JP7615700B2 - How zinc oxide ore is produced - Google Patents

Description

The present invention relates to a method for producing zinc oxide ore.

A commonly used method for producing zinc oxide ore is to remove halogen components from crude zinc oxide dust obtained by a reduction roasting process (Weltz process) in a wet process, and then calcining the zinc oxide-containing wet cake obtained in the wet process (hereinafter referred to as "crude zinc oxide cake") in a dry heating rotary kiln (DRK) that performs a dry heating process to obtain zinc oxide ore.

In the above-mentioned zinc oxide ore manufacturing method, the primary raw material is generally steel dust, which is generated from blast furnaces and electric furnaces in the steel industry and contains a considerable amount of zinc in addition to the main component iron oxide.

In addition, in the above-mentioned zinc oxide ore manufacturing method, in addition to the above-mentioned primary raw materials charged in the reduction roasting process, a process is also implemented in which "crude zinc oxide fine powder" refined to a zinc content of 50% or more and an iron content of about 10% is directly charged as a secondary raw material in the downstream drying and heating process (see Patent Document 1).

Patent Document 1 also discloses that when the above-mentioned "crude zinc oxide fine powder" is charged into a dry heating rotary kiln (DRK) that performs the dry heating process, the powdered material is pelletized with specific properties, thereby suppressing the "carry-over rate" in the dry heating process. In this specification, the "carry-over rate" in the dry heating process refers to the weight ratio of dust carried away in the DRK exhaust gas to the weight of all raw materials charged into the DRK.

However, even with the method described in Patent Document 1, the "carry-over rate" of the powdered material fed into the drying and heating process still exceeds 2% (see Example 1 in paragraph [0056] [Table 2] of the specification of Patent Document 1).

In a manufacturing method for zinc oxide ore that sequentially involves a reduction roasting process, a wet process, and a dry heating process, there was a need for a means to further reduce the "carry-over rate" when powdered raw materials are directly charged into the dry heating process.

JP 2018-104733 A

The present invention aims to reduce the "carry-over rate" when powdered raw materials are directly charged into the dry heating process in a method for producing zinc oxide ore that sequentially involves a reduction roasting process, a wet process, and a dry heating process.

The inventors discovered that when the powdered raw material to be directly charged to the drying and heating process is pelletized before being charged, the quality of the iron component, which was previously eliminated as an impurity as much as possible, can be maintained at a certain level or higher, thereby reducing the "carry-over rate," and thus completed the present invention. More specifically, the present invention provides the following:

(1) A method for producing zinc oxide ore, comprising a reduction roasting process for reducing and roasting a primary raw material containing zinc to reduce and volatilize the zinc contained in the primary raw material to obtain crude zinc oxide dust, a wet process for wet-treating the crude zinc oxide dust to remove water-soluble impurities contained in the crude zinc oxide dust to obtain a crude zinc oxide cake, and a dry heating process for calcining the crude zinc oxide cake to obtain zinc oxide ore, in which in the dry heating process, a secondary raw material containing zinc and iron is charged into a dry heating rotary kiln together with the crude zinc oxide cake, and the secondary raw material is a pellet having a moisture content of 14% by weight or more and 19% by weight or less, an iron content of more than 10% by weight, and a zinc content of less than 50% by weight.

(2) The method for producing zinc oxide ore described in (1), wherein the secondary raw material is a pellet having a particle size distribution in which the proportion of particles having a particle diameter of 4 mm or more is 30% by weight or more.

(3) The method for producing zinc oxide ore according to (1) or (2), wherein the secondary raw material is a pellet obtained by granulating powdered iron and steel dust.

(4) The method for producing zinc oxide ore according to any one of (1) to (3), wherein the secondary raw material has a chlorine content of 3% by weight or more, and the proportion of the secondary raw material in the material to be treated that is charged into the dry heating rotary kiln in the dry heating process is 30% by weight or less.

(5) A method for producing zinc oxide ore according to any one of (1) to (4), wherein the secondary raw material has a zinc content of 35% by weight or more.

According to the present invention, in a method for producing zinc oxide ore in which a reduction roasting process, a wet process, and a dry heating process are carried out in sequence, it is possible to reduce the "carry-over rate" when powdered raw materials are directly charged into the dry heating process.

1 is a flowchart showing an example of an embodiment of a method for producing zinc oxide ore of the present invention.

A preferred embodiment of the method for producing zinc oxide ore of the present invention will be described below. However, the present invention is not limited to the following embodiment.

