Preparation of rare earth concentrate slag by de-ironing and phosphorus removal of medium and low grade rare earth concentrate

Fine rare earth smelting slag is an important raw material of the rare earth silicon iron alloy. The rare earth concentrate is obtained from the tailings of the Baiyun Obo rare earth iron ore after iron ore processing. With the continuous improvement and improvement of mineral processing technology , the rare earth oxide content of rare earth concentrates can reach more than 60%. However, the smelting of rare earth ferrosilicon alloys with high-grade rare earth concentrates is economically unreasonable and thus has not been applied in industrial scale production. At present, a large number of low-grade rare earth concentrates from Baiyun Obo are used to smelt rare earth ferrosilicon alloys. The chemical compositions are shown in Table 1.

Table 1 Chemical composition of Baotou rare earth concentrate Unit: %

Low-grade rare earth concentrate from the chemical composition can be seen, rare earth concentrate contains more impurities, especially phosphorus-containing higher amounts of these oxides, in addition to reductant in the smelting process to consume a certain amount, is not conducive to Improve the rare earth content of rare earth ferrosilicon alloy, and have a very bad impact on product quality. Therefore, rare earth concentrates must be treated to reduce the cost of slagging.

The medium and low grade rare earth concentrates are generally below 20mm in size and contain high water content. Therefore, the rare earth concentrate must be agglomerated and dried before it can be desulfurized and dephosphorized.

I. Agglomeration of rare earth concentrates Commonly used methods for agglomerating rare earth concentrates include pellet method and briquetting method.

(I) Preparation of rare earth concentrate pellets According to the different consolidation temperature of rare earth concentrate pellets, they are divided into low temperature solidified pellets and high temperature calcined pellets.

Preparation of low temperature consolidated pellets The preparation process of rare earth concentrate pellets is shown in Figure 1.

Figure 1 Schematic diagram of the preparation process of low temperature consolidated pellets

Low-temperature consolidated rare earth concentrate pellets need to be selected with suitable binders, such as water glass (Na ₂ SiO ₃ ) and hydrated lime [Ca(OH) ₂ ]. The spheroidal process is simple and easy. Firstly, the rare earth concentrate with less than 8% moisture enters the water glass powder which accounts for 5% of the mass, mixes in the drum mixer, and then is made into a rare earth essence of 15~25mm by a pelletizer. Mineral pellets. The raw ball is dried in a drying oven for 40 minutes, and the temperature of the pellet at the bottom of the drying oven is controlled to be 120 to 150 °C. The dried rare earth concentrate pellets can have a compressive strength of 390 N/ball or more.

Another method of preparing low temperature consolidated rare earth concentrate pellets is carbonation cold consolidation. The process is to add 10% to 15% slaked lime and a small amount of water glass to the dried rare earth concentrate, and mix evenly, and then use a pelletizer to make a pellet of 15 to 25 mm, and the ball formation rate is 70% to 80%. After natural drying, the green ball has a compressive strength greater than 50 N/ball, and the dried pellets are put into a carbonation tank and passed into a hot furnace exhaust gas (CO ₂ >20%, 50-80 ° C). The treated pellets can have a compressive strength of 30 to 50 N/ball. The slaked lime in the rare earth concentrate pellets not only participates in the carbonation reaction, but also acts as a flux to increase the alkalinity of the pellets.

Preparation of high temperature calcined pellets Rare earth concentrates are free of binders or only a small amount of flux. After mixing, a pellet of 15 to 25 mm is formed by a pelletizer, and the slag phase formed by the pellet itself is consolidated by high temperature roasting. There are various methods for high-temperature calcination, and rare earth concentrate pellets are commonly used in a sintering furnace roasting method and a rotary kiln roasting method.

The pelletizing and roasting of the rare earth concentrate was carried out using a 1600 mm disc pelletizer and a 0.24 m3 sintering furnace, and the pelletizer disc angle was 45°. With a side height of 180mm and a rotational speed of 17r/min, a 1.0-1.1t green pellet can be produced per ton of rare earth concentrate with a productivity of 900kg/(m2·h). The ball performance is listed in Table 2. The sintering furnace is shown in Figure 2. The wind pressure of the fan used is 4000Pa, the rotational speed is 2850r/min, and the power is 2.8kW. Each furnace can produce rare earth concentrate balls 0.8~1.0t, and the return rate is 5%~8%. The technical conditions of the calcination and the properties of the cooked balls are shown in Tables 3 and 4, respectively.

