JP7571591B2 - Method for manufacturing pellets containing reducing agent, and method for manufacturing zinc oxide ore - Google Patents

Description

The present invention relates to pellets containing a reducing agent for use in the production of zinc oxide ore. More specifically, the present invention relates to a method for producing pellets containing a reducing agent, which can be suitably used in a method for producing zinc oxide ore, including a process for volatilizing and separating and recovering zinc components contained in steel dust and the like by a reducing roasting process, and a method for producing zinc oxide ore using the pellets.

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 calcinate the zinc oxide-containing wet cake obtained in the wet process (hereinafter referred to as "crude zinc oxide cake") in a dry heating process to obtain zinc oxide ore.

In the above-mentioned zinc oxide ore manufacturing method, 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, is widely used as a raw material.

In the production of zinc oxide ore using steel dust as a raw material, the steel dust is fed into a reduction roasting rotary kiln (RRK) where reduction roasting is performed together with a carbonaceous reducing agent such as carbon. In order to further improve the zinc recovery rate, the steel dust fed into the rotary kiln is commonly mixed with the carbonaceous reducing agent and granulated, and formed into pellets with a size of about 5 to 10 mm that already contain the reducing agent (hereinafter referred to as "reducing agent-containing pellets") (see Patent Document 1).

In addition, a method has been proposed for manufacturing the above-mentioned "reducing agent-containing pellets" that are fed into the RRK where the reduction roasting process is performed, in which the crushing strength of the "reducing agent-containing pellets" is adjusted to a low strength range close to the lower limit of the allowable strength range for pellets, thereby promoting the volatilization of zinc and thereby improving the zinc recovery rate (Patent Document 2).

Here, steel dust usually contains a certain amount of chromium (Cr). On the other hand, the reducing agent contained in the "reducing agent-containing pellets", such as coke powder, has poor wettability, and even if steel dust is pelletized by adding water, the pellets themselves gradually disintegrate over time. For this reason, when the "reducing agent-containing pellets" that have completed the necessary drying are fed into a rotary kiln that performs the reduction roasting process using the Weltz method, the above-mentioned disintegrated parts of the pellets are carried over and mixed into the downstream process, and are ultimately distributed to the zinc oxide ore side. However, in the past, the Cr content of steel dust was rarely high enough to cause problems in the production of crude zinc oxide using the Weltz method, and in terms of quality control of zinc oxide ore, it was sufficient to check the Cr content of steel dust at the material stage before it was granulated into pellets and to eliminate the inclusion of particularly high-quality materials.

However, in recent years, the amount of chromium contained in steel dust has tended to increase overall, and there have been frequent cases where the chromium content of the final product, zinc oxide ore, has increased beyond the acceptable range due to powder from the "reducing agent-containing pellets" being carried over from the rotary kiln. As a result, there has been a demand for a means to more effectively suppress the carryover of the "reducing agent-containing pellets."

JP 2008-261005 A JP 2015-120948 A

The present invention aims to provide "reducing agent-containing pellets" that can stably maintain the chromium content of the final product, zinc oxide ore, in a desirable low-grade range in the production of zinc oxide ore using steel dust as a raw material.

The inventors discovered that by manufacturing the "reducing agent-containing pellets" to be fed into the reduction roasting process using the unique process described below, the chromium content of the final product, zinc oxide ore, can be stably maintained within a desirable low-grade range, and thus completed the present invention. More specifically, the present invention provides the following:

(1) A method for producing pellets containing a reducing agent, which is used in a method for producing zinc oxide ore from steel dust, includes a kneading step of adding water to the steel dust and the reducing agent and kneading them, a granulation step of further adding water to the kneaded product obtained in the kneading step to obtain water-containing pellets with a moisture content of 10% to 20% and a drying step of drying the water-containing pellets until the moisture content of the water-containing pellets is 5% to less than 10%, to obtain pellets containing a reducing agent, and in the kneading step and the granulation step, 20% or more of the water of the total water added to the steel dust and the reducing agent or the kneaded product thereof is added to the steel dust and the reducing agent in the kneading step, and in the kneading step and the granulation step, the rotation speed of the kneader for kneading and the granulator for granulation are adjusted so that the particle size distribution of the water-containing pellets has a ratio of particles with a particle size of less than 1 mm of 4% or less.

