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FROTH FLOTATION
Introduction
Flotation is undoubtedly the most important and versatile mineral-processing technique, and both use and application are being expanded to treat greater tonnages and to cover new areas.
Originally patented in 1906,flotation has permitted the mining of low-grade and complex ore bodies which would have otherwise been regarded as uneconomic. In earlier practice the tailings of many gravity plants were of a higher grade than the ore treated in many modern flotation plants.
Flotation is a selective process and can be used to achieve specific separations from complex ores such as lead-zinc, copper-zinc, etc. Initially developed to treat the sulphides of copper, lead, and zinc, the field of flotation has now expanded to include the oxidized minerals and nonmetallic, including-fine coal.
Principles of Flotation
The theory of froth flotation is complex and is not completely understood. The subject has been reviewed comprehensively by Klassen and Mokrousov and by Glembotskii et al. and will only be briefly dealt with here.
Froth flotation utilizes the differences in physic-chemical surface properties of particles of various minerals. After treatment with reagents, such differences in surface properties between the minerals within the flotation pulp become apparent and, for flotation to take place, an air-bubble must be able to attach itself to a particle, and lift it to the water surface (Fig.12.1). The process can only be applied to relatively fine particles, as if they are too large the adhesion between the particle and the bubble will be less than the particle weight and the bubble will therefore drop its load.
In flotation concentration, the mineral is usually transferred to the forth, or float fraction, leaving the gangue in the pulp or tailing. This is direct flotation as opposed to reverse flotation, in which the gangue is separated into the float fraction.
The air-bubbles can only stick to the mineral particles if they can displace water from the mineral surface, which can only happen if the mineral is to some extent water repllent or hydrophobic. Having reached the surface, the air-bubbles can only continue to support the mineral particles if they can form a stable forth, otherwise they will burst and drop the mineral particles. To achieve these conditions it is necessary to use the numerous chemical reagents known as flotation reagents.
The activity of a mineral surface in relation to flotation reagents in water depends on the forces which operate on that surface. The forces tending to separate a particle and a bubble are shown in Fig.12.2. The tensile forces lead to the development of an angle between the mineral surface and the bubble surface. At equilibrium,
γS/A=γS/W+γW/Acosθ
FROTH FLOTATION
Collectors
All minerals are classified into non-polar, or polar types according to their surface characteristics. The surfaces of non-polar minerals are characterized by relatively weak molecular bonds. The minerals are composed of covalent molecules held together by van der Waals forces, and the non-polar surfaces do not readily attach to the water dipoles, and in consequence are hydrophobic. Minerals of this type, such as graphite, sulphur, molybdenite, diamond, coal, and talc, thus have high natural floatabilities with contact angles of between 60 and 90. Although it is possible to float these minerals without the aid of chemical agents, it is universal to increase their hydrophobicity by the addition of hydrocarbon oils or frothing agents. Creosote, for example, is widely used to increase the floatability of coal. Use is made of the natural hydrophobicity of diamond in grease tabling, a classical method of diamond recovery which is still used in many plants. The pre-concentrated diamond ore slurry is passed over inclined vibrating tables, which are covered in a thick layer of petroleum grease. The diamonds are attracted to and become embedded in the grease because of their water-repellency, while the water-wetted gangue particles are washed off the table. The grease is skimmed off the table either periodically, or continuously, and placed in perforated pots(Fig.12.3), which are immersed in boiling water. The grease melts, and runs out through the perforations, and is collected and re-used, while the pot containing the diamonds is transported to the diamond-sorting section.
Minerals with strong covalent or jonic surface bonding are known as polar types, and exhibit high free energy values at the polar surface. The polar surfaces, react strongly with water molecules, and these minerals are naturally hydrophilic.