Table 2 Performance of rare earth concentrate ball

Table 3 Roasting technical conditions

Table 4 Performance of roasted balls

Figure 2 Schematic diagram of the sintering furnace

1-furnace; 2-exhaust port; 3-dusting valve; 4-handwheel

The main parameters of the rotary kiln are: kiln body diameter 700mm, length 12000mm, effective volume 4.6m3, lining refractory brick thickness 115mm, kiln body inclination angle 5°, rotation speed 0.465r/min, 0.58r/min and 1.2r/min respectively . Coal to coke oven gas as fuel. Air combustion. The kiln maintains a weak negative pressure, and the flame is weakly oxidized. A bell feeder is provided at the end of the kiln. The pellet is fed into the kiln through a mm120mm discharge bend, and the finished pellet discharged from the kiln is stored in the hopper. Practice shows that when the rotary kiln is inclined at 5°, the rotational speed is 0.58r/min, and the sintering temperature is 1115～1130°C, the production index with the utilization coefficient of 1.45～1.54t(m3·d) and the yield of 87.4%～91.1% can be obtained. The pellet has a compressive strength of about 1100 N/ball.

In actual production, the roasting of rare earth concentrate pellets by rotary kiln or sintering furnace can meet the needs of de-ironing and phosphorus removal and smelting rare earth ferrosilicon alloy.

(II) Rare earth concentrate briquetting The rare earth concentrate briquetting process is simple and easy. The rare earth concentrate and slaked lime (added in an amount of 8% to 10% of the concentrate) are uniformly mixed in a mixer, and then sent to a briquetting machine for press molding. The size of the rare earth concentrate compact can be changed according to the production requirements by changing different molds, generally controlled at 65mm × 110mm × 240mm. After the natural compaction of the compact, the strength can meet the requirements of iron removal in the electric furnace. This method is simple and convenient to operate, but the strength of the compact is low, which may cause damage during long-term storage and transportation, and thus the use is limited.

(III) Mineral composition of rare earth concentrate pellets The mineral composition of rare earth concentrate pellets depends largely on the calcination temperature and alkalinity. Consolidation low rare earth mineral concentrate pellets substantially maintaining the original composition of the rare earth ore, the main mineral monazite, bastnaesite, hematite (Fe ₂ O _3), magnetite (Fe ₃ O ₄₎ , fluorite (CaF2) and barite (BaSO ₄ ).

The mineral composition of the high-alkali rare earth concentrate pellets at high temperature is different from that of low-temperature consolidation pellets. The main reason is that under the high temperature roasting conditions, some physical and chemical changes have taken place inside the pellets, and the mineral composition is mainly red iron. Mineral (Fe ₂ O ₃ ), calcium ferrite (CaO·Fe ₂ O ₃ ), fluorite (CaF2) and barite (BaSO ₄ ), gunite (3CaO·CaF ₂ ·SiO ₂ ) and eucalyptus (Ce ₂ O ₃ ) and the like. The appearance of tassel is apparently caused by the decomposition of monazite and bastnasite during roasting. Calcination of high alkalinity (CaO/SiO ₂ >1.87) and high temperature (1100-1200 °C) is a necessary condition for the production of tassel. High-alkali high-temperature calcined rare earth concentrate pellets are very beneficial for the production of high-quality rare earth concentrate slag and smelting rare-earth ferrosilicon alloy, and should be promoted in industrial production.

Second, rare earth concentrate pellets de-iron removal of phosphorus The rare earth concentrate pellets through the electric arc furnace, ore furnace de-iron removal of phosphorus to prepare rare earth concentrate slag, is an important part of smelting qualified rare earth ferrosilicon alloy. The following focuses on the process and principle of preparing rare earth concentrate slag by de-ironing and dephosphorization of electric arc furnace.

(1) Process for de-ironing and dephosphorization of rare earth concentrate pellet arc furnace Desulfurization and dephosphorization of rare earth concentrate by using electric arc furnace to prepare rare earth concentrate slag, which has the advantages of simple process, convenient operation and high equipment utilization rate, and thus is in industrial production. use. The process flow is shown in Figure 3. The equipment used is an electric arc furnace for smelting rare earth ferrosilicon alloy, and the slag iron tank is a high temperature resistant iron casting. After the slag iron in the tank is statically cooled for more than 8 hours, it can be completely separated. Note that high-phosphorus iron cannot be mixed into the slag.

