Rare earth extractant compounds having the following structure: ##STR00001##
wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are independently selected from alkyl groups containing 1-30 carbon atoms and optionally containing an ether or thioether linkage connecting between carbon atoms, provided that the total carbon atoms in R.sup.1, R.sup.2, R.sup.3, and R.sup.4 is at least 12; R.sup.5 and R.sup.6 are independently selected from hydrogen atom and alkyl groups containing 1-3 carbon atoms; and provided that at least one of the conditions (i)-(iv) apply as follows: presence of a distal branched group in at least one of R.sup.1-R.sup.4 (condition i), asymmetry in R.sup.1-R.sup.4 (condition ii), presence of amine-containing ring (condition iii), or presence of lactam ring (condition iv). Also described are hydrophobic water-insoluble solutions containing at least one extractant compound of Formula (1), as well as method for extracting rare earth elements from aqueous solution by contacting the aqueous solution with the water-insoluble solution.

The extraction system contains secondary amides and alkyl alcohols which are separately used as the extractants for extracting lithium and boron and consist of a single compound or a mixture of two or more compounds, and the total number of carbon atoms in their molecules are 1218 and 820 respectively; the extraction system has a freezing point less than 0 C. With a volume ratio of an organic phase and a brine phase being 110:1, at a brine density of 1.251.38 g/cm.sup.3, at a brine pH value of 07 and at a temperature of 050 C., a single-stage or multi-stage countercurrent extraction and a stripping are conducted to obtain a water phase with a low magnesium-lithium ratio, which is subjected to concentration, impurity removal and preparation to get lithium chloride, lithium carbonate, lithium hydroxide and boric acid respectively. Water is used for stripping, greatly reducing the consumption of acid and base.

Proposed are a positive electrode active material using a spent battery leachate for secondary batteries and a method of preparing the same. Using a spent battery leachate enables the positive electrode active material for secondary batteries, the positive electrode active material having a composition of Li(Ni.sub.aCo.sub.bAl.sub.c)O.sub.2 (where a+b+c=1) including Ni, Co, and Al and being prepared from a precursor having a composition of Ni.sub.aCo.sub.b (where a+b=1), to be prepared. As a result, some raw materials can be replaced with the spent battery leachate when preparing the positive electrode active material for secondary batteries, thereby reducing manufacturing costs and solving environmental problems.