau siv dav hauv kev lag luam tshuaj, khoom noj khoom haus, pov tseg rov ua dua tshiab, carbon dub thiab lwm yam.