The polar group of minerals have been subdivided into various classes depending on the magnitude of polarity, which increases from groups 1 to 5(Table 12.1)Minerals in group 3 have similar degrees of polarity, but those in group3(a) can be rendered hydrophobic by sulphidisation of the mineral surface in an alkaline aqueous medium. Apart from the native metals, the minerals in group 1 are all sulphides, which are only weakly polar due to their covalent bonding, which is relatively weak compared to the ionic bonding of the carbonate and sulphate minerals. In general, therefore, the degree of polarity increases from sulphide minerals, through where γS/A, γS/W and γW/A are the surface energies between solid-air, solid-water and water-air, respectively, and θ is the contact angle between the mineral surface and the bubble.
The force required to break the particle-bubble interface is called the work of adhesion, WS/A and is equal to the work required to separate the solid-air interface and produce separate air-water and solid-water interfaces, i.e.
WS/A=γW/A+γS/W-γS/A
Combining with equation(12.1)gives
WS/A=γW/A(1-cosθ)
It can be seen that the greater the contact angle the greater is the work of adhesion between particle and bubble and the more resilient the system is to disruptive forces. The ftbatability of a mineral therefore increases with the contact angle; minerals with a high contact angle are said to be aerophilic, i.e. they have a higher affinity for air than for water. Most minerals are not water repellent in their natural state and flotation reagents must be added to the pulp. The most important reagents are the collectors, which adsorb on mineral surfaces, rendering them hydrophobic (or aerophilic) and facilitating bubble attachment. The frothers help maintain a reasonably stable froth. Regulators are used to control the flotation process; these either activate or depress mineral attachment to air-bubble and are also used to control the pH of the system.
浮 選
引言
浮選無疑是最重要和最廣泛的選礦技術(shù)，以及這種方法的使用和應(yīng)用正在不斷擴(kuò)大，以更好的用途與加工方法，來開拓新的領(lǐng)域。
最初的專利產(chǎn)生于1906年，浮選允許開采低品位復(fù)雜礦體但是這使得其被視為不經(jīng)濟(jì)的方法。那是以往的想法。尾礦中有許多由植物轉(zhuǎn)變的更高品位的礦石，所以許多現(xiàn)代浮選廠就被開設(shè)起來。
浮選是一個(gè)選擇性的過程，可用于實(shí)現(xiàn)具體的分離從復(fù)雜的礦石如鉛，鋅，銅，鋅等初步形成加工過的硫化物的銅，鉛，鋅，浮選領(lǐng)域現(xiàn)已擴(kuò)大到包括氧化礦物和非金屬，包括精細(xì)煤。
浮選原理
浮選理論是復(fù)雜的，不能完全解釋。這個(gè)問題已全面討論過的克拉森、莫科盧瑟弗和克萊姆布特斯克等，只簡單地處理了這里。
浮選采用了不同的理化性能的粒子表面的各種礦物質(zhì)。浮選用試劑，這種在表面性能之間存在差異的礦物在浮選礦漿內(nèi)變得明顯，并發(fā)生浮選，空氣泡沫必須能夠帶動附著在自己表面的粒子，并使其在水面形成泡沫層（圖.12.1 ）。
這個(gè)過程只適用于比較細(xì)的顆粒，因?yàn)槿绻麄冞^于龐大，粒子粘附在泡沫上的重量大于泡沫能夠承受的負(fù)荷的話泡沫就會無法上升。
浮選濃度，礦物通常轉(zhuǎn)移到第四，或固液比例，使煤矸石在礦漿中形成尾礦。這是直接浮選，而不是反浮選，其中煤矸石分為浮選尾礦。
空氣泡沫使得疏水的礦粒脫離水而附著在空氣表面，而這只能取決于礦物的親水性和疏水性。到達(dá)泡沫層，空氣氣泡能繼續(xù)支持礦物顆粒直到他們能夠形成穩(wěn)定的泡沫層，否則將會破裂使附著的礦物顆粒又回落到礦漿中。
為了實(shí)現(xiàn)這些條件，有必要使用大量化學(xué)試劑稱為浮選藥劑。
這個(gè)過程的礦物表面與浮選藥劑在水中的作用效果都是作用在礦粒表面，單獨(dú)粒子和泡沫中顯示如圖（Fig.12.2 ）。拉伸力量導(dǎo)致拉伸的角度使礦物與氣泡表面拉力平衡，
γS/A=γS/W+γW/Acosθ
浮選
捕收劑
所有礦物分為非極性或極性是根據(jù)其表面特性。表面非極性礦物的特點(diǎn)是相對比較弱的分子量。礦物組成，共價(jià)分子總共的范德華力，和非極性表面不容易吸引水偶極子，并因此具有疏水性。
這種類型的礦物，如石墨，硫磺，輝鉬礦，金剛石，煤，滑石，從而具有較高的自然浮選能力而且接觸角在60到90之間。雖然是有可能的浮選，但這些礦物有化學(xué)制劑的援助，這是為了增加其疏水性，增加碳?xì)溆皖惢蚱鹋輨?