(2) The method for producing reducing agent-containing pellets described in (1), in which, of the total water added to the steel dust and reducing agent, or to the mixture thereof, 30% to 80% of the water is added to the steel dust and reducing agent in the kneading process and the granulation process.

(3) The method for producing reducing agent-containing pellets according to (1) or (2), in which the composition of the pellet raw materials is adjusted so that the particle size distribution of the entire pellet raw materials, including the steel dust and the reducing agent, fed into the kneading process, is such that the proportion of pellets with particle sizes less than 1.0 mm is 30% or more.

(4) A method for producing zinc oxide ore, comprising: a reduction roasting process in which a reducing agent-containing pellet obtained by the method for producing reducing agent-containing pellets described in any one of (1) to (3) is fed into a reduction roasting furnace to obtain crude zinc oxide dust; a wet process in which the crude zinc oxide dust is subjected to a wet treatment to obtain crude zinc oxide cake; and a dry heating process in which the crude zinc oxide cake is subjected to a dry heating treatment to obtain zinc oxide ore, wherein the chromium content of the zinc oxide ore is 0.1% by weight or less.

According to the present invention, in the production of zinc oxide ore using steel dust as a raw material, it is possible to provide "reducing agent-containing pellets" that can stably maintain the chromium content of the final product, zinc oxide ore, within a desirable low-grade range.

The following describes preferred embodiments of the method for producing the reducing agent-containing pellets of the present invention and the method for producing zinc oxide ore that can be carried out using the "reducing agent-containing pellets" of the present invention. However, the present invention is not limited to the following embodiments.

<Overall process>
The "reducing agent-containing pellets" obtainable by the present invention can be preferably used as a raw material to be input into the overall process for producing zinc oxide ore, which will be described below. In this process, a reduction roasting process is carried out in which a primary raw material, at least a part of which is the "reducing agent-containing pellets", is reduced and roasted to obtain crude zinc oxide, a wet process is carried out in which chlorine, fluorine and cadmium are separated and removed from the crude zinc oxide dust obtained in the reduction roasting process to obtain a crude zinc oxide cake, and a dry heating process is carried out in which the crude zinc oxide cake obtained in the wet process and the secondary raw material are dry-heated to obtain zinc oxide ore (calcined ore).

<Reducing agent-containing pellets and their manufacturing method>
[Reducing agent-containing pellets]
The reducing agent-containing pellets of the present invention (hereinafter also simply referred to as "reducing agent-containing pellets") can be used as a primary raw material to be fed into a rotary kiln performing a reducing roasting step in a method for producing zinc oxide ore from iron and steel dust.

The "reducing agent-containing pellets" can be obtained by the "reducing agent-containing pellet manufacturing method" of the present invention, which specifies the manufacturing conditions for kneading, granulation, and drying within unique ranges. After properly drying, the "reducing agent-containing pellets" have a particle size distribution immediately before being charged into a rotary kiln with a ratio of -1 mm of 6% or less. It is more preferable that this particle size distribution is 3% or less. By using such "reducing agent-containing pellets", unnecessary carryover is suppressed, and chromium components that should be distributed to the clinker side as residue are prevented from being distributed to the crude zinc oxide dust side, and the chromium content of the zinc oxide ore can be maintained in a preferable low-grade range. Specifically, by using the "reducing agent-containing pellets" of the present invention, the chromium content of the zinc oxide ore can be reduced to 0.1% by weight or less, and more preferably to 0.05% by weight. In addition, while monitoring the chromium content of the zinc oxide ore product, it is possible to increase the processing even for steel dust containing 1% chromium, thereby improving the productivity of the product.

[Method of manufacturing reducing agent-containing pellets]
The "reducing agent-containing pellets" are produced in a process which sequentially includes a kneading process in which water is added to steel dust and a reducing agent and the mixture is kneaded, a granulation process in which further water is added to the mixture obtained in the kneading process to obtain hydrous pellets, and a drying process in which the hydrous pellets are dried to obtain reducing agent-containing pellets.

