【発明の詳細な説明】
本発明はバイヤー法のアルミナ製造において、
工程中を循環しているアルミン酸ソーダ溶液（以
下、循環母液という）中の有機物、特に高分子の
有機物を酸化分解する方法に関するものである。
バイヤー法ではボーキサイト中に含まれる微量
の高分子有機物が循環母液中に溶出し、該母液を
繰り返し使用するに伴い次第に該母液に蓄積され
るが、該高分子有機物は赤泥の沈降や水酸化アル
ミニウムの析出に悪影響をおよぼすことが知られ
ている。
循環母液中に溶出した高分子有機物は酸化分解
すると分子量の小さい有機物に変化し、最終的に
は蓚酸塩、炭酸塩などの低分子量化合物になる。
この様にして生成した蓚酸塩および炭酸塩は既知
の方法によつても該母液から除去することができ
るが、一般に該母液中での蓚酸塩の溶解度は小さ
いので溶解度以上に生成した蓚酸塩は該母液中で
自然に析出して赤泥や析出水酸化アルミニウムと
共にバイヤー工程の系外へ排出される。この様な
性質を利用して従来から循環母液中の高分子有機
物を酸化分解して蓚酸塩として析出除去する方法
がいろいろ提案されている。即ち、本発明者らに
よる循環母液に加圧酸素を接触させる方法（特公
昭45−30458）、240℃以上で加圧空気と接触させ
る方法（特開昭55−62809）、120〜350℃で加圧酸
素または酸素を含有するガスを接触させる方法
（特開昭55−71626）などである。
しかし乍ら、これらの方法で有機物の酸化分解
反応を促進するためには高圧容器内（通常、高圧
蒸気パイプを内部に設けて、それとの熱交換によ
つて、ないしは生蒸気を直接吹き込んで昇温す
る）に封入した循環母液の温度を高温にしなけれ
ばならない。その結果、酸素または酸素を含有す
るガス（以下、酸化ガスという）を高圧容器内へ
吹き込むための装置および高圧容器に大きな耐圧
が要求され、設備に多大な費用を要するという欠
点を持つていた。
本発明者等は種々研究の過程で加圧容器内に封
入した循環母液に水蒸気を添加した酸化ガスを圧
入、分散させたところ、単に酸化ガスのみを圧
入、分散させた場合よりも該母液中の高分子有機
物の酸化分解が著しく早く進行することを見いだ
し、本発明を完成した。
本発明の目的は、従来方法より効率よく高分子
有機物を酸化分解させることであつて、循環母液
の温度より高い露点を有する酸化ガスによつて該
母液を処理することを特徴とするものである。
本発明において、水蒸気を含んだ酸化ガスの温
度は該母液の温度より高いことが必要であること
は勿論のこと、水蒸気を含有する該酸化ガスの露
点は該母液の温度より高くなければならない。水
蒸気を含有した酸化ガスの露点が循環母液の温度
より低いと、高分子有機物の酸化分解効率の向上
は認められない。好ましくは、酸化ガスは水蒸気
で飽和されていることである。
水蒸気を添加した酸化ガスを循環母液に圧入す
る時点で、酸化ガスと水蒸気とは出来るだけ均一
に混合されていることが望ましい。酸化ガス（露
点が循環母液の温度より低い）と水蒸気とを高圧
容器内へ封入した循環母液に、各々異なる個所か
ら圧入、分散したのでは、本発明の効果は達せら
れない。
本発明方法による処理を行うことによつて、高
分子有機物は酸化分解されて最終的には蓚酸塩、
炭酸塩などとなり、既述のように、容易にバイヤ
ー工程から系外に排出されるので、系内の有機物
の総量も低下することになる。
本法と従来法とで同一の有機物分解率を達成し
ようとすると、本法での方が循環母液の処理温度
は低くて済むこととなり、従つて蒸気の使用量は
少なくなつて設備に求められる耐圧強度も小さく
なる。また、両方法で同一の温度で循環母液を処
理する時は、有機物の酸化分解率は本法での方が
大きくなり、従つてその分、設備規模が小さくて
済むことになる。
ところで、本法によつて循環母液中の高分子有
機物の酸化分解率が大巾に向上する理由は必ずし
も明らかではないが、次の如く推察される。
即ち、水蒸気を含有した酸化ガスは、循環母液
中に圧入、分散されると該母液と接触して冷却さ
