Preparation of enriched cerium oxide from bastnasite with hydrochloric acid by two-step leaching
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I. INTRODUCTION
AS a main industrial mineral, bastnasite has the largest proven reserve among all rare earth (RE) minerals in the world.[1] About 70 pct of RE products come from bastnasite production. The RE elements are extracted from bastnasite conventionally, by roasting the bastnasite with concentrated sulfuric acid[2] or sodium carbonate. The HF, which pollutes air seriously, is produced in the former process. The latter technology is focused on by many scientists, because it is friendly to the environment and offers many benefits in increasing the recovery of RE elements. After being roasted with sodium carbonate, bastnasite is converted to calcine, which contains rare earth oxides (REO). The RE elements in the calcine are extracted by leaching it with hydrochloric acid. The RE elements are then recovered from the leachate by precipitation, using oxalic acid.[3] In the roasting, the cerium (III) is oxidized to cerium (IV), existing as cerium oxide (CeO2), which cannot be dissolved with dilute hydrochloric acid. On the contrary, other RE elements existing as trivalence RE oxidants can be leached out easily, as can the impurities, such as Fe, Ca, Mg, etc. Thus, cerium is mostly left in the sludge. In the concentrated hydrochloric acid, cerium (IV) is a kind of strong oxidant, so it can be easily reduced to cerium (III), using hydrogen peroxide as a reducing agent. Consequently, an enriched cerium compound is prepared. Cerium oxide is excellent when used as a kind of polishing powder, because of its high chemical activity and unique crystal structure.[4,5,6] It possesses many advantages, such as long life, high polishing efficiency, and low remainder. It is gradually replacing the traditional abrasives. The RE polR. CHI, Professor, Z. LI, C. PENG, and Z. XU, Postgraduate Students, and H. GAO, Instructor, are with the Wuhan Institute of Chemical Technology, Hubei Key Laboratory of Novel Chemical Reactors and Green Chemical Technology, Wuhan, Hubei, People’s Republic of China 430073. Contact e-mail: [email protected] Manuscript submitted May 12, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
ishing powder is classified into two types: one is known as the high-cerium abrasive, in which the cerium oxide occupies over 80 pct; the other is known as the low-cerium abrasive, in which cerium oxide occupies from 48 to 50 pct. The polishing powder, which is used especially in chemical machine polishing (CMP), requires a high partitioning in cerium oxide. At present, the polishing powder containing low-cerium oxide is predominant in the China market, occupying 90 pct of the total yield in the world. Using traditional technology to prepare a polishing powder containing high cerium, in which the high purity of RE compounds act as a raw material,[7] is very costly The products coming directly from the bastnasite concentrate basically belong to the low-cerium polishing powder. Bastnasite is a typical mineral of light RE elements, of which cerium oxide occupies 50 pct. To produce high-cerium polishing powder from bastnasite,
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