<Overall process>
The overall process carried out in a zinc oxide ore manufacturing plant using the Weltz method is performed in the following order: a reduction roasting step S10 in which primary raw materials such as steel dust are reduced and roasted to obtain crude zinc oxide; a wet process S20 in which chlorine, fluorine and cadmium are separated and removed from the crude zinc oxide dust obtained in the reduction roasting step S10 to obtain a crude zinc oxide cake; and a dry heating step S30 in which the crude zinc oxide cake obtained in the wet process S20 and secondary raw materials are dry heated to obtain zinc oxide ore (calcined ore).

In the overall process described above, primary raw materials containing zinc, such as steel dust, are charged into the upstream process of the reduction roasting process S10. Separately, secondary raw materials containing zinc may be directly charged into the downstream process of the drying and heating process S30. The method for producing zinc oxide ore of the present invention is thus a widely applicable method for cases in which secondary raw materials are directly charged into the drying and heating process S30.

The main feature of the method for producing zinc oxide ore of the present invention is that it is a process that uses, as the secondary raw material, materials whose moisture content and iron content are adjusted to specific ranges.

<Reduction roasting process>
The reduction roasting step S10 is a step of obtaining crude zinc oxide dust by reducing and roasting a primary raw material containing zinc. The reduction roasting process is carried out by a reduction roasting rotary kiln (RRK).

In the reduction roasting process S10, steel dust is widely used as the primary raw material charged into the RRK. The steel dust is continuously charged into the RRK together with carbonaceous reducing agents such as coal and coke, and limestone. Inside the RRK body, the maximum temperature of the material to be treated is controlled to about 1100°C or higher and 1200°C or lower by the combustion of heavy oil and the charged carbonaceous reducing agents.

The steel dust is reduced and roasted inside the RRK, and the metallic zinc that is reduced and volatilized is reoxidized in the exhaust gas to become powdered zinc oxide. The small amount of lead contained in the steel dust volatilizes as lead oxide. The powdered zinc oxide and lead oxide are introduced into a dust collector together with the exhaust gas from the RRK, where they are captured and recovered as crude zinc oxide dust. This crude zinc oxide dust generally contains halogen impurities such as chlorine at a concentration of between 8% and 20% by weight.

<Wet process>
The wet process S20 is a process for removing water-soluble impurities from the crude zinc oxide dust recovered in the reduction roasting process S10 by wet treatment to obtain a crude zinc oxide cake.

In the wet process S20, the crude zinc oxide dust is repulped to produce a slurry containing crude zinc oxide. This process distributes the halogen impurities, mainly chlorine and other impurities, and cadmium contained in the crude zinc oxide dust into the treatment liquid. The above slurry, from which the impurities have been sufficiently removed, is then dehydrated using a vacuum suction dehydrator or the like to produce a wet cake containing zinc oxide (crude zinc oxide cake).

<Drying and heating process>
The dry heating step S30 is a step of obtaining zinc oxide ore (calcined ore) by calcining the crude zinc oxide cake obtained in the wet step S20. The calcination process is carried out in a dry heating rotary kiln (DRK).

In the drying and heating step S30, the crude zinc oxide cake is charged into a DRK and calcined to further reduce the cadmium and fluorine concentrations, producing zinc oxide ore (calcined ore). The main feature of the manufacturing method of the present invention is that in this drying and heating step S30, a pellet-shaped "secondary raw material" having unique properties is charged into the DRK together with the crude zinc oxide cake. The properties of the "secondary raw material" unique to the present invention will be described in detail later.

The pellet-like "secondary raw material" is transported to the vicinity of the DRK using conveyor belts or other transport equipment, and then loaded into the DRK together with the crude zinc oxide cake using a fixed-amount loading device such as a screw feeder.

The heating temperature in the drying and heating step S30 is preferably maintained so that the temperature of the ore burnt when discharged from the DRK is 1100° C. or more and 1150° C. or less. Generally, the fluorine still remaining in the crude zinc oxide cake charged to the DRK is mostly in the form of lead chloride fluoride (PbFCl) and partly in the form of calcium difluoride (CaF ₂ ). Of these, PbFCl is almost entirely decomposed and volatilized by maintaining the temperature of the ore burnt within the above temperature range, and the chlorine and fluorine derived from PbFCl are almost entirely discharged to the exhaust gas treatment facility.

The moisture content of the crude zinc oxide cake to be treated in the drying and heating step S30 is generally 20% to 30% by weight. The general composition of this crude zinc oxide cake is 61% to 68% by weight zinc, 7% to 10% by weight lead, 0.3% to 0.9% by weight chlorine, and 0.2% to 1.5% by weight fluorine (all on a dry weight basis).