れる。その時、酸化ガスと循環母液との気液界面
では該ガス中に含まれる水蒸気が凝縮して大量の
凝縮熱を発生し、該気液界面近傍で局部的に該母
液の温度が上昇する。その結果、該母液中に存在
する有機物の酸化分解反応が増大する。このよう
に、極めて局部的かつ急速に有機物の酸化分解反
応が起こると、その時発生する反応熱は短時間の
うちには放散しがたいので該反応部の温度は更に
上昇し、よつて酸化分解反応が一層加速されるこ
とになる。
なお、水蒸気を含有する酸化ガスの露点が処理
する循環母液の温度より低い場合は、大きな熱量
である水蒸気の凝縮熱を利用することが出来ない
ので有機物の酸化分解効率の向上が認められない
ものと考えられる。
本発明法を実施するに当つては循環母液をバイ
ヤー工程の系外へ取り出して処理するだけでな
く、ボーキサイトからアルミナ分を抽出する工程
において、例えばオートクレーブ中に水蒸気を含
有する酸化ガスを圧入してアルミナ抽出と同時に
有機物を分解させることもできる。
以下に比較例とともに本発明の実施例を示す。
ここでOCとは被処理液中に含まれる分子量500以
上の有機物による有機炭素の濃度（ｇ／）であ
り、その測定法は既知の方法（LEVER、G.、
LIGHT METALS 107、71（1978））によつて、
限外過を行い、被処理液中の分子量500以上の
有機物を分画回収して、この有機物の有機炭素を
OC分析計（例えば、島津TOC−10B）に注入し
て燃焼させて炭素量を定量するものである。
実施例 １
実開昭55−2468の原理に基づく回転かご型気液
接触装置を内包したステンレス鋼製加圧容器（内
径16CM、内高60CM）内にバイヤー工程から採
取した析出終了液（Na2O130ｇ／、Al2O371
ｇ／、OC9.2ｇ／）７を封入し、加圧容器
を外部から加熱または冷却して被処理液の温度を
200℃に保持した。該接触装置に内蔵する回転か
ごを單磁駆動装置により1000RPMで回転しなが
ら、加圧容器の底部に設けた気体送入管を通じて
水蒸気を飽和した250℃の空気を３／分（常圧、
常温の乾き空気換算、以下同様）の流速で容器中
へ120分間圧入した。被処理液へ吸収されずに残
つた空気は加圧容器の上部に設けた保圧弁を通じ
て外部へ放出した。なお、保圧弁の放出圧は析出
液の飽和水蒸気圧と空気圧（常温での８Kg／cm2
（絶対圧）に相当する所定の処理液温度での空気
圧）との複合圧となるように設定した。酸化分解
処理中は被処理液の温度が一定に保たれるように
制御した。処理終了後、冷却してから被処理液を
取り出して、室温で過飽和の蓚酸塩等を自然に析
出させて除去した後、被処理液中のOC（ｇ／）
を測定した。
実施例 ２
温度250℃、露点230℃の空気を酸化ガスとして
使用した以外は実施例１と全く同じ条件で実施し
た。
実施例 ３
被処理液の温度を170℃に保持した以外は実施
例１と全く同じ条件で実施した。
実施例 ４
水蒸気で飽和した酸素（常温換算での酸素の絶
対分圧は６Kg／cm2）を酸化ガスとして使用した以
外は実施例１と全く同じ条件で実施した。
比較例 １
温度250℃の乾き空気を３／分の流量で、ま
たこれとは異る個所を通じて温度250℃の飽和水
蒸気を3.3ｇ／分（実施例１における水蒸気流量
と同じ）の流量で、酸化ガスとして使用した以外
は実施例１と全く同じ条件で実施した。
比較例 ２
温度250℃、露点170℃の空気を酸化ガスとして
使用した以外は実施例１と全く同じ条件で実施し
た。
前記の各実施例および各比較例で得られたデー
タを第１表へ示した。
以上の結果から、本願発明による有機物の酸化
分解方法は従来法による有機物の酸化分解方法の
持つ欠点を大巾に改善していることが明らかであ
る。
【表】[Detailed Description of the Invention] The present invention provides a method for producing alumina using the Bayer method.
