Cov kav dej thiab cov khoom siv dej yuav tsum tau ntxuav ntawm impurities thiab av hauv cov yeeb nkab ua ntej kev teeb tsa; tiv thaiv cov khoom sib khi ntawm lub qhov ncauj ntawm cov yeeb nkab kom tsis txhob poob rau hauv cov yeeb nkab, kom tsis txhob thaiv cov yeeb nkab thiab cov khoom siv dej; Kev tawg ib ntus thaum lub sijhawm teeb tsa cov kav dej yuav tsum tau thaiv lub sijhawm kom tsis txhob muaj lwm yam Tom qab kev tsim kho cov kav dej tiav, cov kav dej yuav tsum tau muab tshem tawm kom txog thaum tsim nyog.