Fig. 3 Schematic diagram of the process of preparing rare earth concentrate slag in electric arc furnace

(II) Basic principle of de-ironing and dephosphorization of rare earth concentrate pellets The low-grade rare earth concentrate pelletizing arc furnace de-ironing and dephosphorization, the reducing agent used is mainly carbon. EAF at a temperature, carbon may be reduced iron, manganese, phosphorus, titanium, and niobium oxides such as, but not reducing rare earth oxide, rare earth oxide remains in the slag leaving the original shape.

Since the coke density is much smaller than the slag, the coke tends to float on the surface of the slag during smelting, and even if the agitation is enhanced, the contact between the slag and the coke is not satisfactory. Thus the kinetic conditions of the reduction are insufficient. In addition, it is also difficult to completely remove oxides which are difficult to be reduced, such as TiO ₂ , with carbon. In order to improve the reducing conditions, a certain amount of ferrosilicon is added as an auxiliary reducing agent in actual production. The reduction effect of ferrosilicon is better than that of coke, but because of the high price, the excessive amount will lead to the increase of the cost of rare earth concentrate slag and the partial rare earth. Reduction, under the premise of ensuring the quality of rare earth concentrate slag, should use as little as possible.

Reduction of Iron Oxide The reduction of iron oxide is carried out step by step, and the iron oxide can be reduced to iron in the order of Fe ₂ O ₃ → Fe ₃ O ₄ → FeO → Fe by carbon or silicon. In actual production, the reduction rate of iron is very fast. After stirring for 10 minutes with compressed air, the iron content in the slag can be reduced from 10% to less than 0.5%. Since the CO gas escapes during the reaction, it acts as an auxiliary agitation, which is beneficial to accelerate the reduction process.

Reduction of phosphorus oxides Phosphorus in rare earth concentrates is mainly present in the form of carbonates such as monazite (CePO ₄ ), calcium phosphate (3CaO·P ₂ O ₅ ), and laminite (3FeO·P ₂ O ₅ · H ₂ O) and the like. Under the conditions of electric arc furnace smelting, when SiO ₂ and CaO are involved, the monazite will be decomposed [P ₂ O ₅ produced by the reaction formula (1)] is reduced by carbon [reaction formula (2)] or silicon [reaction formula (3) )].

2CePO ₄ +3CaO+2SiO ₂ =3CaO·Ce ₂ O ₃ ·2SiO ₂ +P ₂ O ₅ (1)

P ₂ O ₅ +5C=2P+5CO↑ (2)

2P ₂ O ₅ +5Si=4P+5SiO ₂ (3)

At 1200 ~ 1500 ° C, calcium phosphate can be reduced by carbon:

3CaO·P ₂ O ₅ +5C=3CaO+2P+5CO↑ (4)

In the presence of SiO2, calcium phosphate will decompose:

2(3CaO·P ₂ O ₅ )+3SiO ₂ =3(2CaO·SiO ₂ )+2P ₂ O ₅ (5)

Silicon reduced iron oxide and other oxides produced by SiO ₂ can promote the decomposition of phosphate, thereby accelerating the reaction process.

When the iron content of the rare earth concentrate pellets is low, a small amount of pig iron is added, so that the phosphorus produced by the reduction is dissolved in the iron to form a stable Fe ₃ P phase.

Reduction of Manganese Oxide Manganese oxide is carried out step by step as well as reduction of iron oxide. However, the affinity of manganese for oxygen, and the content of manganese in rare earth concentrates is much lower than that of iron, and the thermodynamic conditions of reduction are poor, so manganese oxides are more difficult to reduce than iron oxides. The high-valence oxide of manganese reduced by carbon can be carried out at a lower temperature for the low-valent oxide, and the reaction of carbon and silicon to reduce MnO is shown in the reaction formula (6) and the reaction formula (7).

(MnO)+C=[Mn]+CO↑ (6)

2MnO+Si=2Mn+SiO ₂ (7)

â–³G ⁰ =-123090+18.79T

The SiO _{2 in the} slag can react with MnO to form MnSiO ₃ which is difficult to reduce. Therefore, the addition of an appropriate amount of CaO can promote the decomposition of MnSiO ₃ and increase the activity of MnO in the slag.

MnSiO ₃ +CaO=CaSiO ₃ +MnO (8)

â–³G ⁰ =-75470-5.31T

The reduction rate of manganese in the process of de-ironing and dephosphorization of rare earth concentrate pellets can reach more than 80%.

The reduction of titanium oxide is a relatively high titanium content. When smelting rare earth ferrosilicon alloy for ductile iron, it is required that the titanium content of concentrate slag is as low as possible, so the rare earth concentrate is in the process of dephosphorization and dephosphorization. It is also an important content to reduce the titanium content of its slag.