This invention relates to a method for oxidatively decomposing organic substances, particularly polymeric organic substances, in a sodium aluminate solution (hereinafter referred to as circulating mother liquor) that is circulated during a process. In the Bayer method, trace amounts of high-molecular organic matter contained in bauxite are eluted into the circulating mother liquor, and as the mother liquor is used repeatedly, it gradually accumulates in the mother liquor. It is known to have an adverse effect on aluminum precipitation. When the high-molecular-weight organic substances eluted into the circulating mother liquor undergo oxidative decomposition, they change into organic substances with small molecular weights, and eventually become low-molecular weight compounds such as oxalates and carbonates.
The oxalate and carbonate thus produced can be removed from the mother liquor by known methods, but generally the solubility of oxalate in the mother liquor is low, so the oxalate produced in excess of the solubility It naturally precipitates in the mother liquor and is discharged to the outside of the Bayer process together with red mud and precipitated aluminum hydroxide. Utilizing these properties, various methods have been proposed in the past for oxidatively decomposing high-molecular organic substances in the circulating mother liquor to precipitate and remove them as oxalate. Namely, the method by the present inventors of bringing pressurized oxygen into contact with the circulating mother liquor (Japanese Patent Publication No. 45-30458), the method of bringing the circulating mother liquor into contact with pressurized air at 240°C or higher (Japanese Patent Application Laid-open No. 55-62809), and the method of contacting the circulating mother liquor with pressurized air at 120-350°C. Examples include a method of contacting pressurized oxygen or a gas containing oxygen (Japanese Unexamined Patent Publication No. 71626/1983). However, in order to promote the oxidative decomposition reaction of organic matter with these methods, it is necessary to raise the temperature in a high-pressure container (usually with a high-pressure steam pipe installed inside and exchange heat with it, or by directly blowing live steam into the container). The temperature of the circulating mother liquor sealed in the liquid must be raised to a high temperature. As a result, a device for blowing oxygen or oxygen-containing gas (hereinafter referred to as oxidizing gas) into the high-pressure container and the high-pressure container are required to have a high pressure resistance, and the equipment has the drawback of requiring a large amount of cost. In the process of various studies, the present inventors injected and dispersed an oxidizing gas to which water vapor had been added into a circulating mother liquor sealed in a pressurized container. The present invention was completed based on the discovery that the oxidative decomposition of high-molecular organic substances proceeds extremely quickly. An object of the present invention is to oxidize and decompose polymeric organic substances more efficiently than conventional methods, and is characterized in that the mother liquor is treated with an oxidizing gas having a dew point higher than the temperature of the circulating mother liquor. . In the present invention, the temperature of the oxidizing gas containing water vapor must be higher than the temperature of the mother liquor, and the dew point of the oxidizing gas containing water vapor must be higher than the temperature of the mother liquor. If the dew point of the oxidizing gas containing water vapor is lower than the temperature of the circulating mother liquor, no improvement in the efficiency of oxidative decomposition of the polymeric organic matter will be observed. Preferably, the oxidizing gas is saturated with water vapor. It is desirable that the oxidizing gas and the steam be mixed as uniformly as possible when the oxidizing gas to which water vapor has been added is pressurized into the circulating mother liquor. The effects of the present invention cannot be achieved if oxidizing gas (with a dew point lower than the temperature of the circulating mother liquor) and water vapor are pressurized and dispersed into the circulating mother liquor sealed in a high-pressure container from different locations. By carrying out the treatment according to the method of the present invention, the polymeric organic substances are oxidized and decomposed, and finally oxalate,
The organic matter becomes carbonates, etc., and as mentioned above, is easily discharged from the system from the Bayer process, resulting in a decrease in the total amount of organic matter in the system. If we try to achieve the same organic matter decomposition rate with this method and the conventional method, the treatment temperature of the circulating mother liquor will be lower in this method, and therefore the amount of steam used will be less, which will require less equipment. Compressive strength also decreases. Furthermore, when the circulating mother liquor is treated at the same temperature in both methods, the oxidative decomposition rate of organic matter is higher in this method, and therefore the scale of the equipment can be reduced accordingly. Incidentally, the reason why the oxidative