(Kneading process)
The kneading step is a step of adding water to the steel dust and the reducing agent and kneading them. Specifically, this step can be carried out by adding water to the steel dust and the reducing agent and kneading them using, for example, a non-uniform speed twin-screw kneader, which is a kneader that has stirring blades attached to twin shafts and kneads them by rotating them.

In the manufacturing method of the present invention, in the entire process of adding moisture to the pellet raw materials (i.e., the granulation process carried out following this kneading process), of the total water added to the pellet raw materials (i.e., the steel dust and reducing agent, or a kneaded product thereof) that form the pellets, 20% or more of the water is added to the pellet raw materials in this kneading process. Moreover, of the total water added to the pellet raw materials, the above-mentioned proportion of water added in this kneading process is more preferably 30% or more and 80% or less, and most preferably 45% or more and 55% or less.

In addition, in the kneading process, the pellets (referred to as "moisture pellets" in this specification) immediately after this process and the subsequent granulation process are kneaded under conditions such that the ratio of particles with a particle size of less than 1 mm is 4% or less, more preferably 1% or less. Specifically, the rotation speed of the kneader (the "unequal speed twin-shaft kneader" mentioned above is adjusted so that the particle size distribution is such a distribution. In addition, it is preferable to promote granulation to the extent that the ratio of particles with a particle size of 4.0 mm or more is 50% or more on the upper side of the particle size distribution of the moisture pellets mentioned above, but on the other hand, from the viewpoint of promoting zinc volatilization, it is preferable to keep the ratio of particles with a particle size of 11.2 mm or more to 20% or less, more preferably 15% or less. It is more preferable to set the rotation speed of the kneader to the highest rotation speed as long as the above particle size distribution can be achieved.

In the manufacturing method of the present invention, it is preferable to adjust the blending of the raw pellet materials used so that the particle size distribution of the entire raw pellet materials, including the steel dust and reducing agent fed into the kneading process, is such that the proportion of particles with a particle size of less than 1.0 mm is 30% or more. For example, PC carbon obtained from petroleum coke satisfies the above particle size distribution requirements and is preferable as the reducing agent primarily used in the present invention. In this way, by mixing a certain amount of raw material with a small particle size, the collapse of the reducing agent-containing pellets after drying can be minimized, even if the other parts of the raw material have poor wettability.

[Granulation process]
The granulation process is a process in which water is further added to the mixture obtained in the kneading process to obtain "water-containing pellets". The moisture content of the "water-containing pellets" may be 10% to 20%, and preferably 12% to 18%. Specifically, this process can be carried out by adding water to the mixture of pellet raw materials that has been kneaded in the kneading process, and granulating the mixture using, for example, a non-uniform speed twin-screw granulator that is equipped with blades on two shafts and rotates them to granulate the pellets.

In the manufacturing method of the present invention, of all the water added to the pellet raw materials (i.e., the kneading process and the granulation process) that form the pellets, 80% or less of the water is added to the pellet raw materials that have been through the kneading process in the granulation process. Of all the water added to the pellet raw materials, the proportion of water added in the granulation process is preferably 20% or more and 70% or less, and most preferably 45% or more and 55% or less.

In the granulation process, the particle size distribution of the "water-containing pellets" immediately after this process is such that the ratio of particles with a particle size of less than 1 mm is 4% or less, more preferably 1% or less. Specifically, the rotation speed of the granulator (the above-mentioned "unequal speed twin-axis granulator" is used as an example) is adjusted so that the particle size distribution is as described above. In addition, it is preferable to promote granulation to the extent that the ratio of particles with a particle size of 4.0 mm or more is 50% or more on the upper end side of the flow distribution of the water-containing pellets described above, but on the other hand, from the viewpoint of promoting zinc volatilization, it is preferable to keep the ratio of particles with a particle size of 11.2 mm or more to 20% or less, more preferably 15% or less. It is more preferable to set the rotation speed of the granulator to the highest rotation speed as long as the above particle size distribution can be achieved.