In the drying and heating process S30, the size of the zinc oxide ore discharged from the DRK is generally 1 mm to 6 mm, and its general composition is 60% to 70% by weight zinc, 3% to 5% by weight lead, 0.5% to 1.5% by weight chlorine, and approximately 0.6% by weight or less fluorine (all on a dry weight basis).

<Secondary raw materials>
In the manufacturing method of the present invention, the "secondary raw material" which is charged into the DRK for carrying out the drying and heating step S30 together with the crude zinc oxide cake derived from the primary raw material is a "zinc-containing pellet" characterized in that it contains a predetermined amount or more of iron in addition to zinc and has a moisture content adjusted to a specific range.

The particle size distribution of the "zinc-containing pellets" used as the "secondary raw material" should be such that the proportion of particles with a particle diameter of 1 mm or more is 30% by weight or more, and more preferably the proportion of particles with a particle diameter of 4 mm or more is 30% by weight or more. The particle size of the "zinc-containing pellets" after granulation varies depending on the granulation time, and can be adjusted to the above range, for example, by appropriately optimizing the rotation speed of the agitator blades of the granulator. By maintaining the particle size distribution of the "zinc-containing pellets" within the above range, carryover can be suppressed, and dust generation from the conveyor of the transport device during transport of the finely powdered crude zinc oxide powder can also be suppressed.

The moisture content of the "zinc-containing pellets" used as the "secondary raw material" should be between 14% and 19% by weight, and preferably between 16% and 19% by weight. If the moisture content of the "zinc-containing pellets" is less than 14% by weight, the proportion of powdered pellets will increase, resulting in increased dust generation during transportation and carryover in the DRK. On the other hand, if the moisture content of the "zinc-containing pellets" exceeds 19% by weight, the pellets will not be able to be granulated due to the excess moisture and will turn into a sediment-like material, increasing the amount of adhesion to the transport belt and excavation hopper, making transportation difficult.

The "zinc-containing pellets" used as the "secondary raw material" are characterized in that, although zinc is the main component, the zinc content is less than 50% by weight, and the iron content is more than 10% by weight. The zinc content of these "zinc-containing pellets" is preferably 35% by weight or more, and more preferably 40% by weight or more. The iron content is preferably more than 10% by weight and 30% by weight or less, and more preferably 12% by weight or more and 20% by weight or less.

As the zinc grade of the "zinc-containing pellets" used as the "secondary raw material" decreases, the zinc grade of the crude zinc oxide cinder obtained from the DRK decreases, leading to a deterioration in the zinc recovery efficiency when obtaining zinc from the crude zinc oxide cinder. For this reason, materials with a zinc grade of 50% by weight or more have been widely used as the "secondary raw material" in the past. In contrast, according to the present invention, by maintaining the iron content at a specified amount or more, it is possible to suppress material carryover, so that the zinc grade of the crude zinc oxide cinder can be maintained at 60% or more, as in the past, while using "secondary raw material" with a zinc grade of less than 50% by weight. In other words, according to the present invention, it has become possible to select "secondary raw material" with a lower zinc grade than before, while maintaining the zinc recovery efficiency at a good level equivalent to the past.

As mentioned above, in the production of zinc oxide ore, high-quality "crude zinc oxide fine powder" with a zinc content of 50% by weight or more has traditionally been used as the "secondary raw material" to be fed into the DRK that performs the drying and heating process S30. These "secondary raw materials" have been refined to reduce the content of iron, an impurity, as much as possible in order to increase the zinc content of the final product, zinc oxide ore. Powdered material with increased zinc content and no iron has been pelletized to reduce carryover, but despite this, as mentioned above, the company has still been forced to accept a "carryover rate" of more than 2%.

In contrast, the manufacturing method of the present invention is a new process that requires intentionally leaving a certain amount of iron in the "secondary raw material" used in the production of zinc oxide ore, which was previously thought to require refining in a way that would remove as much iron as possible. The inventors have newly discovered that when the powdered material used as the "secondary raw material" is pelletized, the carryover rate can be reduced if it contains a certain amount of iron.

The "zinc-containing pellets" having the above particle size distribution, moisture content, and iron content, which are essential in the manufacturing method of the present invention, can be obtained by granulating a powdered material whose main component is zinc, with a zinc content of less than 50% by weight and an iron content of more than 10% by weight. Although steel dust is suitable as the powdered material, the powdered material is not limited to steel dust as long as it satisfies the requirements stipulated by the present invention.