decomposition rate of high-molecular organic substances in the circulating mother liquor is greatly improved by this method is not necessarily clear, but it is speculated as follows. That is, when the oxidizing gas containing water vapor is pressurized and dispersed into the circulating mother liquor, it comes into contact with the mother liquor and is cooled. At this time, water vapor contained in the gas condenses at the gas-liquid interface between the oxidizing gas and the circulating mother liquor, generating a large amount of heat of condensation, and the temperature of the mother liquor locally increases near the gas-liquid interface. As a result, the oxidative decomposition reaction of organic matter present in the mother liquor increases. In this way, when the oxidative decomposition reaction of organic matter occurs extremely locally and rapidly, the reaction heat generated at that time is difficult to dissipate in a short period of time, so the temperature of the reaction area further increases, and the oxidative decomposition occurs. The reaction will be further accelerated. Note that if the dew point of the oxidizing gas containing water vapor is lower than the temperature of the circulating mother liquor being treated, the heat of condensation of the water vapor, which has a large amount of heat, cannot be used, so no improvement in the efficiency of oxidative decomposition of organic matter is observed. it is conceivable that. In carrying out the method of the present invention, not only is the circulating mother liquor taken out of the Bayer process and treated, but also an oxidizing gas containing water vapor is pressurized into an autoclave in the process of extracting alumina from bauxite. It is also possible to decompose organic matter at the same time as alumina extraction. Examples of the present invention are shown below along with comparative examples.
Here, OC is the concentration (g/) of organic carbon due to organic matter with a molecular weight of 500 or more contained in the liquid to be treated, and its measurement method is a known method (LEVER, G.,
LIGHT METALS 107, 71 (1978))
Ultrafiltration is performed to fractionate and recover organic matter with a molecular weight of 500 or more in the liquid to be treated, and the organic carbon of this organic matter is extracted.
The amount of carbon is determined by injecting it into an OC analyzer (for example, Shimadzu TOC-10B) and burning it. Example 1 Precipitation-finished liquid (Na 2 O130g/, Al 2 O 3 71
g/, OC9.2g/) 7 is sealed, and the temperature of the liquid to be treated is controlled by heating or cooling the pressurized container from the outside.
It was maintained at 200°C. While rotating the rotary cage built in the contact device at 1000 RPM using a magnetic drive device, air at 250°C saturated with water vapor was fed at 3/min (normal pressure,
The air was pressurized into the container for 120 minutes at a flow rate of dry air at room temperature (the same applies hereafter). The remaining air that was not absorbed into the liquid to be treated was released to the outside through a pressure-holding valve provided at the top of the pressurized container. The release pressure of the pressure holding valve is the saturated water vapor pressure of the precipitate and the air pressure (8 kg/cm 2 at room temperature).
(air pressure at a predetermined processing liquid temperature) corresponding to the absolute pressure). During the oxidative decomposition treatment, the temperature of the liquid to be treated was controlled to be kept constant. After the treatment is completed, the liquid to be treated is cooled and taken out, and supersaturated oxalate etc. are naturally precipitated at room temperature and removed, and the OC (g/) in the liquid to be treated is
was measured. Example 2 The experiment was carried out under exactly the same conditions as in Example 1, except that air with a temperature of 250°C and a dew point of 230°C was used as the oxidizing gas. Example 3 The experiment was carried out under exactly the same conditions as in Example 1, except that the temperature of the liquid to be treated was maintained at 170°C. Example 4 The experiment was carried out under exactly the same conditions as in Example 1, except that oxygen saturated with water vapor (absolute partial pressure of oxygen calculated at room temperature was 6 kg/cm 2 ) was used as the oxidizing gas. Comparative Example 1 Dry air at a temperature of 250°C was supplied at a flow rate of 3/min, and saturated steam at a temperature of 250°C was supplied at a flow rate of 3.3g/min (same as the steam flow rate in Example 1) through a different location. The experiment was carried out under exactly the same conditions as in Example 1 except that it was used as an oxidizing gas. Comparative Example 2 A test was conducted under exactly the same conditions as in Example 1, except that air with a temperature of 250°C and a dew point of 170°C was used as the oxidizing gas. The data obtained in each of the Examples and Comparative Examples described above are shown in Table 1. From the above results, it is clear that the method for oxidative decomposition of organic matter according to the present invention greatly improves the drawbacks of the conventional method for oxidative decomposition of organic matter. 【table】