[Drying process]
The drying process is a process in which the "moisture-containing pellets" obtained in the granulation process, which have a moisture content of 10% to 20%, are dried until the moisture content is 5% to less than 10%, to obtain "reducing agent-containing pellets." This process is a so-called aging process, and as a guideline, sufficient drying can be achieved by natural drying indoors for 7 days or more.

Here, the moisture content of the steel dust fed unprocessed into the reduction rotary kiln is usually around 7-8%. Conventionally, this steel dust has been able to be transported without problems without adhesion to the transport equipment or dust generation to the reduction roasting rotary kiln. By giving the "reducing agent-containing pellets" a similar moisture content, it is possible to prevent adhesion to the various transport equipment and dust generation. From this perspective, it is more preferable that the moisture content of the "reducing agent-containing pellets" be 5% or more and 8% or less.

<Method of manufacturing zinc oxide ore>
[Reduction roasting process]
The reduction roasting process is a process for obtaining crude zinc oxide dust by reduction roasting the zinc-containing primary raw material including the above-mentioned "reducing agent-containing pellets". The reduction roasting process is carried out by a reduction roasting rotary kiln (RRK). In the reduction roasting process, the "reducing agent-containing pellets" are put into the RRK together with steel dust that has not been subjected to a process for containing a reducing agent, a reducing agent such as coke, and a flux such as limestone, and are roasted in a reducing atmosphere at an average maximum temperature in the furnace of 1050 to 1200°C. In this RRK, the zinc contained in the steel dust is reduced, volatilized, and reoxidized, and is recovered as crude zinc oxide dust containing impurities such as chlorine and fluorine.

[Wet process]
The wet process is a process in which water-soluble impurities are removed from the crude zinc oxide dust recovered in the reduction roasting process by wet treatment to obtain a crude zinc oxide cake.

In the wet process, 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 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). The iron contained as the main component in the steel dust is discharged as clinker together with limestone added as flux.

[Drying and heating process]
The dry heating process is a process in which the crude zinc oxide cake obtained in the wet process is calcined and subjected to a dry heating treatment to obtain zinc oxide ore (calcined ore). The calcination treatment is carried out in a dry heating rotary kiln (DRK). In the dry heating process, the crude zinc oxide cake is put into the DRK and calcined to further reduce the concentrations of halogen impurities such as chlorine and cadmium, to obtain zinc oxide ore (calcined ore).

In addition, in this drying and heating process, crude zinc oxide obtained by reducing and volatilizing zinc-containing raw materials elsewhere can also be processed together as a secondary raw material.

The present invention will be explained in more detail below with reference to examples. However, the present invention is not limited to the following examples.

(Pellet raw material)
Steel dust and PC carbon as a reducing agent were used as pellet raw materials for producing the reducing agent-containing pellets of each Example and Comparative Example. The ratio of the steel dust to the reducing agent in the pellet raw materials was 1130:181 in weight ratio. The amount of reducing agent required to reduce and volatilize the zinc contained in the steel dust was calculated as a carbon rate of 13% using a predetermined calculation method, and the moisture rate of the steel dust was 8.8% and the moisture rate of the PC carbon was 15.3%, and the above content ratio was determined. The chromium content in the above "pellet raw materials" was 0.8% (average).

The particle size distribution of the above pellet raw materials, i.e., the particle size distribution of the entire pellet raw materials, including the steel dust and PC carbon, before being fed into the kneading process, is shown in Table 1 below. As shown in the table, as a result of adding 13% PC carbon, which has a small particle size, the particle size distribution above shows that the proportion of -1 mm particles is 30% or more.

In the present embodiment, the particle size distribution was measured in each of the following ways by sieving a sample extracted from the powder or granules to be measured, changing the mesh size from the largest to smaller mesh sizes one by one, and sieving 10 times for each mesh size.

