The Security Council of the UN is made up of the five permament members and ten other members who are elected on two year terms by the General Assembly.
Most decisions are made by consensus, however when decisions are not consensual, a vote is taken. According to article 27, there is one vote allowed per member. Moreover, there are two kinds of decisions that the Security Council makes, procedural and substantative. A decision by the council takes nine yes votes; on a substantative decision, all five permanent members of the council must also concur, this need not be the case for a procedural decision.
There has been some debate as to what constitutes a procedural or a substantative issue. Between 1946 - 1991, 160 procedural decisions have been recorded and Council practise shows that such a decision is generally on whether to include an agenda item, to decide on where to hold a meeting, or to call an emergency meeting of the General Assembly and to extend participation to other members.
Thus it seems to expel a member would seem to be a substantative decision. It would require a consensual decision of the Council, or failing that, nine yes votes from the fifteen standing members, including all the votes from all five of the permanent members: China, Russia, France, USA & UK.
A member state has, upto now, never been recommended for expulsion. However, the expulsion of South Africa was notably called for many states due to its policy of apartheid. How this was handled in the UN shows what 'expulsion' might mean even in the case of where the Security Council failed to agree.
In 1974, the General Assembly called upon the Security Council to review the membership of South Africa due to its constant violation of the UN Charter and the Declaration of Universal Human Rights. A resolution drafted by Cameroon, Iraq, Kenya and Mauritania recommending immediate expulsion under Article Six was considered but was not adopted due to negative votes by three of the permanent members - France, UK & USA.
Nevertheless, after the Security Council reported back to the General Assembly their failure to adopt the resolution, the President of the General Assembly, Abdelaziz Bouteflika of Algeria ruled that
South Africa was to be barred from participation from the work of the UN. This ruling was upheld by 91 votes to 22, with 19 abstentions. In subsequent missions of the UN, South Africa was not represented.
Following the successful democratic elections in South Africa in May 1994 after the dissolution of apartheid, South Africa was welcomed back to the UN in the following month.