In the drying and heating step S30, the ratio of the "zinc-containing pellets" used as the "secondary raw material" to the treated material charged to the DRK is preferably 30% by weight or less. The "zinc-containing pellets" are placed in the twin-screw conveyor together with the crude zinc oxide cake described above, and are extruded from there to be charged to the DRK. If the ratio of the "zinc-containing pellets" in the charged material increases, the pellets are crushed when extruded by the twin-screw conveyor, and some of them are crushed when they are charged to the DRK. For this reason, by suppressing the ratio of the "zinc-containing pellets" in the amount of processing in the DRK to 30% by weight or less, the crushing of the pellets can be sufficiently avoided, and the effect of reducing the carryover rate can be stably enjoyed.

In the manufacturing method of the present invention, a powder material having a chlorine content of 3% by weight or more can be used as the raw material for the "zinc-containing pellets" used as the "secondary raw material" in a range of 5% by weight or less. For example, when unrefined steel dust is used as the raw material for the "zinc-containing pellets", the chlorine content is often 4% by weight. If the amount of chlorine brought into the DRK increases, the risk of an increase in carryover due to the promotion of chlorination volatilization of lead and the like in the DRK increases. However, by suppressing the proportion of the "zinc-containing pellets" in the processing amount in the DRK to 30% by weight or less, that is, by suppressing the chlorine content of the "zinc-containing pellets" as the "secondary raw material" and the relative usage amount of the "zinc-containing pellets" in the total processing amount in the drying and heating step S30 within the above range, the merit of the present invention, which reduces the carryover rate while maintaining the quality of the zinc oxide ore, can be fully enjoyed.

Specifically, the "zinc-containing pellets" can be obtained by adding water to a powder material containing zinc and iron in the above content ratio range, kneading the mixture, and then granulating and pelletizing the mixture while applying shear force. The amount of water added during granulation can be adjusted so that the final moisture content after granulation is 14% by weight or more and 19% by weight or less. The above kneading and granulation can be performed using granulation equipment consisting of, for example, a dow mixer and a dow pelletizer.

By using the "zinc-containing pellets" obtained in this way as the "secondary raw material", it is possible to reduce the carryover rate when the "secondary raw material" consisting of powdered material is charged into the drying and heating process S30 compared to the conventional method. This also makes it possible to suppress clogging of the wash water piping in the wash tower that treats the DRK exhaust gas, and the progression of adhesion to the exhaust gas duct.

In addition, according to the manufacturing method of zinc oxide ore of the present invention, dust generation during transportation of the "secondary raw material" to the DRK and carryover in the DRK can be suppressed, and unrefined "secondary raw material" derived from steel dust can be directly charged into the DRK without going through the reduction roasting kiln process or the wet process. This can reduce the consumption of carbonaceous reducing agent used in the reduction roasting kiln and can also reduce the operating costs of electricity and the like used in each of these upstream processes. In addition, in the manufacturing method of the present invention, other "secondary raw materials" than the above-mentioned "zinc-containing pellets" may be added supplementarily to the extent that the above-mentioned effects of the present invention are not impaired.

In a crude zinc oxide production plant capable of carrying out the overall process shown in Figure 1, a test run of crude zinc oxide ore production by the production method of the present invention was carried out. The effect of the present invention was verified by measuring the carryover rate of the raw materials in the drying and heating process in the test runs of each Example and Comparative Example, and observing the state of dust generation during transport to the DRK. In each run, "zinc-containing pellets" obtained by pelletizing any of the powder materials of Powders 1 to 4 having the composition shown in Table 1 below were used as the "secondary raw material". The particle size distribution and moisture content of the "zinc-containing pellets" are shown in Table 2, and the measurement and observation results are shown in Table 3 below.

[Example 1]
The powder material (powder 1) was granulated using a granulation facility consisting of a non-uniform speed twin-screw kneader (Dow mixer) and a non-uniform speed twin-screw granulator (Dow pelletizer) described below to obtain "zinc-containing pellets". The moisture content of the pellets after granulation was adjusted to 19% by weight. The rotation speeds of the Dow mixer and Dow pelletizer were each 60 Hz. The particle size distribution after granulation was such that the proportion of particles of 4 mm or more was 87% by weight. The "zinc-containing pellets" were charged into a DRK. In addition, the processing amount of the "zinc-containing pellets" processed as a secondary raw material in a drying heating kiln was 40 t/day, while the processing amount of the crude zinc oxide cake produced from the primary raw material (steel dust) via the reduction roasting kiln was 140 t/day.
Uneven speed twin-shaft kneader: "Dow Mixer (registered trademark)" manufactured by Shin-Nichinan Co., Ltd.
Non-uniform speed twin-shaft pelletizer: "Dow Pelletizer (registered trademark)" manufactured by Shin-Nichinan Co., Ltd.