(Kneading process/granulation process)
The pellet raw material consisting of the steel dust and the reducing agent was mixed by a non-uniform speed twin-shaft mixer (Dow Mixer (registered trademark) manufactured by Shin-Nichinan Co., Ltd.). The kneaded pellet raw material mixture was granulated using a non-uniform speed twin-axis granulator (Dow Pelletizer (registered trademark) manufactured by Shin-Nichinan Co., Ltd.) to obtain pellets of the respective Examples and Comparative Examples. In the above series of steps (kneading step and granulation step), the throughput of the pellet raw material per unit time was 16.6 wt/h for steel dust and 2.67 wt/h for PC carbon. /h.

The total amount of water (fresh water) added per unit time in the above series of steps (kneading step and granulation step) was 1.3 ^m3 . The ratio of the total amount of water added in each step (weight ratio of the amount added per unit time) was set as shown in Table 2 below for each Example and Comparative Example.

The rotation speeds of the kneaders and granulators for each Example and Comparative Example were as shown in Table 2 below.

(Drying process)
In Table 2, each of the water-containing pellets obtained under the four different kneading and granulation conditions shown as Examples 1 to 3 and Comparative Example was subjected to a drying process indoors (temperature 20° to 40°, humidity 40% to 80%) for 4 days to a maximum of 17 days to obtain reducing agent-containing pellets of each Example and Comparative Example.

(Pellets containing reducing agent)

In Table 2, the moisture content of the hydrous pellets obtained under the four different kneading and granulation conditions shown as Examples 1 to 3 and Comparative Example was measured on the first, third, seventh, and seventeenth days of the drying process. The moisture content was measured by drying at 105°C for 24 hours and calculating from the weight difference before and after drying. The changes in moisture content during the drying process of the hydrous pellets (or reduced agent-containing pellets that had been dried) of each Example and Comparative Example are shown in Table 3.

The particle size distribution of each of the above-mentioned hydrous pellets was measured on the first day and the 17th day of the drying process. The particle size distribution was measured by carrying out the above-mentioned "particle size distribution measurement method" by sieving on samples of 500 g each extracted from the hydrous pellets (first day of the drying process) or from the sufficiently dried reducing agent-containing pellets (17th day of the drying process). The change in particle size distribution of the hydrous pellets (or the dried reducing agent-containing pellets) of each Example and Comparative Example before and after the drying process is shown in Table 4.

(Zinc Oxide)
In a manufacturing plant for manufacturing zinc oxide ore by the above-mentioned "overall process", i.e., a process in which the reduction roasting process, the wet process, and the dry heating process are sequentially performed, a test operation was carried out using the reducing agent-containing pellets manufactured under the respective manufacturing conditions of Examples 1 to 3 and the Comparative Example, and the chromium content in the final product (zinc oxide ore) was measured for each case in which different reducing agent-containing pellets were used. The chromium content was measured by fluorescent X-ray analysis. The results are shown in Table 5. Table 5 also shows the proportion of -1.0 mm fine powder (particle size distribution (%)) for the reducing agent-containing pellets of each Example and Comparative Example.

(Changes in moisture content of wet pellets during the drying process)
As shown in Table 3, the moisture content on the first day of the drying process was 15.7% on average for all of the Examples and Comparative Examples. On the 17th day after the drying process was completed, the moisture content was the same average of 6.2%. For all of the hydrous pellets produced under different production conditions, the moisture content decreased relatively rapidly to an average of 7.1% during natural drying up to the 7th day, and thereafter the rate of decrease in moisture content became relatively slow up to the 17th day. From this, it can be seen that the number of days for which the drying process is carried out in the production of the reducing agent-containing pellets of the present invention is preferably 7 days for natural drying indoors.

