Physical method solar cell polysilicon

What is the slagging and standing clarification method?

admin on December 31, 2021 0 Comments • Tags: #dephosphorizingagent #precipitationfluxdeslaggingrefiningmethod #slaggingagent

What is the slagging and standing clarification method?

The slag-making static clarification method is to add a slag-making agent to keep the melt for a period of time under the condition of maintaining the refining temperature and flux coverage to make the inclusions float or sink and remove.

The low melting point flux added to the metal melt combines with non-metallic inclusions at high temperatures. This driving force mainly comes from the reduction of the interface energy. The adsorption capacity of a solvent depends on its chemical composition. The solvent is non-toxic due to the source requirement, is not easy to react with silicon, and is easy to solidify, float, and then be removed. The solvent should be heated to remove moisture when in use, and it can be stirred at the same time. The silicon liquid is stirred during the refining process outside the furnace to homogenize the silicon liquid composition and temperature and promote the reaction. When the silicon liquid is in a static state, the inclusions are removed by floating upwards, which obeys Stokes’ law, and the removal speed is slow: when the silicon liquid is stirred, the removal of the inclusions is accelerated. It is best to periodically change the direction and speed of stirring to avoid forced convection caused by stirring and prevent natural convection. Every time the silicon water rises to a certain temperature, heat preservation and oxygen blowing, the temperature rise time period should maintain a low oxygen blowing pressure to ensure the smooth flow of the air inlet, so that the repeated operation will increase the temperature to 2000°C. The non-metallic impurities in the silicon water will float on the surface of the silicon water and be removed by other processes.

(1) Soldering agent method
If the density of slag inclusions is less than that of silicon melt, they tend to gather on the upper part and surface of the molten pool. In this case, the flux method should be used. The flux used in the upper flux method has a density lower than that of the molten metal at the melting temperature. The flux is added to the surface of the molten pool, and the inclusions in the upper layer of the molten pool are in contact with the flux, causing adsorption, dissolution or chemical synthesis to enter the flux. At this time, a thin layer of metallic silicon liquid in contact with the flux is purer and its density is greater than that of the silicon liquid containing slag and moves downward. At the same time, the lower layer of silicon liquid containing more slag rises to contact the flux, and the slag in it is continuously absorbed by the flux.

(2) Lower flux method
If the density of the slag inclusion is greater than that of the molten metal, it will mostly gather in the lower part of the molten pool or the bottom of the furnace, and gradually increase from top to bottom. At this time, the lower flux method, also known as the precipitation flux slag removal refining method, should be used. The flux used in the lower flux method has a density greater than that of the molten metal under smelting conditions. After being added to the surface of the molten pool, they gradually sink to the bottom of the furnace. During the sinking process, they will adsorb, dissolve or combine with the slag. Sink to the bottom of the furnace together.

(3) Total flux method
It uses a bell jar or a porous container to add the flux into the melt, and then fully stir it to make the flux evenly distributed in the entire molten pool. While the flux absorbs the slag, under the action of the density difference, the lighter one will float up, and the heavy one will sink. When using a flux with a lower density, sprinkle the flux on the bottom of the furnace before charging, and the same slag removal effect can be obtained. Compared with the previous two fusion methods, the overall flux method is characterized by increasing the chance of contact between the slag inclusion and the flux, which is beneficial to the adsorption, dissolution or chemical synthesis, and improves the slag removal and purification effect. The slagging agent is composed of some metal oxides and salts, and the composition of the slagging agent is related to the content and composition of impurities in the metal silicon.

Among them, the dephosphorization agent is mainly composed of an oxidant, a slag-forming agent and a co-solvent. Its function is: after blowing oxygen to oxidize the phosphorus in metallurgical silicon water to P2O5, the slag-forming agent combines with P2O5 to form phosphate and stays in the dephosphorization slag.

At present, there are two types of slag-making agents used in industry. One type is soda (sodium carbonate), which can both oxidize phosphorus and generate sodium phosphate and stay in the slag: when no additional oxidant is added, soda can directly supply oxygen and Slagging. The dosage is generally 30~60kg per ton of metallurgical silicon, and the reaction is as follows:

The other type is lime-based dephosphorization agent, which oxidizes phosphorus to P2O5 by calcium oxide and oxygen, and then combines with lime to form phosphoric acid which is left in the slag. If the lime-based dephosphorization agent is equipped with an oxidizing agent or blowing oxygen and a co-solvent, A high dephosphorization rate can be achieved, and at the same time a certain desulfurization rate.

In order to reduce the oxidation of silicon, 1%~10% of silicon carbide particles can be added during refining to control the oxidation of silicon. The temperature of silicon from the T industry is about 1800℃, the oxygen blowing refining temperature in the ladle is controlled between 1600~1800℃, the temperature during air blowing and mixing is controlled between 1500~1700℃, and the slagging temperature is controlled at 1450~1500℃ between. In addition, the large dry 15μm impurities in the silicon liquid can be removed with a ceramic foam filter. Foam ceramic materials have special structures such as a three-dimensional network skeleton structure and high porosity. When the fluid flows through the ceramic foam, it has the advantages of small pressure loss, large surface area, high fluid contact efficiency and light weight. Different from the filter materials used in the past, ceramic foam has the advantages of simple preparation process, energy saving, high temperature resistance, and low pollution of raw materials, and it has higher filtration efficiency.

What is the slagging and standing clarification method?
Silicium Refining furnace

The above method is similar to the out-of-furnace refining of industrial steelmaking. It is necessary to set up a refining furnace next to the submerged arc furnace. After the silicon water comes out of the submerged arc furnace, the silicon water is directly or indirectly injected into the refining furnace by induction heating. Apply heat to keep the silicon liquid in a liquid state.

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