[Comparative Example 1]
A test operation for producing crude zinc oxide ore was carried out under the same conditions as in Example 1, except that "Powder 3 (iron content: 9.0 wt%)" having a lower iron content than in Example 1 was used as the powder material.

[Example 2]
Except for changing the final moisture content to 14% by weight, the procedure was the same as in Example 1. In the particle size distribution after granulation, the proportion of particles of 4 mm or more was 38% by weight.

[Comparative Example 2]
A test operation for producing crude zinc oxide ore was carried out under the same conditions as in Example 2, except that "Powder 3 (iron content: 9.0 wt%)" having a lower iron content than in Example 2 was used as the powder material.

[Example 3]
The same procedure as in Example 1 was repeated except that "Powder 2 (zinc content: 38% by weight)" having a slightly lower zinc content was used as the powder material.

[Comparative Example 3]
The same procedure as in Example 1 was conducted except that the final moisture content was 20% by weight. Granulation was not possible due to the excess moisture, and the mixture became sediment-like, and the amount of the mixture adhering to the conveyor belt and the excavation hopper increased, making it difficult to convey the mixture and making it impossible to charge the mixture into the drying and heating kiln.

[Example 4]
As the powder material, "Powder 4 (zinc content: 48 wt %, iron content: 11 wt %)" having a zinc content of less than 50 wt % and an iron content of more than 10 wt % was pelletized under the same conditions as in Comparative Example 1, and a test operation for producing crude zinc oxide ore was carried out under the same conditions as in Example 1 and Comparative Example 1.

[Comparative Example 4]
The powder materials were charged into the DRK in powder form without granulation.

From the comparison between Example 1 and Comparative Example 1, and the comparison between Example 2 and Comparative Example 2, it can be seen that the carryover of the "zinc-containing pellets" used as the secondary raw material can be reduced when the iron content is more than 10% by weight. Also, from the comparison between Example 4 and Comparative Example 1, it can be seen that the carryover can be reduced when the zinc content is less than 50% by weight, compared to when a material with a high zinc content is used. This is presumably due to an increase in impurities other than zinc, such as Fe, CaO, and SiO ₂ , which increase the strength of the resulting zinc oxide cinder. Therefore, for example, it can be seen that steel dust with an iron content of about 12% by weight and a zinc content of less than 50% by weight can be used as a secondary raw material without being refined to reduce the iron content, while maintaining product quality and productivity comparable to that of a powder material that has been refined to reduce the iron content and increase the zinc content.

The above results show that the manufacturing method of the present invention, which is a method for manufacturing zinc oxide ore that sequentially involves a reduction roasting process, a wet process, and a dry heating process, is a manufacturing method that can reduce the "carry-over rate" when powdered raw materials are directly charged into the dry heating process, and therefore is a manufacturing method that can contribute to improving the productivity of zinc oxide cinders.

S10 Reduction roasting step S10
S20 Wet process S30 Dry heating process

Claims (5)

a reduction-roasting step of reducing and roasting a primary raw material containing zinc to reduce and volatilize the zinc contained in the primary raw material to obtain crude zinc oxide dust;
a wet process for removing water-soluble impurities contained in the crude zinc oxide dust by wet treating the crude zinc oxide dust to obtain a crude zinc oxide cake;
a drying and heating step of calcining the crude zinc oxide cake to obtain zinc oxide ore;
A method for producing zinc oxide ore, comprising:
In the dry heating step, a secondary raw material containing zinc and iron is charged into a dry heating rotary kiln together with the crude zinc oxide cake;
The secondary raw material is a pellet containing moisture, and the moisture content is 14% by weight or more and 19% by weight or less relative to 100% by weight of the pellet. The pellet has an iron content of 12% by weight or more and a zinc content of 38% by weight or more and less than 50% by weight on a dry weight basis.
How zinc oxide ore is produced. The secondary raw material has an iron content of 17% by weight or more and 20 % by weight or less on a dry weight basis in the pellets.
A method for producing the zinc oxide ore according to claim 1. The secondary raw material is a pellet having a particle size distribution in which the proportion of particles having a particle diameter of 4 mm or more is 30% by weight or more.
A method for producing zinc oxide ore according to claim 1 or 2. The secondary raw material is a pellet obtained by granulating powdered iron and steel dust.
A method for producing zinc oxide ore according to any one of claims 1 to 3. The secondary raw material has a chlorine content of 3% by weight or more,
The ratio of the secondary raw material to the material to be treated charged into the dry heating rotary kiln in the dry heating step is 30% by weight or less.
A method for producing zinc oxide ore according to any one of claims 1 to 4.

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