(Changes in particle size distribution of wet pellets during drying process)
Example 1: On the first day of the drying process (state of the hydrous pellets immediately after granulation), the ratio of +4.0 mm to +6.7 mm was about 50%, which confirmed that granulation had progressed sufficiently. On the 17th day of the drying process (state of the reducing agent-containing pellets after sufficient drying), the ratio of -1 mm remained below 2%.
Example 2: On the first day of the drying process (the state of the water-containing pellets immediately after granulation), the ratio of +4.0 mm to +6.7 mm was about 50%, and it was confirmed that granulation had progressed sufficiently. However, the fact that +11.2 mm also increased to about 20% is somewhat disadvantageous in terms of promoting zinc volatilization compared to Example 1. However, on the 17th day of the drying process (the state of the reducing agent-containing pellets after sufficient drying), the ratio of -1 mm remained within a range slightly exceeding 2%.
Example 3: On the first day of the drying process (in the state of the water-containing pellets immediately after granulation), the ratio of +1.0 mm to +2.0 mm or +4.0 mm to +6.7 mm is about 40%, so granulation has progressed, but the ratio of +1.0 mm is 10% or more, which is somewhat insufficient compared to Examples 1 and 2. Also, the fact that +11.2 mm has also increased to nearly 20%, is somewhat disadvantageous in terms of promoting zinc volatilization, as in Example 2. However, on the 17th day of the drying process (in the state of the reducing agent-containing pellets that have been sufficiently dried), the ratio of -1 mm is about 5%, which is less than 1/3 of Comparative Example 1.
Comparative Example 1: On the first day of the drying process (the state of the water-containing pellets immediately after granulation), the ratio of +1.0 mm to +2.0 mm was about 50%, and granulation did not progress to +4.0 mm to +6.7 mm. Then, on the 17th day of the drying process (the state of the reducing agent-containing pellets after sufficient drying), the ratio of -1 mm increased to over 18%. As a result, the chromium content of the zinc oxide ore increased to 0.28%. This is considered to be due to the fact that the fine particle size of the granulated steel dust and PC carbon peeled off or collapsed during the drying process, and as a result, some of the reducing agent-containing pellets that were introduced were carried over.

(Chromium content in zinc oxide ore)
From Table 5, it can be seen that by using the reducing agent-containing pellets (Examples 1 to 3) of the present invention as the primary raw material to be input into the reduction roasting process in the production of zinc oxide ore, even when steel dust with a high chromium content is used, unnecessary carryover of the reducing agent-containing pellets can be suppressed and the chromium content of the zinc oxide ore can be controlled to a low grade of 0.1 wt % or less.

Claims (4)

A method for producing reducing agent-containing pellets for use in a method for producing zinc oxide ore from iron and steel dust, comprising the steps of:
a kneading step of adding water to the steel dust and the reducing agent and kneading the mixture;
a granulation step of further adding water to the kneaded product obtained in the kneading step to obtain wet pellets with a moisture content of 10% or more and 20% or less;
a drying step of drying the water-containing pellets until the moisture content of the water-containing pellets becomes 5% or more and less than 10% to obtain reducing agent-containing pellets;
Equipped with
When the total amount of water added in the kneading step and the granulating step is taken as 100% , water in a proportion of 20% or more is added to the steel dust and the reducing agent in the kneading step,
In the kneading step and the granulation step, the rotation speed of a kneader for kneading and a granulator for granulation are adjusted so that the particle size distribution of the water-containing pellets is such that the ratio of particles having a particle size of less than 1 mm is 4% or less.
A method for producing reducing agent-containing pellets. When the total amount of water added in the kneading step and the granulation step is taken as 100% , water is added to the steel dust and the reducing agent in the kneading step in a proportion of 30% to 80%.
The method for producing the reducing agent-containing pellet according to claim 1 . The composition of the pellet raw materials is adjusted so that the particle size distribution of the entire pellet raw materials, including the steel dust and the reducing agent, to be fed into the kneading step is such that the ratio of pellets with a particle size of less than 1.0 mm is 30% or more.
The method for producing the reducing agent-containing pellet according to claim 1 or 2. A reducing and roasting step of reducing and roasting the reducing agent-containing pellets obtained by the method for producing reducing agent-containing pellets according to any one of claims 1 to 3 to obtain crude zinc oxide dust;
a wet process for subjecting the crude zinc oxide dust to a wet process to obtain a crude zinc oxide cake;
A drying and heating step of subjecting the crude zinc oxide cake to a dry and heat treatment to obtain zinc oxide ore,
The chromium content of the zinc oxide ore is 0.1% by weight or less.
How zinc oxide ore is produced.