木餾油，例如，被廣泛用于增加浮煤。使用的天然鉆石的疏水性油脂出臺，一個(gè)經(jīng)典方法鉆石的發(fā)現(xiàn)仍然是使用在許多植物中。集中金剛石礦泥漿是通過傾斜振動篩，這是覆蓋著一層厚厚的石油潤滑脂。這些鉆石石油潤滑脂，并成為中嵌入油脂，因?yàn)樗鼈冇惺杷裕衩喉肥w粒。油脂是脫脂定期或不斷，并放置在穿孔盆（ Fig.12.3 ） ，這是沉浸在沸騰的水中進(jìn)行。油脂融化，并運(yùn)行通過穿孔，并收集和再利用，而鍋含有鉆石的物質(zhì)被運(yùn)送到鉆石排料部分。
礦物強(qiáng)烈結(jié)合的表面共價(jià)鍵被稱為極性共價(jià)鍵，在能源價(jià)值高的極地表面。極地表面，作出強(qiáng)烈的反應(yīng)與水分子，這些礦物質(zhì)是親水性。
極地礦物群已分為各類不同的規(guī)模的極性，從而增加從群體1至5 （表12.1 ）礦物在第3組也有類似程度的極性，但那些在組3可以使疏水性由生硫化物的礦物表面在堿性水溶液中。除了本地金屬，
礦物在第1組都是金屬硫化物，這時(shí)分子間力極弱，由于其共價(jià)鍵，這是相對比較弱的離子鍵的碳酸鹽和硫酸鹽礦物。因此，總的說來，極性程度增加從硫化礦物，
通過在γS /A年， γS / W及γW /A是固體表面能之間的關(guān)系，固態(tài)水和水氣，分別是與θ之間的接觸角和礦物表面的泡沫。式子中所需中斷粒子泡沫接口是被稱為粘附的工作，是WS/A和平等的工作需要單獨(dú)的固體空中接口并產(chǎn)生不同的水氣和固體水界面，即
WS/A=γW/A+γS/W-γS/A
結(jié)合方程（ 12.1 ）得知
WS/A=γW/A(1-cosθ)
可以看出，更多的接觸角較大的工作面是粘附粒子和氣泡以及更具彈性的系統(tǒng)，憑借強(qiáng)大的分子間力。油這種性質(zhì)的礦產(chǎn)因此增加，接觸角;礦物高接觸角被認(rèn)為需要?dú)怏w的 ，也就是說，它們對空氣的親和力比水的更高。
大多數(shù)礦產(chǎn)不親水的自然狀態(tài)和浮選藥劑必須添加到礦漿。最重要的試劑是捕收劑，其中的礦物表面吸附，使它們的疏水性 （或需要?dú)怏w的）和促進(jìn)泡沫附著。該過程有助維持一個(gè)相當(dāng)穩(wěn)定的泡沫。穩(wěn)壓器是用來控制浮選工藝;這些要么激活或抑制礦粒附著氣泡，也可用來控制pH值的系統(tǒng)。