TiO ₂ is a very stable oxides, carbon reduction of TiO ₂ [Reaction formula (9)], an onset temperature of 1684 deg.] C, at which temperature conditions EAF is quite difficult. However, if ferrosilicon is added to the charge and the reaction [10] is reduced with silicon, the reaction proceeds smoothly.

1/2 TiO ₂ +C=1/2 Ti+CO↑ (9)

â–³G ⁰ =-341290-174.05T

[RESi]+[Si]=[RESi ₂ ] (10)

(III) Calculation of ingredients in the process of de-ironing and dephosphorization In the process of preparing rare earth concentrate slag by demineralization and dephosphorization of rare earth concentrate pellets, the ratio of various raw materials to the furnace must be accurately calculated, and the raw materials used should be chemically analyzed. When iron, phosphorus, manganese and titanium in the rare earth concentrate pellets are all reduced by carbon, the amount of coke can be calculated by the following formula.

In the formula:

C-coke into the furnace, kg;

Q-Rare earth concentrate pellets into the furnace, kg;

C _Solid - carbon content in coke,%;

A-coke burning loss, %;

Fe, Mn, P, and Ti- are iron, manganese, phosphorus, and titanium elements, respectively, which are converted from rare earth concentrate pellets based on chemical analysis data.

In actual production, in order to simplify the calculation process, the amount of coke added can be calculated according to the following empirical formula:

1.2Q(0.58Fe+0.32P) (12)

In the formula:

C-coke into the furnace, kg;

Q-Rare earth concentrate pellets into the furnace, kg;

Fe and P- are the iron and phosphorus content in the rare earth concentrate pellets, respectively.

In order to increase the reduction ratio of iron, manganese and titanium, 75 ferrosilicon which accounts for 2% to 3% of the total amount of the rare earth concentrate pellets may be added to the furnace. If the rare earth concentrate contains less than 6% iron, it can be added to pig iron or scrap which accounts for 2% to 5% of the rare earth concentrate pellets.

(IV) Operation of the process of de-ironing and dephosphorization of rare earth concentrate pellets Firstly, according to the capacity of the electric arc furnace, the addition amount of rare earth concentrate pellets (clamps) is determined, and the amount of coke, ferrosilicon and pig iron (scrap) is calculated. . After the arc furnace is started with a low voltage, all the prepared coke is added to the furnace, and the rare earth concentrate pellet (clamp) is added to the coke. After 30 minutes, the power is supplied with a high voltage. The current gradually reaches full load. At any time, observe the smelting situation in the furnace. When the charge is melted, stack the charge to the high temperature zone as much as possible to accelerate the melting process and avoid the lining of the charge.

When the charge is melted by 80% to 90%, ferrosilicon is added to the furnace, and if necessary, pig iron or scrap steel is added. Note that the reduction of coke should be used as much as possible, and the timing of adding ferrosilicon should not be too early.

The furnace temperature is controlled at 1400 ~ 1500 ° C, supplemented by moderate agitation. When the slag Fe is less than 0.5%, it can be discharged, and the rare earth rich slag and the high phosphorus iron are cooled and separated in the tank.

(V) Technical and economic indicators for preparing rare earth concentrate slag Rare earth concentrate pellets for deironing and dephosphorization of rare earth concentrates for the preparation of rare earth concentrate slag. An important technical index is the slag formation rate, that is, the slag produced by the unit rare earth concentrate pellets. the amount. The level of slag formation depends mainly on the content of elements such as phosphorus and manganese in the rare earth concentrate pellets, and is also related to the production operation. The slag formation rate of low-grade rare earth concentrate pellets of Bayan Obo is generally 70%-80%.

Another important technical indicator is the recovery rate of rare earths. The level of rare earth recovery reflects the level of production technology and operation level. When the rare earth concentrate slag is prepared by de-ironing and dephosphorization of the Baiyun Ebo rare earth concentrate pellet arc furnace, the rare earth recovery rate is above 80%.

The chemical composition of the grade rare earth concentrate pellets and de-ironing slag in Baotou is shown in Table 5. The distribution ratios of the main elements in the slag and iron phase of the de-ironing and phosphorus removal process are listed in Table 6. The technical and economic indicators of production are listed in the table. 7.

Table 5 Chemical composition of grade rare earth concentrate pellets and de-ironing slag Unit: %

Table 6 Distribution of main elements in the process of detachment of iron and phosphorus in slag and iron phase Unit: %

Table 7 Technical and economic indicators of iron furnace de-ironing

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