1. Field of the Invention
The present invention relates to a method of refining a ferrous ion-containing acid solution, and more particularly to a method which is used for eliminating impurities (such as silicon) from an iron ion-or iron salt-containing acid solution (such as a waste liquid which remains after steel is washed with a hydrochloric acid), so as to obtain a high-purity iron oxide.
2. Description of the Related Art
A waste liquid which remains after steel is washed with a hydrochloric acid (such a liquid will be hereinafter referred to simply as a "waste liquid") includes not only iron components but also impurities such as silicon, as is shown in Table 1 below.
TABLE 1 __________________________________________________________________________ Waste liquid Fe SiO.sub.2 Mn Al Cr Free-HCl remaining after 10.about.12% 80.about.110 ppm 250.about.290 ppm 45.about.50 ppm very small 1.5.about.2.2% washing steel amount.about. 5 ppm with acid Recovered Fe.sub.2 O.sub.3 SiO.sub.2 Mn Al Cr Cl- Iron oxide 93.about.96% 0.045.about.0.06% 0.18.about.0.22% 0.022.about.0.03% &lt;0.002 0.1.about.0.3% __________________________________________________________________________ (note) %: percentage by weight
Normally, a hydrochloric acid is recovered from the waste liquid by a roasting process, and the iron oxide powder (.alpha.-Fe.sub.2 O.sub.3) produced secondarily to the hydrochloric acid recovery is utilized for obtaining ferrite. However, since the waste liquid includes impurities, the iron oxide powder also includes them. For example, the SiO.sub.2 content in the iron oxide powder is in the range of 0.04 to 0.06% by weight.
In order to obtain high-quality ferrite, it is necessary to improve the purity, the grain size and other characteristics of the iron oxide powder. To obtain high-class soft ferrite, in particular, the iron oxide powder should be as pure as possible, and its SiO.sub.2 content, for example, should be not more than 0.01% by weight and preferably within the range of 0.005 to 0.007% by weight or less.
In a widely-adopted conventional method for producing such a high-purity iron oxide, purification of an iron hydroxide is performed when the iron hydroxide is crystallized by adding an alkali liquid to an iron sulfate solution, and the resultant high-purity iron hydroxide is thermally oxidized for deriving a high-purity iron oxide. More specifically, in the conventional method, an iron salt-crystallizing step is repeated under the same condition, or is combined with that performed under different conditions, so as to obtain a high-purity iron oxide. However, this conventional method does not allow recovery of an acid in many cases. In addition, it requires a complicated process, due to the inclusion of the step of crystallizing the iron salt and the step of thermally oxidizing the iron salt, so that production of a high-purity iron oxide requires a high cost.
In recent years, new roasting processes has been developed, so as not only to recover a hydrochloric acid from a waste liquid but also to produce a high-purity iron oxide at a low cost. According to the new methods, the waste liquid is subjected to a roasting process after silicon components are eliminated therefrom mainly in the form of SiO.sub.2. These new methods are disclosed in the following references, for example:
Japanese Patent Publication No. 59-73439 discloses a method wherein a waste liquid is ultrafiltered to eliminate silicon components therefrom and is then subjected to a roasting or crystallizing process. However, this method has the problem that the grain size of silica ions eliminatable from the waste liquid is limited, so that it is difficult always to control the SiO.sub.2 content in a-Fe.sub.2 O.sub.3 to be less than 0.01% by weight. In addition, since the productivity of the method is not good, due to the principle of the ultrafiltration. In the light of economical points, it is practically impossible to eliminate silicon components from all amount of the waste liquid.
Japanese Patent Publication No. 58-151335 discloses a method wherein a cationic polymer coagulant is added to a waste liquid, to thereby cause the silicon components (SiO.sub.2) to coagulate, and then the waste liquid is ultrafiltered. However, since this method utilizes ultrafiltration as in the method noted above, its productivity is not good, either.
Japanese Patent Disclosure No. 51-14898 discloses a method wherein a waste liquid is neutralized by adding an alkali solution thereto, thereby producing a ferrous hydroxide, a ferric hydroxide, or a mixture of these, and the silicon components in the waste liquid are filtered out after they are coprecipitated with the iron hydroxides. In this method, however, the silicon components are not coprecipitated with high efficiency. Therefore, the iron hydroxides must be produced such that they account for 8 to 10% by weight of the waste liquid. As a result, the amount of iron oxide obtainable will be lost in an amount corresponding to the produced iron hydroxides.
As mentioned above, the methods available at present do not simultaneously enable both high efficient impurity (e.g., Si) elimination from a waste liquid and low-cost production of a high-purity iron oxide.